Human diseases of carbohydrate metabolism
Inherited enzyme deficiencies
Mutations that change enzyme function or abolish enzyme activity
Most are recessive since only one functional copy of gene is sufficient
for needed activity
Diabetes
Lactose intolerance
Galactosemia
Glycogen storage disease
Monosaccharides - Aldoses
# Isomers = 2n where
n = # of chiral carbons
Enantiomer
Distant chiral C
From most oxidized
Epimers – differ
in configuration
at only one chiral
carbon
Not all made in nature
Monosaccharides - Ketoses
# Isomers = 2n where
n = # of chiral carbons
Cyclization - aldohexose
Draw most oxidized carbon (C1
aldose and C2 ketose) on right and
number C clockwise
In ring most oxidizes carbon new
chiral center (anomeric C)
Transfer information from Fisher
projections
-OH on right then down in Haworth
-OH on left then up in Haworth
Bulky substituent on highest numbered
carbon points up
rapid equilibrium
Anomers
Cyclization - aldopentose
Equilibrium
Anomeric C
Hemiacetal
Haworth
projection
Anomers
Glycolysis: Steps 2 and 3
Opens the chain
during the rxn
PFK-1
CH2OH
OH
utilizes 100% -anomer
Stereospecific: uses -Glc; produces
100% -D-fructose-6-phosphate
36% -fructose
64% -fructose
Glycoside Bonds – Disaccharides
No open chain equil
non-reducing
reducing
Hemiacetals -a reactive
carbonyl that can be oxidized.
 anomer: refers to free C1 OH
non-reducing sugar
Glycoside Bonds – Disaccharides
epimer
Most abundant disacc. in nature (plants)
Polysaccharides – Structure
Cellulose
-(1-4) linkage
180 deg rotation
300- 15,000 Glc residues
Rigid extended conformation
H-bonding
Forms bundles or fibrils
Plant cell walls, stems
and branches
Humans don’t have
-glucosidases
Microbe that live in ruminants
do
termites
Amylose
Polysaccharides – Glucose Storage
• Plant starch –
mixture of amylose and
amylopectin
• Animals glycogen
Homoglycans- one type of monosaccharide
Amylose
100-1000 glucose residues (maltose units)
Amylopectin
and
Glycogen
Amylopectin:
branch every 25 residues
Glycogen:
branch every 8-12 residues
10% mass of liver
No template (ie no gene)
Polysaccharides -Starch Degradation
Know how starch is broken down !
• Humans digest starch via
two enzymes:
– α -amylase endoglycosidase of α-(1-4)
linkages (random)
– debranching enzyme
(cleaves limit dextrans)
• Higher plants have
– β- amylase exoglycosidase
of α- (1-4) linkages,
releasing the disaccharide
maltose
Single reducing end
multiple non-reducing end
Glycogen Metabolism
Synthesis:
Different enzymes for
syn and degradation
Driven by PPi hydrolysis
Major regulatory step
(hormonally regulated)
Key regulation
by phosphorylation
Pre-existing
Glycogenin primer
Amylo-(1,4 1,6)-transglycolase catalyzes the branch point. (Alpha 1-6 link)
Degradation:
Two subunits, two catalytic sites, allosteric sites.
AMP- activator; ATP & Glc-6-P – inhibitor.
Phosphorolysis rxn.
Generates phosph-sugar not free glc
Phosphorylation: active (phosphorylase a).
Dephosphorylated: less active (phosphorylase b).
Primary regulation
Branching inc speed of
syn and degradation
phosphorolytic
Reg by ATP and G-6-P
Sequential removal of Glc
From non-reducing end
Primarily by phosphorylation
Stops 4 Glc from branch pt
Consequences of branch
Reducing vs non-reducing ends
solubility
Rate of syn/degradation
Energy yield from glycogen
Higher than from glc
Human diseases of carbohydrate metabolism
Inherited enzyme deficiencies
Mutations that change enzyme function or abolish enzyme activity
Most are recessive since only one functional copy of gene is sufficient
for needed activity
Diabetes
Lactose intolerance
Galactosemia
Glycogen storage disease
α -amylase
lactase
Glc + Gal
Absorbed from intestine
Major source of energy for nursing animals
In liver
20% of caloric intake of infants
Glc-6-P
Glucose metabolism
lactase
glucogen
glucogen
Glc 1-P
Glc + Gal
Glc
Glc 6-P
Fruc 6-P
Fruc 1,6-P
Glc
X
Absorbed from intestine
Major source of energy for nursing animals
In liver
20% of caloric intake of infants
X
Two inherited metabolic errors
Hypolactasia (lactose intolerance)
Galactosemia
Glc-6-P
Lactose Intolerance
X
Single gene defect
Normal decrease in enzyme by 6 yrs old
10% of original activity
(Northern Europeans are lactase producing adults)
Mutation in chromosome 2
Lactase deficient people:
Lactose passes intact into colon
bacteria in colon ferment to lactic acid, methane and H gas
Bloated/ gas and diarrhea
Can also hinder absorption of other nutrients
Avoid dietary lactose
Take enzyme substitute
Galactosemia
X
Galactose 1-P accumulates in liver cells (high galactose in blood and urine)
Decrease liver function and cataracts
death
CNS damage and mental retardation (even if avoid milk)
cataracts
Cataracts (clouding)
due to high galactose in eye.
