Biochemistry ~ Carbohydrates
OVERVIEW
A.) Types:
MONOSACCHARIDES
- compounds that contain at least 3 carbon atoms + hydroxyl groups and usually an
aldehyde or ketone group
glucose
= major product of digestion (some galactose and fructose also produced)
= major fuel source oxidized by cells for energy
= stored as glycogen or triacylglycerols after a meal
= converted to glucose 6-phosphate when enters cell, then G6P enters glycolysis ( NADH
& ATP) or pentose phosphate pathway ( NADPH)
Glycogen:
- major storage form of carbs in animals
- largest stores in mm and LV
- mm glycogen used to generate ATP for mm contraction
- LV glycogen used to maintain blood glucose during fasting or exercise
- LV produces glucose from glycogenolysis & gluconeogenesis
- cannot be reduced or hydrolyzed into smaller or simpler sugars
- produced by the breakdown of dietary carbohydrates
- major energy source for the body
- usually found as D-series vs its enantiomer (mirror image) L-series
Structure: (formula = CH2O)n
- all of the above are hexose (6carbon) monosaccharides)
- mainly ring structures where aldehyde or ketone group reacts with hydroxyl group
- structural formula is C6H12O6 (differ in the position of the =O)
- epimers: stereoisomers that differ in the position of the hydroxyl group at only 1 assymetric
carbon
- aldose or ketose depending on aldehyde or ketone as most oxidized functional group
Fructose glucose
- glucose,
- fructose,
- galactose,
- mannose
(all isomers of
eachother)
Glucose: - most important & abundant
- body uses glucose to synthesize other carbs
- major source of fuel in the body
Fructose: can be converted to glucose
- a ketose (all others = aldoses)
- forms furanose rings vs. pyranose rings
- metabolized mainly in the liver  glyceraldehydes 3-P (intermediate of glycolysis)
- can be produced from sorbitol (generated from glucose)
Galactose:
- converted to glucose when taken into the body
Mannose:
- constituent found in the glycoproteins of plant gums
DISACCHARIDE:
- sucrose,
lactose,
maltose,
isomaltose
- 2 monosaccharides covalently bonded via glycosidic attachment
- glycosidic attachment can also bond carbs to purines, pyrimidines, aromatic rings, lipids etc…
- bond is between anomeric carbon of one molecule & different C of 2nd molecule
Sucrose: (glucose + fructose)  table sugar
- both use anomeric carbon  ring can’t open  NO alpha or beta ring structure
Lactose: (glucose β-1,4 linked to galactose) - milk sugar
- galactose uses anomeric carbon
- glucose does not use anomeric C, so can open and form alpha or beta ring structure
Maltose: (glucose + glucose)
- one glucose uses anomeric carbon
- other glucose does not use anomeric C, so can open and form alpha or beta ring structure
Isomaltose: (glucose + glucose) – like maltose
- bound by alpha-1,6 glycosidic linkage (vs. alpha-1,4 in maltose)
POLYSACCHARIDE:
Amylase,
amylopectin,
glycogen, inulin
>6 monosaccharides
- can be linear or branched
- major role = storage of monosaccharides (starch, amylase, amylopectin)
- major role = structural support (cellulose, chitin)
1
Amylose: ( lots of glucose molecules)
- non branching (unbranched) starch of alpha-1,4 glycosidic linkages
- spiraling helical shape or coil
- abundant in many plants
Amylopectin:
- branching starch of alpha-1,4 glycosidic linkages
- branch points = alpha- 1.6 linkages
- branch points at about every 24- 30 glucose residues (not how described in H.M. pg 125)
Glycogen:
- storage molecule of glucose in animals / humans
- more highly branched than amylopectin
- branch points every 8-12 glucose residues (so more branches than amylopectin)
- has many non-reducing ends  rapid mobilization of glucose in times of metabolic need
* enzymes work at the ends …so there are more places for enzymes to work on glycogen
because of the branch points
- α-1,6 branches cleaved by glycogen debranching enzyme
Inulin:
- starch found in many roots
- made of repeating fructose molecules (vs. glucose)
- excreted almost completely in urine with no reabsorption in renal tubules
- gold standard for determining glomerular filtration rate of KI
B.)
Digestion
Overview
- principle sites = mouth and intestinal lumen
- major dietary carbs = starch, sucrose, lactose
- final products = glucose, fructose & galactose
- final products  absorbed by intestinal epithelial cells & enter blood
DIGESTIVE
ENZYMES
SUGARS:

simple sugars are disaccharides

complex sugars – starch, amylopectin

only monosaccharides can be absorbed

if there is a  1-4 linkage, as in cellobiose, enzyme can’t break this down
DIGESTION:

for simple sugars (i.e. disaccharides), digestion is not in the mouth or stomach

must be broken into monosaccharides in the small intestine by membrane bound
enzymes

for membrane bound enzymes, the active sites must face lumen (external side of GIT)

