Lecture 20
Enzyme & metabolism
Pages 51-53
&
913-917
in Ch.24
ATP synthesis
Enzymes
• Biological catalysts
– Lower the activation energy, increase the speed of
a reaction (millions of reactions per minute!)
How Enzymes Work
 Enzymes = large protein molecules that
function as biological catalysts.
 Catalyst = chemical that speeds up a reaction
without being consumed
 NOTE: enzyme names end in –ase and are
often named after their substrates
 The enzyme that hydrolyzes sucrose is sucrase
 Hydrolases, Add water during Hydrolysis
reactions
Energy of activation(Ea)
• There is an energy barrier that must be
overcome before a chemical reaction can
begin. This is called The energy of activation
• Enzymes speed up the reaction by lowering
the Ea barrier
• Energy of Activation
How Enzymes Work
– Enzymes lower the barrier
– Eg.Mexican jumping bean
How Enzymes Work
The effect of an enzyme on the energy of activation
WITHOUT ENZYME
WITH ENZYME
Activation
energy
required
Less activation
energy required
Reactants
Reactants
Product
PLAY
Product
Animation: Enzymes
Figure 2.20
How Enzymes Work
 Enzymes
 Very selective
 3D shape that determines its specificity for a
substrate
 Substrate = the substance that the enzyme works
on
 Substrate binds to the enzyme in the active site
 Pocket or groove on protein surface where binding
occurs
Enzyme action
Three basic steps involves in enzyme action:
1) The enzyme active site binds to the
substrate.
2)The enzyme –substrate complex undergoes
internal rearrangement that form the product.
3)The enzyme releases the the product of the
reaction.
How Enzymes Work
Substrates (S)
e.g., amino acids
+
Product (P)
e.g., dipeptide
Energy is
absorbed;
bond is
formed.
Water is
released.
H2O
Peptide
bond
Active site
Enzyme (E)
Enzyme-substrate
complex (E-S)
1 Substrates bind
2 Internal
at active site.
rearrangements
Enzyme changes
leading to
shape to hold
catalysis occur.
substrates in
proper position.
Enzyme (E)
3 Product is
released. Enzyme
returns to original
shape and is
available to catalyze
another reaction.
Figure 2.21
Substrates (S)
e.g., amino acids
+
Active site
Enzyme (E)
Enzyme-substrate
complex (E-S)
1 Substrates bind
at active site.
Enzyme changes
shape to hold
substrates in
proper position.
Figure 2.21, step 1
Substrates (S)
e.g., amino acids
+
Energy is
absorbed;
bond is
formed.
Water is
released.
H2O
Active site
Enzyme (E)
Enzyme-substrate
complex (E-S)
1 Substrates bind
2 Internal
at active site.
rearrangements
Enzyme changes
leading to
shape to hold
catalysis occur.
substrates in
proper position.
Figure 2.21, step 2
Substrates (S)
e.g., amino acids
+
Product (P)
e.g., dipeptide
Energy is
absorbed;
bond is
formed.
Water is
released.
H2O
Peptide
bond
Active site
Enzyme (E)
Enzyme-substrate
complex (E-S)
1 Substrates bind
2 Internal
at active site.
rearrangements
Enzyme changes
leading to
shape to hold
catalysis occur.
substrates in
proper position.
Enzyme (E)
3 Product is
released. Enzyme
returns to original
shape and is
available to catalyze
another reaction.
Figure 2.21, step 3
Characteristics of Enzymes
• Often named for the reaction they catalyze;
usually end in -ase (e.g., hydrolases, oxidases)
• Some enzymes are purely protein.
• Some functional enzymes (holoenzymes)
consist of two parts:
– Apoenzyme (protein)
– Cofactor (metal ion) or coenzyme (a vitamin)
Characteristics of Enzymes
• Cofactors
– Nonprotein helpers
• May be inorganic substances
– Zinc, iron or copper
– Magnesium is a cofactor essential for proper functioning
of chlorophyll
• Coenzymes
– Organic helpers
• Vitamins
– Vitamin B6 is used to convert one amino acid to another
• Enzyme activity is affected by its
environment
– Temperature affects molecular motion
• Enzyme’s optimal temperature is when there is the
highest rate of contact between the enzyme and
substrate
• Temperature too high – denaturation
– Changes the shape and the function of the enzyme
– Salt concentration
• Salt interferes with some of the chemical bonds
that maintain protein shape
– pH
• Same is true for pH outside of the 6-8
range.eg.digestive enzyme produce in pancreas are
activated in small intestine.