Converted to galactol allowing
diffusion of water into eye
Glucose metabolism
glucogen
glucogen
Glc 1-P
Glc
Glc 6-P
Fruc 6-P
Fruc 1,6-P
Glc
Glycolysis and Cancer
Defined: Glucose is converted anaerobically to the
three carbon acid pyruvate
Net Reaction:
Glucose + 2 ADP + 2 NAD+ + 2 Pi 
2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
Generates ATP at higher rate than Oxid Phosp
Oxidative phosphorylation: allows more energy extracted
from Glc
Otto Warburg-cancer cells utilize glycolysis even in presence of O2
Aerobic glycolysis: Warburg effect
Initial stages of tumor growth vessels grow at slower rate:
cells deprived of O2
Cells switch to reliance to glycolysis
Max energy when pyruvate from glycolysis enters
Citric Acid Cycle
Glycolysis and Cancer
Continue to rely of Glycolysis even when O2 restored to tumor
Can visualize tumors based on inc sugar uptake (PET scan)
Treatment?? Blocking lactose dehydrogenase (block NAD regeneration turn off Glycolysis)
ATP
ADP
Glucose
Hexokinase
Phosphorylation
Glucose-6-phosphate
Know key reg steps!
Glucose-6-phosphate isomerase
Isomerization
Fructose-6-phosphate
ATP
Phosphorylation
Phosphofructokinase-1
ADP
Fructose-1,6-bisphosphate
Aldolase Cleavage
Dihydroxyacetone phosphate
Isomerization
Glyceraldehyde-3-phosphate
NAD+ + Pi
NADH + H+
1,3-bisphosphoglycerate
ADP
Glyceraldehyde-3-phosphate
Oxidation and
Glyceraldehyde-3-phosphate NAD+ + Pi
+
NADH + H
dehydrogenase
Phosphorylation
1,3-bisphosphoglycerate
Phosphoglycerate kinase
ATP
Trios phosphate isomerase
Substrate Level
Phosphorylation
ADP
ATP
3-phosphoglycerate
3-phosphoglycerate
Phosphoglycerate mutase
2-phosphoglycerate
Rearrangement
2-phosphoglycerate
Enolase
H2O
Dehydration
H2O
phosphoenolpyruvate
phosphoenolpyruvate
ADP
ADP
ATP
Pyruvate kinase
pyruvate
ATP
pyruvate
Substrate Level
Phosphorylation
Enzymatic Regulation of Glycolysis
Not moving forward,
stop converting ATP
Cellular rxns are
converting ATP and
ADP, make more ATP
You’ve committed!
Bi-phosphated
furanoses, keep
pathway moving
CAC intermediates,
slow down, there is
already adequate
supply of energy
Glycolysis: Hexokinase Isozymes
Hexokinases (I-III)
-regulated negatively by Glc-6-P
-if later steps slow down, Glc-6P builds up
Glucokinase (IV) in Liver
-regulated negatively by Fru-6-P
-pulls glucose out of bloodstream until equil
-liver can produce more Glc-6-P
-converts Glucose to Glycogen storage
Glc
glucogen
Glc 1-P
Glc 6-P
I-III
IV
Isozymes
Different inhibition profiles
Location, Km
Control point
Glc
Can’t leave the cell with
negative charge
Fruc 6-P
Fruc 1,6-P
in Liver
Hormones Involved
High blood [Glc], insulin released
Low blood [Glc], glucagon released
Insulin Dependent Uptake
Muscle
Adipose
Major function of liver: maintain constant level of Glc in blood
Release Glc (from glycogen) during muscle activity and between meals
Glucose 6-phosphatase
Most cases Glc-6-P is end product---used in other pathways
- glycogen, starch, pentose, hexose synthesis
Enzyme only found in liver, kidney, small intestines
Bound to ER with active site towards lumen
Hydrolysis of phosphate irreversibly forms glucose
Secretory pathway exports to blood stream for other tissues
Body does not transfer pyruvate
Lactate- produced in RBC and Muscle
Lactate to pyruvate in liver
Cori cycle
Major function of liver: maintain constant level of Glc in blood
Release Glc during muscle activity and between meals
Breakdown of glycogen to Glc 6-P (does not leave the cell)
Liver contains glc 6-phoshatase enzyme
Glc not major fuel in liver
Regulation of Phosphofructokinase-1
Large oligomeric enzyme
bacteria/mammals - tetramer
yeast - octamer
ATP - product of pathway
- allosteric inhibitor
AMP - allosteric activator
- relieves inhibition by ATP
Citrate - feedback inhibitor
- regulates supply of pyruvate
- links Glycolysis and CAC
Fru-2,6-bisphosphate
- strong activator
- produced by PFK-2 when excess
fru-6-phosphate
- indirect means of substrate
stimulation or feed forward
activation