only soluble enzymes are broken down in the mouth

for complex sugars, they start to be broken down in the mouth by -amylase

-amylase in pancreas is where the majority of starch digestion takes place
MOUTH:
Salivary α-amylase:
- contained in saliva & cleaves starch & glycogen
- hydrolizes α-1,4 glycosidic linkages (between glucose residues)
- enzyme deactivated by lower pH(high acidity) when food enters stomach
- dextrins = major product that enter the stomach
INTESTINES
1.) Pancreatic enzymes:
- pancreas and GB secrete bicarbonate to neutralize acidic stomach contents
- have to raise pH for pancreatic enzymes to work
Pancreatic α-amylase:
- interacts with dietary starch & glycogen not fully broken down by salivary α-amylase
- acts in lumen of intestine
- cleaves α-1,4 glycosidic linkages (like salivary amylase)
- products = disaccharides, trisaccharides & small oligosaccharides  require different
enzymes in the small intestine brush border to be broken down
2.) enzymes of intestinal cells:
Small intestine:
- secretion of Brunner & Lieberkuhn glands on the brush border (RATE LIMITING STEP)
- disaccharidases (alpha-glucosidase or maltase)
- oligosaccharidases (alpha-dextrinase)
2
- remove glucose molecules from non-reduced ends  produce monosaccharides
- complete the conversion of starch/glycogen to useable forms of glucose
Sucrase: sucrose  glucose + fructose
Lactase: lactose  glucose + galactose
Isomaltase: cleaves cleaves α-1,6 linkages releasing glucose from oligosaccharides
Glucoamylase & maltase: cleave glucose residues from non-reducing ends of
Oligosaccharides
- cleaves α-1,4 from maltose  releases 2 glucose residues
Large Intestine:
- indigestible polysaccharides (eg. β-1,4 bond of cellulose) = dietary fiber
- aide in passage of stool
C.
Absorption
OVERVIEW
- absorption begins once carbs broken down to monosaccharides
- monosaccharides enter portal venous system & proceed to the liver
- duodenum & jejunum absorb bulk of dietary sugars (insulin is not required for glucose uptake)
- glucose, fructose & galactose = breakdown products absorbed in intestines
-
ABSORPTIVE
PROTEINS
Recognition:
- transmembrane & peripheral membrane proteins act as recognition molecules
Transport:
- free glucose  across epithelium, from lumen of SI into the blood
- sodium dependent hexose transporter
- transport protein requires concurrent uptake of sodium ions = 2ndary active transport
- a Na+, K+ ATPase pumps Na+ into the blood, & glucose moves down [ ] gradient from cell
into the blood
D.
Metabolism
Role of the Liver
(where carb
metabolism
happens)
Role of
Hormones
- converts glucose  glycogen (glycogenesis) when blood glucose levels are high
- converts glycogen  glucose (glycogenolysis) when blood glucose is low
- capable of interconverting sugars (eg. fructose & galactose into glucose)
- converts glucose into fat
- converts amino acids & other molecules  glucose when glycogen stores low
(gluconeogenesis)
Insulin:
- produced by beta cells of islets of Langerhans in the pancreas
- increases rate of glucose use via:

Oxidation

Glycogenesis

Lipogenesis
- increase blood glucose & amino acids (arginine & leucine) stimulate the release of insulin
- works to reduce blood glucose by:

Increasing facilitated diffusion of glucose into muscle and adipose cells

Promoting storage of glucose & glycogen in liver and muscle cells

Enhancing uptake of glucose by adipose & liver cells to convert to fat
* liver, brain & red blood cells do not need insulin for uptake of glucose
- insulin effects the long term enhancement of glucose uptake in the liver by affecting the
synthesis of enzymes for controlling glycolysis, glycogenesis & gluconeogenesis
Glucagon:
- produced by alpha cells of the islets of Langerhans
- increases blood glucose levels
- in metabolic pathways in the liver:

Glucagon stimulates breakdown of glycogen (glycogenolysis) stored in the liver

Glucagon activates hepatic glucogeogenesis
- has minor effect of enhancing lipolysis of triglycerides in adiptose tissue  provides fatty acid
fuel to most cells to conserve glucose if needed
secreted in response to:

Low blood glucose

Eleveated blood amino acids (protein rich meal, or after exercise)
* insulin & somatostatin inhibit glucagon secretion
3
Cortisol:
- glucocorticoid produced by adrenal cortex
- released in low blood glucose states
- stimulates processes needed to increase and maintain normal [glucose] In the blood via:

Stimulation of gluconeogenesis (particularly in the liver)

Mobilization of amino acids from extra-hepatic tissues

Inhibition of glucose uptake in muscle and adipose tissue

Stimulation of fat breakdown in adipose tissue
Epinephrine:
- produced by the adrenal medulla
- favors breakdown of liver and muscle glycogen (glycogenolysis) via production of cyclic
adenosine monophosphate (cAMP) in liver & muscle cells
- decreases release of insulin
GLYCOLYSIS
(glucose
catabolism)
Major fates of glucose:
1.) stored as polysaccharide or sucrose
2.) oxidized to 3 carbon compound (pyruvate)  via glycolysis
3.) oxidized to pentoses  via then pentose phosphate pathway
Overview:
- a molecule of glucose is degraded ina series of enzyme-catalyze rxns  2 pyruvate molecules
- during the reaction, some of the free energy release is conserved as ATP
- breakdown of a 6C glucose  2x 3C pyruvate in ten steps
Preparatory phase: = first 5 steps
Payoff phase : energetic gain happens here
- overall end products = ATP, NADH & pyruvate
- 3 reactions are irreversible & are regulated by the needs of the cell
GLYCOLYSIS:
 3 irreversible steps – all kinases
 PFK 1 – most important for glycolysis – the switch
 glucokinase in liver metabolism not inhibited by product
 PFK 1 asks the cell – do you need energy? do we need glycolysis?
a.) Location:
- cytoplasm (cytosol) of the cell in all body tissues
- pyruvate = end product of cells with mitochondria
- lactate = end product of cells lacking mitochondria (eg. red blood cells)
PHASE I GLYCOLYSIS:
GLUCOKINASE (liver)/ HEXOKINASE(all tissues): (Glucose  glucose 6-P)

add P to C-6 (ATP ADP)

IRREVERSIBLE

adding P traps glucose molecule in cell
PHOSPHOGLUCOSE ISOMERASE: (Glucose 6-P  Fructose 6-P)

REVERSIBLE

how does enzyme know what to do?