Characteristics of Enzymes
• Sometimes enzymes are inactivated
immediately after they have performed .eg.
blood clot formation.
Cellular respiration
Chloroplasts and Mitochondria
• Energy, enzymes, and
membranes
– Important parts of the
functioning of
chloroplasts and
mitochondria
• Photosynthesis and
cellular respiration are
linked
Same for cells
Figure 5.4A ATP structure and
hydrolysis
Adenosine
Adenosine diphosphate
Triphosphate
Phosphate
groups
P
Adenine
P
H2 O
P
P
Hydrolysis
Ribose
ATP
ADP
P
+
P
+
Energy
Adenosine Triphosphate (ATP)
• Adenine-containing RNA nucleotide with two
additional phosphate groups
High-energy phosphate
bonds can be hydrolyzed
to release energy.
Adenine
Phosphate groups
Ribose
Adenosine
Adenosine monophosphate (AMP)
Adenosine diphosphate (ADP)
Adenosine triphosphate (ATP)
Figure 2.23
Function of ATP
• Phosphorylation:
– Terminal phosphates are enzymatically transferred
to and energize other molecules
– Such “primed” molecules perform cellular work
(life processes) using the phosphate bond energy
Solute
+
Membrane
protein
(a) Transport work: ATP phosphorylates transport
proteins, activating them to transport solutes
(ions, for example) across cell membranes.
+
Relaxed smooth
muscle cell
Contracted smooth
muscle cell
(b) Mechanical work: ATP phosphorylates
contractile proteins in muscle cells so the
cells can shorten.
+
(c) Chemical work: ATP phosphorylates key
reactants, providing energy to drive
energy-absorbing chemical reactions.
Figure 2.24
Metabolism
• Metabolism: biochemical reactions inside cells
involving nutrients
• Two types of reactions
– Anabolism: synthesis of large molecules from
small ones
– Catabolism: hydrolysis of complex structures to
simpler ones
Metabolism
• Cellular respiration: catabolism of food fuels
and capture of energy to form ATP in cells
• Enzymes shift high-energy phosphate groups
of ATP to other molecules (phosphorylation)
• Phosphorylated molecules are activated to
perform cellular functions
Stages of Metabolism
•
Processing of nutrients
1. Digestion, absorption and transport to tissues
2. Cellular processing (in cytoplasm)
•
•
Synthesis of lipids, proteins, and glycogen, or
Catabolism (glycolysis) into intermediates
3. Oxidative (mitochondrial) breakdown of
intermediates into CO2, water, and ATP
Stage 1 Digestion in
GI tract lumen to
absorbable forms.
Transport via blood to
tissue cells.
PROTEINS
CARBOHYDRATES
Amino acids
Glucose and other sugars
Stage 2 Anabolism
Proteins
(incorporation into
molecules) and
catabolism of nutrients
NH3
to form intermediates
within tissue cells.
FATS
Glycerol
Glycogen
Glucose
Fatty acids
Fats
Pyruvic acid
Acetyl CoA
Stage 3 Oxidative breakdown
of products of stage 2 in
Infrequent
mitochondria of tissue cells.
CO2 is liberated, and H atoms
removed are ultimately delivered
to molecular oxygen, forming
water. Some energy released is
used to form ATP.
