BIOL 410 FINAL EXAM STUDY GUIDE
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Glucogen - ANSWERPolymer of glucose residues linked by
- alpha(1-->4) glycosidic bonds, primary
- alpha(1-->6) glycosidic bonds, at branch points
Glycogen is needed to maintain blood glucose levels
Glucose is stored as glycogen predominantly in liver and muscle cells
Glycogen Degradation - ANSWERGlucose residues are removed from glycogen by
glycogen phosphorylase
Glycogen phosphorylase catalyzes the addition of phosphate to glycogen in order to
remove a terminal glucose residue from the non-reducing end of a glycogen chain,
releasing glucose-1-phosphate as reaction product
Glucose-1-phosphate is isomerized to glucose-6-phosphate with phophoglucomutase
for metabolism
Glucose residues are removed from glycogen by glycogen phosphorylase -
ANSWERGlycogen is converted to monosaccharide units by glycogen phosphorylase.
Glycogen phosphorylase is the key enzyme in regulation of glycogen degradation
Glycogen phosphorylase catalyzes a hydrolytic cleavage of alpha(1-->4) bonds
The reaction that cleaves a bond by the addition of orthophosphate is called
phosphorolysis
This phosphorolysis may be compared to hydrolysis:
(1) Hydrolysis: R-O-R' + HOH --> R-OH + R'-OH
(2) Phosphorolysis: R-O-R' + HO-PO3^2- --> R-OH + R'OPO3^2-
**Instead of the addition of water, there is an addition of phosphate group
Dealing with branch points in glycogen - ANSWERGlycogen phosphorylase works on
nonreducing ends until it reaches four residues from an alpha(1-->6) branch point (limit
branch)
Debranching enzyme is required to cleave alpha-1,6-glycosidic branches in glycogen
,Debranching enzyme is a bifunctional enzyme. It has 2 independent active sites:
transferase and alpha(1-->6) glucosidase are present in a single polypeptide chain
(1) The transferase of the debranching enzyme transfers 3 glucose residues from a 4
residue limit branch to the end of another branch, diminishing the limit branch to a single
glucose residue
(2) The alpha(1-->6) glucosidase moiety of the debranching enzyme then catalyzes
hydrolysis of the alpha(1-->6) linkage, yielding free glucose
Glucose-1-phosphate must be isomerized to glucose-6-phosphate for metabolism -
ANSWERConversion of glucose 1-phosphate to glucose 6-phosphate is carried out by
the enzyme phosphoglucomutase
Phosphoglucomutase catalyzes the reversible reaction:
glucose-1-phosphate <--> glucose 6-phosphate
Reaction with serine exchanges phosphate between C1 to C6
Glucose-6-phosphate may enter glycolysis or (mainly liver) be dephosphorylated for
release to the blood
Regulation of Glycogen Degradation - ANSWERGlycogen phosphorylase is the key
enzyme in regulation of glycogen degradation.
Glycogen phosphorylase is regulated by
(1) Allosteric effectors
- ATP/AMP or NADH/NAD+
- High ratios have enough energy; low ratios need more energy so breakdown more
glycogen for glucose
(2) Reversible phosphorylation
Glycogen phosphorylase exists in two interconvertible forms:
(1) Phosphorylase a - usually in R state, active
(2) Phosphorylase b - usually in T state, inactive
Phosphorylase kinase activate glycogen phsophorylase because phosphorylation of
phosphorylase converts phosphorylase b to phosphorylase a (inactive form to active
form)
Inactive b --kinase--> active a with P
Regulation of Glycogen Degradation: Signalling - ANSWERGlucagon and epinephrine
are the critical hormones that signal for glycogen breakdown
- Glucagon: glucose is gone so need more!
- Epinephrine: high stress so need more glucose for energy!
Glucagon/Epinephrine signaling pathway
, - Starts phosphorylation cascade vis cAMP
- Activates glycogen phosphorylase
Glycogen phosphorylase cleaves glucose residues off glycogen, generating glucose-1-
phosphate
Glycogen Synthesis - ANSWERUridine diphosphate glucose (UDP-glucose), an active
form of glucose, is the glucose donor for the biosynthesis of glycogen
UDP-glucose is formed from glucose-1-phosphate
Glucose-1-phosphate + UTP --> UDP-glucose
UDP-glucose + glycogen (n residues) --> UDP + glycogen (n+1 residues)
Glycogen Synthesis: Glycogenin - ANSWERGlycogenin initiates glycogen synthesis
Glycogenin catalyzes attachment of a glucose molecule to one of its own tyrosine
residues
Glycogenin then catalyzes glucosylation at C4 of the attached glucose to yield a
disaccharide with alpha (1-->4) glycosidic linkage
This is repeated until a short linear glucose polymer with alpha(1-->4) glycosidic
linkages is built up on glycogenin
Glycogenin is both the primer on which new glycogen chains are initiated and the
enzyme that catalyzes their assembly
Glycogen Synthesis: Enzyme - ANSWERGlycogen synthase then catalyzes elongation
of glycogen chains initiated by glycogenin
Glycogen synthase catalyzes transfer of the glucose moiety of UDP-glucose to the
hydroxyl at C4 of the terminal residue of the glycogen chain to form an alpha(1-->4)
glycosidic linkage
Glycogen synthase is the key regulatory enzyme that primarily controls the rate of
glycogen synthesis
A branching enzyme transfers a segment from the end of a glycogen chain to the C6
hydroxyl of a glucose residue of glycogen to yield a branch with an alpha (1-->6) linkage
Why need branching? Branching increases the rates of glycogen synthesis and
degradation by increasing the potential sites of action
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Glucogen - ANSWERPolymer of glucose residues linked by
- alpha(1-->4) glycosidic bonds, primary
- alpha(1-->6) glycosidic bonds, at branch points
Glycogen is needed to maintain blood glucose levels
Glucose is stored as glycogen predominantly in liver and muscle cells
Glycogen Degradation - ANSWERGlucose residues are removed from glycogen by
glycogen phosphorylase
Glycogen phosphorylase catalyzes the addition of phosphate to glycogen in order to
remove a terminal glucose residue from the non-reducing end of a glycogen chain,
releasing glucose-1-phosphate as reaction product
Glucose-1-phosphate is isomerized to glucose-6-phosphate with phophoglucomutase
for metabolism
Glucose residues are removed from glycogen by glycogen phosphorylase -
ANSWERGlycogen is converted to monosaccharide units by glycogen phosphorylase.