[ ] of substrates and products – whatever product is being used, more of that one will be
made = Le Chatelier’s Principle
PHOSPHOFRUCTOKINASE 1: (Fructose 6-P + ATP  fructose 1,6-bispohsphate + ADP)

it costs to trap (i.e. to put on a P)

this is the KEY CONTROLLING ENZYME/ SWITCH for glycolysis

IRREVERSIBLE

if PFK is working, you will have glycolysis

when you trap, you can put glucose 6-phospate into other pathways other than glycolysis
(i.e. hexokinase)

add another P to destabilize the molecule

2 –vely charged P on opposite sides will pull the structure apart into 2 separate
molecules
4
PHASE II A GLYCOLYSIS:
ALDOLASE: (fructose 1,6 bisP  glyceraldehyde 3P + dihydroxyacetone phosphate-DHAP)

splits into 2 different molecules

REVERSIBLE

1 ketone (50%) and 1 aldehyde (50%) – each with phosphate attached

with phosphate sequestered in cell, the molecule is sequestered in the cell because of
its’ –ve charge
TRIOSE PHOSPHATE ISOMERASE: (DHAP glyceraldehydes 3-P)

50/ 50 ratio becomes 95/ 5 – making more dihydroxyacetone as using more
glyceraldehyde

as remove glyceraldehydes, the equilibrium shifts to make more dihydroxyacetone which
is then converted to glyceraldehydes

this is Le Chatelier’s Principle – using glyceraldehyde so will make more glyceraldehydes
(a reversible reaction will go in the direction in which you are removing the product)
NET:
- 2 molecules of glyceraldehyde 3-P formed from 1 glucsoe
PHASE II B:
GLYCERALDEHYDE 3-P DEHYDROGENASE: (G 3-P  1,3 bisphosphoglycerate

add phosphate

reduce NAD+  2 NADH

REVERSIBLE
PHOSPHOGLYCERATE KINASE: (1,3 bisphosphoglycerate + ADP  3-phosphoglycerate +
ATP)

REVERSIBLE

most kinases are irreversible reactions
PHOSPHOGLYCEROMUTASE: (3-phosphoglycerate  2- phosphoglycerate)

REVERSIBLE

give up P to give to ADP (transfers P from carbon 3  carbon 2)
ENOLASE: (2- phosphoglycerate  PEP via dehydration)

REVERSIBLE
PYRUVATE KINASE: (phosphoenolpyruvate + ADP  pyruvate + ATP)

IRREVERSIBLE
B. CO-ENZYMES + COFACTORS)
1.) hexokinase, glucokinase & pFK-1 need Mg + ATP
2.) the enzymes that make ATP need ADP & Mg
3.) pyruvate kinase needs K+
4.) G3-P dehydrogenase needs NAD (step # 6)
C.) REGULATION OF RATE-LIMITING ENZYMES:
1.) Hexokinase or glucokinase
Glucokinase (the turtle)
- found in liver
- functions at high rate only after a meal
When glucose levels in hepatic portal vein are
high
- induced when insulin is high
- not inhibited by its product (G 6-P)
- low affinity for glucose therefore just traps
and traps and traps
- will keep trapping and if too many carbs,
body will make fats
-liver is normally a glucose-producing rather
than a glucose-using organ
- however, after a meal containing
carbohydrate, the liver becomes a net
consumer of glucose, retaining roughly 60%
of every 100g of glucose presented to portal
system
Hexokinase (the rabbit)
- found in extrahepatic cells
- functions when glucose 6-P is being rapidly
used
- inhibited by its product (G 6-P) 
preserving a supply of glucose
- high affinity for glucose, but inhibited by
product which means it is sensitive to need
5
2.) Phosphofructokinase: (Fructose 6-P + ATP  fructose 1,6-bispohsphate + ADP)
- key controlling enzyme of glycolysis
- most strongly activated by Fructose 2, 6-P

- question enzymes asks cell – do you need ATP?

inhibited allosterically by elevated levels of ATP

activated allosterically by high [ ] of AMP, which signals that the cell’s energy stores
are depleted