Krebs
cycle
H
CO2
Oxidative
phosphorylation
(in electron
transport chain)
O2
H2O
Catabolic reactions
Anabolic reactions
Figure 24.3
Oxidation-Reduction (Redox) Reactions
• Oxidation; gain of oxygen or loss of hydrogen
• Oxidation-reduction (redox) reactions
– Oxidized substances lose electrons and energy
– Reduced substances gain electrons and energy
Oxidation-Reduction (Redox) Reactions
• Coenzymes act as hydrogen (or electron)
acceptors
– Nicotinamide adenine dinucleotide (NAD+)
– Flavin adenine dinucleotide (FAD)
When glucose is converted to carbon dioxide
•
It loses hydrogen atoms, which are added to
oxygen, producing water
Loss of hydrogen atoms
(oxidation)
C6H12O6 +
6 O2
6 CO2
Glucose
6 H2O
+
Energy
(ATP)
Gain of hydrogen atoms
(reduction)
Figure 6.5A
+
– Glucose loses hydrogen atoms = oxidization
(oxidation is loss)
– Oxygen gains hydrogen atoms = reduction
(reduction is gain)
– “OIL RIG”
Loss of hydrogen atoms
(oxidation)
C6H12O6 +
6 O2
6 CO2
Glucose
6 H2O
+
Energy
(ATP)
Gain of hydrogen atoms
(reduction)
Figure 6.5A
+
ATP Synthesis
•
Two mechanisms
1. Substrate-level phosphorylation
2. Oxidative phosphorylation
Substrate-Level Phosphorylation
• High-energy phosphate groups directly
transferred from phosphorylated substrates to
ADP
• Occurs in glycolysis and the Krebs cycle
Catalysis
Enzyme
Enzyme
(a) Substrate-level phosphorylation
Figure 24.4a
Oxidative Phosphorylation
• Chemiosmotic process
– Couples the movement of substances across a
membrane to chemical reactions
Oxidative Phosphorylation
• In the mitochondria
– Carried out by electron transport proteins
– Nutrient energy is used to create H+ gradient
across mitochondrial membrane
– H+ flows through ATP synthase
– Energy is captured and attaches phosphate groups
to ADP
High H+ concentration in
intermembrane space
Membrane
Proton
pumps
(electron
transport
chain)
ATP
synthase
Energy
from food
ADP +
Low
concentration
in mitochondrial matrix
(b) Oxidative phosphorylation
H+
Figure 24.4b
Carbohydrate Metabolism
• Oxidation of glucose
C6H12O6 + 6O2  6H2O + 6CO2 + 36 ATP + heat
• Glucose is catabolized in three pathways
– Glycolysis
– Krebs cycle
– Electron transport chain and oxidative
phosphorylation
Chemical energy (high-energy electrons)
Chemical energy
Glycolysis
Glucose
Cytosol
Krebs
cycle
Pyruvic
acid
Mitochondrial
cristae
Via substrate-level
phosphorylation
1 During glycolysis,
each glucose
molecule is broken
down into two
molecules of pyruvic
acid in the cytosol.
Electron transport
chain and oxidative
phosphorylation
Mitochondrion
2 The pyruvic acid then enters
the mitochondrial matrix, where
the Krebs cycle decomposes it
to CO2. During glycolysis and
the Krebs cycle, small amounts
of ATP are formed by substratelevel phosphorylation.
Via oxidative
phosphorylation
3 Energy-rich electrons picked up by
coenzymes are transferred to the electron transport chain, built into the cristae
membrane. The electron transport chain
carries out oxidative phosphorylation,
which accounts for most of the ATP
generated by cellular respiration.
Figure 24.5
Thank you
Video:
• Oxidation-reduction reactions involve the loss
and gain of electrons. The reactant oxidized
will lose electrons, while the reactant reduced
will gain electrons.
In biological oxidation-reduction reactions the
loss and gain of electrons is often associated
with the loss and gain of hydrogen atoms.
Electrons are still being transferred since the
hydrogen atom contains an electron.