Glycogen phosphorylase is the key enzyme in regulation of glycogen degradation
Glycogen phosphorylase catalyzes a hydrolytic cleavage of alpha(1-->4) bonds
The reaction that cleaves a bond by the addition of orthophosphate is called
phosphorolysis
This phosphorolysis may be compared to hydrolysis:
(1) Hydrolysis: R-O-R' + HOH --> R-OH + R'-OH
(2) Phosphorolysis: R-O-R' + HO-PO3^2- --> R-OH + R'OPO3^2-
**Instead of the addition of water, there is an addition of phosphate group
Dealing with branch points in glycogen - ANSWERGlycogen phosphorylase works on
nonreducing ends until it reaches four residues from an alpha(1-->6) branch point (limit
branch)
Debranching enzyme is required to cleave alpha-1,6-glycosidic branches in glycogen
,Debranching enzyme is a bifunctional enzyme. It has 2 independent active sites:
transferase and alpha(1-->6) glucosidase are present in a single polypeptide chain
(1) The transferase of the debranching enzyme transfers 3 glucose residues from a 4
residue limit branch to the end of another branch, diminishing the limit branch to a single
glucose residue
(2) The alpha(1-->6) glucosidase moiety of the debranching enzyme then catalyzes
hydrolysis of the alpha(1-->6) linkage, yielding free glucose
Glucose-1-phosphate must be isomerized to glucose-6-phosphate for metabolism -
ANSWERConversion of glucose 1-phosphate to glucose 6-phosphate is carried out by
the enzyme phosphoglucomutase
Phosphoglucomutase catalyzes the reversible reaction:
glucose-1-phosphate <--> glucose 6-phosphate
Reaction with serine exchanges phosphate between C1 to C6
Glucose-6-phosphate may enter glycolysis or (mainly liver) be dephosphorylated for
release to the blood
Regulation of Glycogen Degradation - ANSWERGlycogen phosphorylase is the key
enzyme in regulation of glycogen degradation.
Glycogen phosphorylase is regulated by
(1) Allosteric effectors
- ATP/AMP or NADH/NAD+
- High ratios have enough energy; low ratios need more energy so breakdown more
glycogen for glucose
(2) Reversible phosphorylation
Glycogen phosphorylase exists in two interconvertible forms:
(1) Phosphorylase a - usually in R state, active
(2) Phosphorylase b - usually in T state, inactive
Phosphorylase kinase activate glycogen phsophorylase because phosphorylation of
phosphorylase converts phosphorylase b to phosphorylase a (inactive form to active
form)
Inactive b --kinase--> active a with P
Regulation of Glycogen Degradation: Signalling - ANSWERGlucagon and epinephrine
are the critical hormones that signal for glycogen breakdown
- Glucagon: glucose is gone so need more!
- Epinephrine: high stress so need more glucose for energy!
Glucagon/Epinephrine signaling pathway
, - Starts phosphorylation cascade vis cAMP
- Activates glycogen phosphorylase
Glycogen phosphorylase cleaves glucose residues off glycogen, generating glucose-1-
phosphate
Glycogen Synthesis - ANSWERUridine diphosphate glucose (UDP-glucose), an active
form of glucose, is the glucose donor for the biosynthesis of glycogen
UDP-glucose is formed from glucose-1-phosphate
Glucose-1-phosphate + UTP --> UDP-glucose
UDP-glucose + glycogen (n residues) --> UDP + glycogen (n+1 residues)
Glycogen Synthesis: Glycogenin - ANSWERGlycogenin initiates glycogen synthesis
Glycogenin catalyzes attachment of a glucose molecule to one of its own tyrosine
residues
Glycogenin then catalyzes glucosylation at C4 of the attached glucose to yield a
disaccharide with alpha (1-->4) glycosidic linkage
This is repeated until a short linear glucose polymer with alpha(1-->4) glycosidic
linkages is built up on glycogenin
Glycogenin is both the primer on which new glycogen chains are initiated and the
enzyme that catalyzes their assembly
Glycogen Synthesis: Enzyme - ANSWERGlycogen synthase then catalyzes elongation
of glycogen chains initiated by glycogenin
Glycogen synthase catalyzes transfer of the glucose moiety of UDP-glucose to the
hydroxyl at C4 of the terminal residue of the glycogen chain to form an alpha(1-->4)
glycosidic linkage
Glycogen synthase is the key regulatory enzyme that primarily controls the rate of
glycogen synthesis
A branching enzyme transfers a segment from the end of a glycogen chain to the C6
hydroxyl of a glucose residue of glycogen to yield a branch with an alpha (1-->6) linkage
Why need branching? Branching increases the rates of glycogen synthesis and
degradation by increasing the potential sites of action