only get AMP is build-up of ADP

if make AMP, it means there is not enough ATP
in fed state, insulin: activates phosphatases to dephosphorylate PFK-2  Fructose 2, 6-P 
increased activity of PFK-1
- PFK 1 is inhibited by ATP & citrate (regulatory mechanisms in muscle)
- if ATP high, cell does not need anymore - glycolysis inhibited
3.) Pyruvate Kinase:
- transfers P from phosphoenolpyruvate to ADP  ATP
- activated by fructose 1,6-bispohsphate (product of PFK-1 rxn)
- activated in fed state (insulin stimulates phosphatases that dephosphorylate & activate PK)
- glucaon  increase intracellular cAMP  activates protein kinase A  inactivates pyruvate
kinase  pyruvate for gluconeogenesis
- inhibition of pyruvate kinase promotes gluconeogenesis
D. FUNCTIONS: (Production of energy + end products)
a.) Anaerobic energy:
- in tissues without mitochondria
- or, under anaerobic conditions
Result = 2 lactate + 2 ATP
Aerobic energy: (stage I consumes 2 ATP, Stage II produces 4 ATP = NET 2 ATP)
- in tissues with mitochondria
Result = 2 ATP + 2NADH + 2 pyruvate
- pyruvate enters Kreb’s cycle  more ATP via NADH + FADH2  NADH enters electron
transport chain  ATP
E. INTEGRATION WITH OTHER PATHWAYS:
- G 6-P is an important compound at the crossroad of a number of pathways:
 glycolysis, gluconeogenesis, pentose phosphate pathway, glycogenesis, glycogenolysis
Fate of pyruvate:
GLYCOLYSIS
GLUCOSE
↓
2 PYRUVATE
2 ETHANOL + 2CO2
(anerobic)
↓
2 LACTATE
(anerobic)
KREB’ CYCLE
2 ACETYL-CoA
(aerobic)
↓
4CO2 + 4H2O
6
Krebs Cycle
(TCA)
** Pyruvate
Dehydrogenase
is required to
convert Pyruvate
 Acetyl Co-A
for the Kreb’s
(see last page of
notes)
OVERVIEW
- their carbon skeletons converted  CO2 + H2O
- oxidation provides energy for the production of the majority of ATP in most animals
- totally in mitochondria
- aerobic pathway because O2 is required as final electron acceptor
- pyruvate from aerobic glycolysis is transported into the mitochondrion before entering Krebs
- pyruvate converted to acetyl CoA by pyruvate dehydrogenase complex
- handle = oxaloacetate made from pyruvate  no oxaloacetate = NO TCA
- fats give Acetyl-CoA, but not the handle, so … “fats burn in the flame of carbohydrates”
- intermediates made in Kreb’s are used in other rxns too (eg. oxaloacetate in gluconeogenesis)
7
a.) LOCATION
cellular : occurs in soluble matrix area of mitochondria to liberate hydrogen ions for production of
ATP in ETC
tissue : only occurs in tissues with mitochondria (eg. not in the RBC)
b.) CO-ENZYMES + CO-FACTORS:
- NAD + FAD needed in several reactions
- Manganese needed in 1 rxn
- Iron in 1 rxn
- CoASH, TPP + lipoic acid in for the 2 dehydrogenase enzymes
- GDP needed for 1 rxn
1)







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
Can I Keep
Selling Sex For
Money, Officer?
2)



citrate synthase (Acetyl CoA + oxaloacetate - citrate)
condensing enzyme
oxaloacetate must bind to enzyme before Acetyl-CoA can attach
irreversible
no ATP – anabolic rxn
acetyl CoA has energy which is used to attach to handle
citric acid can leave mitochondria if builds up
has transporter
oxaloacetate and acetyl CoA can’t leave – no transporter
how do we get CoA in mitochondria if we don’t have a transporter?

make it

synthesized in mitochondria
aconitase (citrate  isocitrate)
moves OH group – isomer
required to prepare molecule to lose carbon
COO
can leave with appropriate enzymes
3)
isocitrate dehydrogenase (isocitrate  α-ketoglutarate)

forming reduced cofactor

irreversible

lose CO2

forms NADH
4)
 - ketoglutartae dehydrogenase (α-ketoglutarate  succinyl CoA)

complex

similar to pyruvate dehydrogenase

lose CO2

forms NADH

steps 3 and 4 release CO2 from citric acid
5)
succinate thiokinase (succinyl CoA  succinate)

CoA linkage has energy

energy transferred to GDP to make GTP (high energy molecule)

SCoA has energy!
6)
succinate dehydrogenase (succinate  fumarate)

- oxidation

fatty acid  acetyl CoA is - oxidation

this enzyme is in the only one in the inner mitochondrial membrane vs. matrix
7)
fumarase (fumarate  malate)

addition of water across a double bond
8)
malate dehydrogenase (malate  oxaloacetate)

2 H+ are passed to NAD+  NADH + H
8
C. REGULATION OF RATE LIMITING ENZYMES:
** TCA cycle regulated by cells’s need for energy (when ADP is high)
** Acts in concert with the ETC to produce ATP
CONTROLS:
1.) oxaloacetate

limiting

only small amounts present in mitochondria
2.) citrate synthase (acetyl CoA  citrate)

inhibited by succinyl-CoA, citrate, NADH, ATP

controlled by availability of substrates (acetyl-CoA & oxaloacetate)
3.) isocitrate dehydrogenase

first committed step of Krebs

citric acid can leave Krebs

once at isocitrate, good chance will have Krebs

inhibited by NADH, ATP

stimulated by isocitrate, ADP & NAD

only burn calories that we need

store extra calories as fat  obesity

only burn nutrients if need ATP

in theory, reaction is reversible

in practice, not reversible once lost CO2

4.)  - ketoglutarate

similar to pyruvate dehydrogenase

senses product and cofactors

not directly sensitive to ATP

inhibited by succinyl-CoA and NADH, ATP & GTP
D. FUNCTIONS  ENERGY PRODUCTION & END PRODUCTS
- 3 x NADH(3 ATP) = 9 ATP
- 1 x FADH2 (2 ATP) = 2 ATP
- 1 x GTP (1 ATP)
= 1 ATP
TOTAL
= 12 ATP (per turn of the cycle)
2.) END PRODUCTS:
* 3 molecules of CO2 also produced
* intermediates are either siphoned off for use in, or replenished by other metabolic processes
- thus, the cycle is integrally connected to other pathways
* intermediates of the TCA cycle used in fasting state in the liver to make glucose (via
gluconeogenesis) and in fed state to make fatty acids
* intermediates also used to make amino acids
3.) Electron
Transport Chain
* you might
want to consult
another diagram,
as this one is
missing FADH2
9
* e- = electrons
OVERVIEW:
 energy derived from transfer of e- through the ETC is used to pump H+ across the inner
mitochondrial membrane from the matrix to the cytosolic side
 NADH & FADH2 donate a pair of e- to carriers of the ETC
 As e- move down the chain, they lose energy that is captured by the production of ATP
 H+ move back into the matrix via ATP synthase complex producing ADP  ATP
 ATP is transported from matrix to the cytosol in exchange for ADP via the ATP-ADP antiport system
 NADH & FADH2 are oxidized only if ADP is available for conversion to ATP
 Because energy generated by the transfer of e- through the ETC to O2 to make ATP, it =
oxidative phosphorylation
 The innter mitochondrial membrane is impermeable to H+. So, H+ re-enter only through ATP
synthase  generation of ATP from ADP