Glycolysis
•
•
•
•
•
10-step pathway
Anaerobic
Occurs in the cytosol
Glucose  2 pyruvic acid molecules
Three major phases
1. Sugar activation
2. Sugar cleavage
3. Sugar oxidation and ATP formation
Phases of Glycolysis
1. Sugar activation
– Glucose is phosphorylated by 2 ATP to form
fructose-1,6-bisphosphate
Phases of Glycolysis
2. Sugar cleavage
– Fructose-1,6-bisphosphate is split into 3-carbon
sugars
•
•
Dihydroxyacetone phosphate
Glyceraldehyde 3-phosphate
Phases of Glycolysis
3. Sugar oxidation and ATP formation
– 3-carbon sugars are oxidized (reducing NAD+)
– Inorganic phosphate groups (Pi) are attached to
each oxidized fragment
– 4 ATP are formed by substrate-level
phosphorylation
Glycolysis
Krebs
cycle
Electron transport chain
and oxidative
phosphorylation
Carbon atom
Phosphate
Glucose
Phase 1
Sugar
Activation
Glucose is
activated by
2 ADP
phosphorylation
and converted
to fructose-1,
Fructose-1,66-bisphosphate
bisphosphate
Figure 24.6 (1 of 3)
Glycolysis
Krebs
cycle
Electron transport chain
and oxidative
phosphorylation
Carbon atom
Phosphate
Fructose-1,6bisphosphate
Phase 2
Sugar
Cleavage
Fructose-1,
6-bisphosphate
is cleaved into
two 3-carbon Dihydroxyacetone
fragments
phosphate
Glyceraldehyde
3-phosphate
Figure 24.6 (2 of 3)
Glycolysis
Krebs
cycle
Electron transport chain
and oxidative
phosphorylation
Carbon atom
Phosphate
Dihydroxyacetone
phosphate
Glyceraldehyde
3-phosphate
Phase 3
Sugar oxidation
and formation
2 NAD+
of ATP
4 ADP
The 3-carbon fragments are oxidized
2 NADH+H+
(by removal of
hydrogen) and 4 ATP
molecules are formed
2 Pyruvic acid
2 NADH+H+
2 NAD+
2 Lactic acid
To Krebs
cycle
(aerobic
pathway)
Figure 24.6 (3 of 3)
Glycolysis
• Final products of glycolysis
– 2 pyruvic acid
• Converted to lactic acid if O2 not readily available
• Enter aerobic pathways if O2 is readily available
– 2 NADH + H+ (reduced NAD+)
– Net gain of 2 ATP
Krebs Cycle
• Occurs in mitochondrial matrix
• Fueled by pyruvic acid and fatty acids
Krebs Cycle
•
Transitional phase
– Each pyruvic acid is converted to acetyl CoA
1. Decarboxylation: removal of 1 C to produce acetic acid
and CO2
2. Oxidation: H+ is removed from acetic acid and picked
up by NAD+
3. Acetic acid + coenzyme A forms acetyl CoA
Krebs Cycle
• Coenzyme A shuttles acetic acid to an enzyme
of the Krebs cycle
• Each acetic acid is decarboxylated and
oxidized, generating:
– 3 NADH + H+
– 1 FADH2
– 2 CO2
– 1 ATP
Krebs Cycle
• Does not directly use O2
• Breakdown products of fats and proteins can
also enter the cycle
• Cycle intermediates may be used as building
materials for anabolic reactions
PLAY
Animation: Krebs Cycle
Glycolysis
Krebs
cycle
Electron transport chain
and oxidative
phosphorylation
Cytosol
Pyruvic acid from glycolysis
Transitional
phase
Carbon atom
Inorganic phosphate
Coenzyme A
Mitochondrion
(matrix)
NAD+
CO2
NADH+H+
Acetyl CoA
Oxaloacetic acid
NADH+H+
(pickup molecule)
Citric acid
(initial reactant)
NAD+
Malic acid
Isocitric acid
NAD+
Krebs cycle
CO2
NADH+H+
-Ketoglutaric acid
Fumaric acid
CO2
FADH2
Succinic acid
FAD
GTP
Succinyl-CoA
NAD+
NADH+H+
GDP +
ADP
Figure 24.7
Electron Transport Chain and Oxidative
Phosphorylation
• The part of metabolism that directly uses
oxygen
• Chain of proteins bound to metal atoms
(cofactors) on inner mitochondrial membrane
• Substrates NADH + H+ and FADH2 deliver
hydrogen atoms
Electron Transport Chain and Oxidative
Phosphorylation
• Hydrogen atoms are split into H+ and electrons
• Electrons are shuttled along the inner
mitochondrial membrane, losing energy at
each step
• Released energy is used to pump H+ into the
intermembrane space
Electron Transport Chain and Oxidative
Phosphorylation
• Respiratory enzyme complexes I, III, and IV
pump H+ into the intermembrane space
• H+ diffuses back to the matrix via ATP synthase
• ATP synthase uses released energy to make
ATP
PLAY
Animation: Electron Transport
Glycolysis
Krebs
cycle
Electron transport
chain and oxidative
phosphorylation
Intermembrane
space
Inner
mitochondrial
membrane
Mitochondrial
matrix
2 H+ +
FADH2
NADH +
(carrying
from food)
1
2
ATP
synthase
FAD
H+
NAD+
Electron Transport Chain
Electrons
are transferred from complex to complex and
some of their energy is used to pump protons (H+) into the
intermembrane space, creating a proton gradient.