Regulation of Pyruvate Kinase
+ F 1,6 BP
Allosteric (feed-forward) activation
Fructose-1,6-bisphosphate
-allosterically activates
-produced in step three
-links control steps together
High blood [Glc]
Inactivation by covalent modification
-blood [Glc] drops, glucagon released
-liver protein kinase A (PKA) turned on
-PKA phosphorylates pyruvate kinase
Allosteric inhibition by ATP
-product of pathway and CAC
Low blood [Glc]
Regulation of Glycogen Metabolism
Hormonal Regulation:
Fed state
fasting
Via cAMP
Via PIP3
Insulin: secreted by pancreas when Glc high inc rate of transport into cell and glycogen syn
GLUT4
Glucagon: secreted when Glc low
Epi: released by adrenal gland in response to neural signal (flight or flight)
Sudden energy response
Gluconeogenesis
Liver
glycogen
- 3 places differ- control points in glycolysis
- 4 new enzymes
ATP energy, NADH reducing equivalents
consumed
Glc also syn from pyruvate
(lactate and amino acids)
Liver/kidney
Glc needed in brain/muscle
PPP
Gluconeogenesis: Regulation
Modulate one enzyme and
affect 2 opposing pathways
Sensitive regulatory point
Low [Glc]:
glucagon increases
protein kinase A
(activates Fru-2,6-bisP
phosphatase)
lowering [Fru-2,6bisP].
Activate Glc syn
and
Loss of glycolysis stim
Regulation of Glycogen Metabolism
Hormonal Regulation:
Fed state
fasting
Via cAMP
Via PIP3
Insulin: secreted by pancreas when Glc high inc rate of transport into cell and glycogen syn
GLUT4
Glucagon: secreted when Glc low
Epi: released by adrenal gland in response to neural signal (flight or flight)
Sudden energy response
Intracellular Regulation
of Glycogen Metabolism by
Interconvertible Enzymes:
Low glc activate kinase and breakdown
Low [Glc]
Simultaneous
effect
High [Glc]
Human diseases of carbohydrate metabolism
Inherited enzyme deficiencies
Mutations that change enzyme function or abolish enzyme activity
Most are recessive since only one functional copy of gene is sufficient
for needed activity
Diabetes
Lactose intolerance
Galactosemia
Glycogen storage disease
Understand how enzyme deficiency
leads to accumulation of glycogen
Other symptoms
Treatment, if any
Glycogen storage disease
glucogen
II, III, VI
glucogen X
X
Glc
V
Glc 1-P
X
Glc 6-P
Glc
I
IV
Fruc 6-P
All defects lead to
glycogen accumulation
X
VII
Fruc 1,6-P
I
Glucose-6-Phosphatase in liver (von Gierk’s disease):
hypoglycemia (low blood glc) when fasting
Liver enlargement
IV Branching enzyme in organs (liver) (Andersen’t disease)
Liver dysfunction and early death
V
Glycogen phosphorylase in muscle (McArdle’s disease)
Muscle cramps with exercise
VII Phosphofructokinase in muscle
Glycogen accumulation and Inability to exercise
Glycogen storage disease
Type I:
Glucose-6-Phosphatase deficiency in liver (von Gierk’s disease):
Glc not released into blood
No response to Epinephrine or Glucagon
hypoglycemia (low blood glc) between meals
infant in convulsions
Large amounts of glycogen in liver (G-6-P inhibits breakdown)
Liver enlargement
Glc-6-P increases glycolysis inc lactate/pyruvate in blood
(Lactic acidosis)
Delayed puberty, short stature
Continuous feedings of cornstarch (intragastric feeding)
Drug induced inhibition of Glc uptake by liver
Surgical transplant of portal vein (normally intestine-liver)
Glc to peripheral tissues before liver
Glycogen storage disease
Type IV:
Most severe disease
Branching enzyme deficiency in organs (liver) (Andersen’s disease)
Accumulate abnormal glycogen
Reduced solubility of glycogen
Foreign body immune response??
Liver dysfunction
Failure to thrive----- death 2-5 yrs old
Glycogen storage disease
Type V:
Glycogen phosphorylase deficiency in muscle (McArdle’s disease)
No breakdown of glycogen
Exercise indices muscle cramps
otherwise normal
Effective utilization of muscle glycogen not essential to life
NMR on forearm muscle
Can’t provide fuel for glycolysis to keep up
Demand for ATP
Muscle cramps correlate with inc ADP
Vasodialation-muscle now has access to Glc
and fatty acids in blood