- NADH & FADH2 are produced by glycolysis, β-oxidation of fatty acids, the TCA cycle etc…
- NADH freely diffuses from the matrix to the membrane
- FADH2 is bound to enzymes that produce it within the mitochondrial matrix
The ETC: 3 major stages: ( Complexes I – IV)
1.) Transfer of electrons from NADH to CoEnzyme Q (ubiquinone)

NADH passes e- via NADH dehydrogenase complex to FMN

FMN passes the e- through a series of iron-sulfur (Fe-S) complexes to CoQ

Energy produced by the e- transfers  pumps H+ to cytosolic side
2.) Transfer of electrons from CoQ to cytochrome c

CoQ passes e- through Fe-S centers to cytochromes b and c1, which transfer e- to
cytochrome c
b.) electrons from FADH2 (from oxidation of succinate  fumarate) enter the ETC at the
CoQ level (complex II) – but no H+ is pumped across the membrane at this point
3.) Transfer of electrons from cytochrome c to O2

Cytochrome c transfers e- to the cytrochrome aa3 complex (contains heme and
copper), which transfers them to oxygen, reducing it to water

Catalyzed by cytochrome oxidase

2 e- are needed to reduce one atom of oxygen (so need 1 NADH [which has 2e-] to be
oxidized, & ½ O2 converted to H2O
ATP Production/ Oxidative Phosphorylation
- 1 ATP is generated for every 2 electrons (4 H+) passed on to Oxygen
- 3 ATP/NADH, 2ATP/FADH2
High energy Compounds:
- ATP is the high energy molecule generated by oxidative phosphorylation
- contains a ribose sugar that has an adenine base and 3 attached phosphates
- the phosphate groups have high-energy anhydride bonds that release energy when broken
Location:
Cellular:
- ETC is embedded in the inner mitochondrial membrane (cristae)
- components are arranged in order of increasing redox potential (become more +)
- interior matrix is where the chemical reactions that produce e- donors NADH & FADH2 happen
Tissue:
- ETC is found in all tissues that contain mitochondria
Co-Enzymes & Co-Factors:
- NADH dehydrogenase (in Complex I) contains iron atoms + sulfur atoms  iron-sulfur centers
- the cytochromes contain heme groups made of a porphyrin ring containing an atom of iron
Inhibitors and Uncouplers:
- if there is a block anywhere in the chain, all carriers before will accumulate in their reduced
states, and all carriers after will accumulate in their oxidized states.
- O2 will not be consumed, ATP not generated and the TCA cycle will slow down
- ETC can be inhibited at all stages of the process
10
2,4 Dinitrophenol:

Lipophilic H+ carrier that readily diffuses through the mitochondrial membrane

causes ETC to proceed at a rapid rate without establishing a proton gradient

energy is released as heat ( explains why you get a fever with an overdose of aspirin)
Rotenone & Amytal:

complexes with NADH dehydrogenase  accumulation of NADH (prevent use of NADH)

does not block the transfer of e- from FADH2
Atnimycins: (antibiotics)

block passage of e- through the cytochrome b-c1 complex
Cyanide, azide & carbon monoxide:

block electron flow in cytochrome c oxidoreductase (ie. transfer of electrons to O2)

cyanide poisoning  inhibited respiration (O2 can’t receive e-)  death occurs rapidly
Oligomycin & DCCD:

prevent the influx of protons through the ATP synthase
* if actively respiring mitochondria are exposed to an inhibitor of ATP synthase  ETC stops
operating
* atractyloside (plant glycoside) & bongkrekic acid (antibiotic from mold), inhibit ATP-ADP
translocase, even in low concentrations.
Uncoupling:
- the ETC can be uncoupled by compounds that increase permeability of inner mitochondrial
membrane to H+
- uncoupling proteins create a “proton leak” without capturing energy
- aspirin causes uncoupling
- causes ETC to proceed at a rapid rate without establishing a proton gradient
- the energy produced is released as heat rather than to synthesize ATP

Responsible for the activation of fatty acid oxidation and heat production in brown
adipocytes of mammals.

In hibernating animals  brown adipose is specialized for the process of nonshivering thermogenesis (by using uncoupling proteins of the mitochondrial membrane
in BAT)

Functions:
1- production of energy:
- 1 ATP is generated for every 2 electrons (4 H+) passed on to Oxygen
- 3 ATP/NADH, 2ATP/FADH2
2- production of end products  H2O???
Integration with Other Pathways
4.) Gluconeogenesis
OVERVIEW:
- synthesis of glucose from compounds that are not carbohydrates
- main precursors = lactate, amino acids(which form pyruvate or TCA cycle intermediates) &
glycerol
- some tissues (brain, RBCs, KI, lens & cornea of the eye) need continuous supply of glucose
- liver glycogen is an essential supply of post-prandial glucose
- sometimes liver glycogen cannot meet the body’s need in times of prolonged fast

anabolic process that costs energy

costs 6 ATP to make glucose from lactate or alanine
Substrates:

All intermediates of glycolysis and the Citric Acid cycle

Glycerol, lactate and α-keto acids from deamination of glucogenic amino acids =
most important glucogenic precursors (all amino acids except leucine & lysine)

Alanine = major gluconeogenic amino acid (all aa’s are converted to this first)

All non-carb precursors must be converted to oxaloacetate
* Fatty acids canNOT be used because they break down to Acetyl CoA & there is no pathway
to convert Acetyl Co-A to oxaloacetate
Reactions of Gluconeogenesis:

Similar to reversal of glycolysis, except some of those steps are irreversible, so a few
steps are changed – not just a simple reversal!!