ADP +
Chemiosmosis
ATP synthesis is powered by the
flow of H+ back across the inner
mitochondrial membrane through
ATP synthase.
Figure 24.8
Electron Transport Chain and Oxidative
Phosphorylation
• Electrons are delivered to O, forming O–
• O– attracts H+ to form H2O
Krebs
cycle
NADH+H+
Electron transport chain
and oxidative
phosphorylation
FADH2
Free energy relative to O2 (kcal/mol)
Glycolysis
Enzyme
Complex II
Enzyme
Complex I
Enzyme
Complex III
Enzyme
Complex IV
Figure 24.9
Electronic Energy Gradient
• Transfer of energy from NADH + H+ and FADH2
to oxygen releases large amounts of energy
• This energy is released in a stepwise manner
through the electron transport chain
ATP Synthase
•
Two major parts connected by a rod
1. Rotor in the inner mitochondrial membrane
2. Knob in the matrix
•
Works like an ion pump in reverse
Intermembrane space
A rotor in the
membrane spins
clockwise when H+
flows through it down
the H+ gradient.
A stator anchored in
the membrane holds
the knob stationary.
As the rotor spins, a
rod connecting the
cylindrical rotor and
knob also spins.
ADP
+
Mitochondrial matrix
The protruding,
stationary knob
contains three
catalytic sites that
join inorganic
phosphate to ADP
to make ATP when
the rod is spinning.
Figure 24.11
Cytosol
Mitochondrion
2 NADH + H+
Electron
shuttle across
mitochondrial
membrane
Glycolysis
Glucose
Pyruvic
acid
2 NADH + H+
2
Acetyl
CoA
6 NADH + H+
Krebs
cycle
(4 ATP–2 ATP
used for
activation
energy)
Net +2 ATP
by substrate-level
phosphorylation
2 FADH2
Electron transport
chain and oxidative
phosphorylation
10 NADH + H+ x 2.5 ATP
2 FADH2 x 1.5 ATP
+2 ATP
by substrate-level
phosphorylation
About
32 ATP
+ about 28 ATP
by oxidative
phosphorylation
Maximum
ATP yield
per glucose
Figure 24.12
Glycogenesis and Glycogenolysis
• Glycogenesis
– Glycogen formation when glucose supplies exceed
need for ATP synthesis
– Mostly in liver and skeletal muscle
• Glycogenolysis
– Glycogen beakdown in response to low blood
glucose
Blood glucose
Cell exterior
Hexokinase
Glucose-6(all tissue cells)
phosphatase
(present in liver,
kidney, and
ADP
intestinal cells)
Glucose-6-phosphate
Glycogenolysis
Glycogenesis
Mutase
Mutase
Glucose-1-phosphate
Pyrophosphorylase
Glycogen
phosphorylase
Uridine diphosphate
glucose
Cell interior
2
Glycogen
synthase
Glycogen
Figure 24.13
Gluconeogenesis
• Glucose formation from noncarbohydrate
(glycerol and amino acid) molecules
• Mainly in the liver
• Protects against damaging effects of
hypoglycemia
Lipid Metabolism
• Fat catabolism yields 9 kcal per gram (vs 4 kcal
per gram of carbohydrate or protein)
• Most products of fat digestion are transported
as chylomicrons and are hydrolyzed by
endothelial enzymes into fatty acids and
glycerol