Pyruvate is not being made from glycolysis anymore, and instead comes from lactate,
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








Pathway
Produces
Fresh
Glucose
alanine and amino acids
reversible steps are the same for glycolysis and gluconeogenesis
the point of gluconeogenesis is to make pyruvate to glucose and not to acetyl CoA
therefore, pyruvate dehydrogenase must be blocked because pyruvate needs to go to
glucose
therefore, oxaloacetate will not be available for the Krebs cycle – pyruvate to
oxaloacetate and leaves Krebs
instead, oxaloacetate is used for gluconeogenesis
Krebs makes handle for gluconeogenesis
when gluconeogenesis is occurring, the KC is not used to oxidize fuels – therefore,
acetyl CoA is not being burnt
ETC is being used instead
need 4 new enzymes (to reverse 3 steps)

pyruvate carboxylase – pyruvate  oxaloacetate
 in mitochondria

phosphoenolpyruvate carboxykinase

counterpart to pyruvate kinase in glycolysis

pyruvate kinase is an irreversible enzyme
in glycolysis

in cytosol

fructose 1,6-bisphosphatase

counterpart of PFK 1 in glycolysis

PFK 1 is also irreversible in glycolysis

In cytosol

glucose 6-phosphatase

counterpart of glucokinase in glycolysis

this enzyme is also irreversible in glycolysis

in cytosol
Location:
1-cellular – starts in the mitochondria, but mainly in the cytosol of the cells

remember that oxaloacetate is formed in Kreb’s in the mitochondria, but glycolysis
happens in the cytosol of the cell – will have to use malate dehydrogenase to get
oxaloacetate out of the mitochondria and into the cytosol by converting it to
malate which can cross the mitochondrial membrane
2- tissue

occurs mainly in the liver (90% occurs here during an overnight fast)

Kidneys provide 10% of newly synthesized glucose molecules

Kidneys provide 40% of glucose production during a prolonged fast
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Co-Enzymes & Co-Factors:
Pyruvate carboxylase:

Needs ATP, biotin & carbon dioxide

NADH is required for some other steps
Regulation of Rate Limiting Enzymes
* gluconeogenesis only occurs if pyruvate dehydrogenase, pyruvate kinase, PFK-I,
glucokinase & hexokinase are rendered Inactive

pyruvate kinase will be blocked so PEP is not made into pyruvate – first control switch
of gluconeogenesis

PFK1 will be blocked – fructose 1,6-bisphosphate is made into fructose 6-phosphate via
fructose 1,6-bisphosphatase – second control switch

glucokinase will be blocked – glucose 6-P is made into glucose via glucose 6phosphatase – third and last control step
Pyruvate dehydrogenase:

Inhibited by Acetyl-CoA

Pyruvate is then converted to oxaloacetate instead of Acetyl-CoA
Pyruvate Carboxylase:

Activated by acetyl-CoA, cortisol & ATP

Converts pyruvate  oxaloacetate (co-factors listed above)
Phosphoenolpyruvate carboxylase:

Glucagon elevates cAMP  stimulates conversion of pyruvate kinase to inactive form

This diverts phosphoenolpyruvate to synthesis of glucose
PFK-I:


high ATP & citrate inhibit PFK-I  fructose 1,6 bisphosphatase becomes more active
glucagons lowers the level of fructose 2,6 bisphophate  activation of fructose 1,6
bisphosphatase and inhibition of PFK-I
Glucokinase / Hexokinase:

glucokinase is relatively inactive when the amount of its substrate (glucose) is low

hexokinase is inhibited by its product – glucose 6 phosphate
Overall Hormonal:

glucagons stimulate gluconeogenesis under fasting conditions

glucocorticoids & cAMP induce production of PEPCK

low insulin levels favor mobilization of amino acids from muscle protein
Hormonal control in the Liver:
- includes reciprocal effects of a cyclic AMP cascade
- cascade is triggered by glucagon when blood glucose is low
Glucagon  phosphorylation of cAMP-dependent protein kinase (CDPK)  inihibits
glycolysis  stimulates gluconeogenesis  glucose available for release into the blood
Enzymes that are phosphorylated by CDPK (which was triggered by glucagon):
- pyruvate kinase  glycolysis enzyme that is inhibited by CDPK
- bi-functional enzyme PFK2 & fructose-bisphophatase-2 (make and degrade fructose-2,6bisphophate)
 cAMP-dependent phosphorylation of the bi-functional enzyme activates FBPase2 & inhibits
PFK2  decrease of fructose-2,6-bisphophate in liver cells  glycolysis slows down 
gluconeogenesis increases d/t low [fructose-2,6-bisphophate]
SUM: gluconeogenesis = stimulated
Glycolysis = inhibited
Glycogen synthesis = inhibited
Free glucose formed for release to the blood
Non-Hormonal Regulation:
1- PFK (glycolysis) is inhibited by ATP and stimulated by AMP
2. Fructose 1,6 bisphosphatase (gluconeogenesis) is inhibited by AMP
Positive effectors: High energy state: ATP, Acetyl-CoA & Citrate
Negative effectors: Low energy state & lack of glycolytic intermediates
- AMP, Fructose-1,6 bisphosphate or fructose-2,6-bisphophate & inorganic phosphate
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Functions:
1- consumption of energy:
1 mole of glucose from 2 moles pyruvate needs 6 moles ATP
2.- production of end products

2 moles pyruvate  1 mole glucose

2 moles glycerol  1 mole glucose
Integration with Other Pathways
- integrated with several other pathways:
 glucose can be made from:

Lactate

Glycogenic amino acids

Glycerol

5.) Glycogenesis
E = energy
Pyruvate, oxaloacetate & α-ketoglutarate (all are alpha-ketoacids)
OVERVIEW:

body requires a constant source of glucose

glucose is preferred E for brain, and is required for all cells with no/few mitochondria

essential source for exercising muscle b/c it is the substrate for anerobic glycolysis
Blood glucose from 3 sources: 1- diet, 2- glycogenolysis and 3 – gluconeogenesis

gluconeogenesis is slow in response to falling blood glucose levels

glycogen is rapidly mobilizable form of glucose

in absence of dietary source it is rapidly released from liver and kidney (and muscle
when exercising)
Main stores:

skeletal muscle and the liver
Liver glycogen: used to maintain blood glucose during fasting (days) or exercise
 breakdown is stimulated by glucagon and epinephrine
Muscle glycogen: used to generate ATP for muscle contraction (fuel reserve)
 not affected by short periods of fasting, and only moderately depleted during long fasts (weeks)
Reaction: (UDP glucose = precursor for glycogen synthesis)
 glucose enters the cell






Conversion of glucose to G-6-P: hexokinase (glucokinase in the liver)
Conversion of G-6-P to G-1-P: phosphoglucomutase
Synthesis of UDP-glucose: UDP-glucose pyrophoshorylase catalyzes 2
reactions:
Glucose-1-P + UTP Û UDP-glucose + PPi
Glucose residue transferred from UDP glucose to non-reducing end of
glycogen primer: glycogen synthase
** UDP glucose acts as a donor of glucose to form the glycogen
Branching: occurs when > 11 glucose residues
- an oligomer (6-8 residues) is removed by glycosyl 4:6 transferase that breaks an α-1,4 bond
and reattaches it as an α 1-6 bond
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Location:
Tissue:
- 70g in the liver to maintain blood glucose
- 200g in the muscle for exercise
Cellular:
- glycogenolysis and glycogenesis occur in the cytosol of these cells (liver and muscle cells)
Co-Enzymes & Co-Factors:

UTP is a co-factor  converted to UDP-glucose
Regulation of Rate Limiting Enzymes
1- hormonal:
- elevated glucagon and/or epinephrine cause increased glycogen degradation
- epinephrine stimulates glycogenesis and glycogenolysis
- insulin increases glycogen synthesis
- insulin stimulates transport of glucose into muscle for substrate for glycogen synthesis
2- non-hormonal
- glycogen synthesis is promoted by glycogen synthase and increased [glucose] which enters
the liver cells from the hepatic portal vein
Rate Limiting Enzymes:
1.Glycogen synthase: KEY REGULATOR FOR GLYCOGENESIS




exists in active and inactive forms  regulates synthesis of glycogen
inactivated by phosphorylation via protein kinases
activated by protein phosphatase (insulin activates the phosphatases)
requires a primer (either a glycogen fragment or glycogenin) to initiate chain
synthesis  it cannot start a new chain on its own
Functions:
1-production or consumption of energy

1 ATP is required for glycogen synthesis
2- production of end products
Integration with Other Pathways:
5.)
Glycogenolysis
OVERVIEW:
o Glycogen phosphorylase catalyzes the splitting off of a glucose unit as glucose-1-phosphate
which can be converted into glucose-6-phosphate in the glycolytic pathway,
thus eliminating the first priming step in glycolysis.
 a fall in glucose levels leads to the breakdown of glycogen
Glycogen Degradation
Glycogen phosphorylase cleaves a(1®4) by phosphorylysis to yield glucose-1-phosphate


glucose-1-phosphate has to be converted to glucose-6-phosphate to enter glycolysis
cannot cleave branches and stops 4 residues from branch
Removal of Branches



Debranching enzyme (has both glycosyl 4:4 transferase & α 1-6 glucosidase actvitiy)
Cleaves 3 of the 4 remaining residues as a trisaccharide at the branch point and added
on to another chain by 4:4 transferase (cleaves and forms a new α 1-4 bond)
The last glucose at the branch point linked by an α 1-6 bond Is cleaved by α 1-6
glucosidase  forms a free glucose
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Location:
glycogenolysis and glycogenesis occur in the cytosol of these cells (liver and muscle cells
Co-Enzymes & Co-Factors:
Inorganic phosphate = co-factor for glyconolysis
Regulation of Rate Limiting Enzymes
1- hormonal:
- elevated glucagon and/or epinephrine cause increased glycogen degradation
- glucagon acts on liver cells
- epinephrine acts on both liver and muscle cells
2- non-hormonal:
- AMP & calcium stimulate glycogenolysis
Rate Limiting Enzymes:
1. Glycogen phosphorylase a:

regulates degradation of glycogen

glucagons or epineprhrine binding to receptors activates adenylate cyclase  increase
cAMP  activates protein kinase  phosphorylates phosphorylase kinase 
phosphorylates glycogen phosphorylase b (inactive form)  glycogen phosphorylase a
(active form)
Functions:
1-production or consumption of energy
2- production of end products
- glycogen degradation  glucose 1-Phosphate & free glucose
- free glucose in the liver  maintains blood glucose (via Glucose 6-Phosphatase)
- free glucose in the muscle for contraction of the muscle  glucose 6-P enters glycolysisB
(muscle doesn’t have G-6-phosphatase, so the G6P can’t be used for blood glucose, instead is
used to make ATP)
Integration with Other Pathways:
- this pathway is integrated with the glycolytic pathway (see above)
6.) Hexose
Monophosphate
Shunt
(aka. Pentose
Phosphate
Pathway)
OVERVIEW:
- another route glucose can take once made into glucose 6-phosphate
 The pentose phosphate pathway is an alternate route other than glycolysis used to obtain
energy from the breakdown of sugars.
 The cycle starts with glucose-6-phosphate (G6P) and results in the regeneration of G6P and
the formation of NADH, which can later be used to form ATP
Location:
- in cytoplasm of the cells
- important in RBCs – because it protects from oxidative damage by keeping glutathione reduced
- liver and adipose cells – used in fatty acid and cholesterol synthesis
- adrenal cortex – for synthesis of steroid hormones
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Co-Enzymes & Co-Factors:
Regulation of Rate Limiting Enzymes
Functions:
1-production or consumption of energy

no ATP is directly consumed or produced in this cycle

major producer of NADPH for the body (2NADPH for each G6P molecule)

NADPH is used for reduction reactions of FA, cholesterol & steroid synthesis

NADPH also used for hepatic Phase I detox, and synthesis of tyrosine from
phenylalanine
a.) oxidative ()

2NADPH for each G6P molecule

Ribulose 5-phosphate

CO2
b.) non oxidative :

Ribose residues for nucleotide and nucleic acid synthesis

Fructose 6-P and glyceraldehyde-3-P for entry in glycolysis

2- production of end products

main function is to generate NADPH and ribose-5-phosphate
Integration with Other Pathways:
Other
Carbohydrate
Pathways:
(mannose,
fructose,
galactose)
OVERVIEW:
- other monosaccharides absorbed in the small intestine enter specific pathways in the cytosol of
liver cells to be converted into G-6-P for entry into glycolysis, glycogenesis, or the pentose
phosphate pathway
- Fructose is converted to F-1-P and then to DHAP and G-3-P
 DHAP and G-3-P then go through 3 steps of gluconeogenesis pathway to convert  G-6-P
- Mannose and galactose each go through their own 3 step processes - G-6-P
Location:
Co-Enzymes & Co-Factors:
- all of these processes require ATP
- galactose pathway also requires UDP-glucose
Regulation of Rate Limiting Enzymes
Functions:
1-production or consumption of energy
2- production of end products
Integration with Other Pathways:
Acetyl Co-A
Metabolism
OVERVIEW:
- Acetyl-CoA is completely oxidized by the citric acid cycle
Production:
- FA and most amino acids can be degraded to Acetyl-CoA
- Acetyl CoA is produced from pyruvate through an oxidative decarboxylation by pyruvate
dehydrogenase
- Pyruvate dehydrogenase (EC 1.2.1.51) is an allosteric enzyme that transforms pyruvate into
acetyl-CoA by a process called oxidative decarboxylation. Acetyl-CoA may then be used in the
citric acid cycle to carry out cellular respiration.
Location:
17
Co-Enzymes & Co-Factors:
- Pyruvate Dehydrogenase requires
 niacin (as NAD)
 CoASH (pantothenic acid in active form)
 thiamine (as thiamine pyrophosphate)
 riboflavin as FAD
 lipoic acid
5 different cofactors are required for this complex: (FROM WIKIPEDIA.COM)





TPP (Thiamine pyrophosphate) - Hydroxyethyl Carrier
CoA (Coenzyme A) - Substituted onto the Acetyl group to form Acetyl-CoA
R-Lipoic acid - Utilized as a Lysine Tether to transport Acetyl group to acitve site for CoA
addition
FAD (Flavin Adenine Dinucleotide) - Oxidizing agent to oxidize the lipoyllysine sulfaring
to repeat process.
NAD (Nicotinamide adenine dinucleotide)- Oxidizes FADH2 in order to repeat process.
NADH is then used for oxidative phosphorylation or may be used somewhere else in the
cytosol.
Regulation of Rate Limiting Enzymes
- pyruvate  acetyl CoA is regulated via covalent modification of the pyruvate dehydrogenase
complex (PDC)
- as energy needs of the cell are met, ATP levels increase
- ATP acts as a stimulus to change PDC to its inactive form  pyruvate dehydrogenase
phosphate
- when the body needs ATP, PDC is regenerated to active form via free Ca2+
Functions:
- pyruvate is the end product of glycolysis and some amino acid conversions (alanine,
cysteine, glycine, hydroxyproline, serine and threonine)
- some amino acids can be directly to acetyl-CoA (phenylalanine, tyrosine, tryptophan, lysine, and
leucine)
Utilization of Acetyl CoA:
- several metabolic fates for acetyl-CoA
1- it can combine with oxaloacetate to form citrate and go into the Kreb’s cycle
2- it can be transported out of the mitochondria and used for FA synthesis
3- can be used for ketone body synthesis in hepatic mitochondria
4- it can enter the pathway for cholesterol synthesis
Acetyl CoA acts as a precursor:
- Acetyl CoA can form Fatty acids or ketone bodies
- glucose CANNOT be formed from acetyl-CoA in gluconeogenesis because pyruvate  acetyl
CoA = IRreversible
Integration with Other Pathways:
- the acetyl CoA is used to enter into Krebs as well as the pathways mentioned above
18
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