glycogen metabolism

Question 1

glycogen

Answer 1

glucose storage form in animals and microorganisms
- storage as a polymer avoids a large increase in osmotic pressure
-B- particle contains about 55,000 glucose residues
- 20-40 B particles will cluster together in a a-rosette

Question 2

what do plants use instead of glycogen?

Answer 2

starch

Question 3

what are the two primary storage organs?

Answer 3

muscle and liver

Question 4

glycogen accounts for about 1-2% of this organ’s mass
- overall stores a higher quantity of glycogen due to its larger mass
stores glycogen for itself
provides a quick energy source for aerobic and anaerobic metabolism

Answer 4

muscle

Question 5

glycogen accounts for about 10% of this organ’s mass
stores glycogen for the whole body

Answer 5

liver

Question 6

glycogen stores are quickly _______

Answer 6

depleted; only really enough for 24 hours

Question 7

remember that even though _____ are able to store more energy, they can’t be converted to glucose in mammals and can’t be catabolized anaerobically.

Answer 7

fats

Question 8

2 monosaccharides can be joined by an O-glycosidic bond to form a disaccharide
- this is a ______________ reaction of an alcohol and a hemiacetal
- they can be separated through ___________

Answer 8

condensation; hydrolysis

Question 9

in sugar polymers, there will be a reducing end and a non-reducing end
- the ___________ will have a free hemiacetal that can still be reduced

Answer 9

reducing end
( everything adds to the nonreducing end)

Question 10

glycogen contains both linear and branched chains
- the linear chain has __________ glycosidic bonds
- the branched chain has __________ glycosidic bonds
branching occurs every 8-12 residues

Answer 10

a(1->4)
a(1->6)

Question 11

branching increases the number of ____________ ends
this is a good thing because it increases ____________ and _______________

Answer 11

non-reducing ends; solubility; accessibility

Question 12

glycogen breakdown (glycogenolysis) step 1

Answer 12

enzyme: glycogen phosphorylase
Pi cleaves a (1->4) glycosidic bond on the nonreducing end
- continues until 3 residues away from an a(1->6) glycosidic branch point
- this is a phosphorolysis reaction
some of the bond energy is conserved in phosphate ester formation
enzyme requires pyridoxal phosphate as a cofactor (derivate of vitamin B6)

Question 13

glycogen breakdown (glycogenolysis) step 2

Answer 13

enzyme: debranching enzyme
2 activities of enzyme:
1.) transferase activity
- transfers 3 glucose molecules from the branch to the non-reducing end
2.) a 1->6 activity
- glucose is cleaved and released as free glucose

Question 14

glycogen breakdown (glycogenolysis) step 3

Answer 14

enzyme: phosphoglucomutase
glucose-1-phosphate is converted to glucose-6-phosphate
no energy input needed
reaction is reversible
in muscle, the product can then be used in glycolysis pathway

Question 15

glycogen breakdown (glycogenolysis) step 4

Answer 15

enzyme: glucose-6-phosphetase
enzyme is integral membrane protein of the ER
found in liver and kidneys only
dephosphorylates glucose-6-phosphate to give glucose

Question 16

glycogen synthesis (glycogenesis)

Answer 16

can occur anywhere in the body but it is primarily seen in the liver and skeletal muscles
starting point is glucose-6-phosphate, which is formed from glucose
D-glucose+ATP->D-glucose-6-phosphate+ADP (enzyme: hexokinase I/II for muscle and hexokinase IV/glucokinase for liver)
glucose-6-phosphate=glucose-1-phosphate (enzyme: phosphoglucomutase)

Question 17

glycogenesis step 1

Answer 17

enzyme: UDP-glucose phosphorycase
UDP-glucose is a sugar nucleotide (with increased energy for polymerization)
- joined at anomeric carbon
glucose-1-phosphate+UTP-> UDP-glucose+PPi

Question 18

glycogenesis step 2

Answer 18

enzyme: glycogen synthase
UDP-glucose to non-reducing end of glycogen residue
- makes a new a(1->4) glycosidic bond
- UDP is eliminated

Question 19

glycogenesis step 3

Answer 19

enzyme: glycogen branching enzyme
transfers a fragment from the nonreducing end to the C-6 hydroxy group at a more interior position on the chain
branching increases solubility and accessibility and the number of nonreducing ends

Question 20

regulation of glycogenolysis

Answer 20

regulated both allosterically and hormonally
there are two forms of glycogen phosphorylase that are interconvertible
- phosphorylase a is the active form while phosphorylase b is the less active form
conversion occurs when a specific serine residue in phosphorylase b is phosphorylated

Question 21

hormonal control is dependent on energy demand and blood glucose

Answer 21

glucagon signals low blood sugar (activates glycogenolysis)
insulin signals high blood sugar
glucagon (liver) and epinephrine (muscles) both activate phosphorylase b kinase
insulin actives phosphorylase a phosphatase (PP1)

Question 22

there are two allosteric control mechanisms

Answer 22

Ca2+ binding activates phosphorylase b kinase
- ca2+ signals for muscle contraction (activates glycogenolysis)

AMP binding activates phosphorylase a
- AMP accumulation indicates vigorous muscle contraction
- when ATP levels are sufficient, ATP blocks the allosteric site where AMP binds

Question 23

regulation of glycogenesis

Answer 23

there are two forms of glycogen synthase that are interconvertible
- glycogen synthase a is the active form while glycogen synthase b is the less active form

Question 24

phosphorylation of glycogen synthase A

Answer 24

phosphorylation of glycogen synthase a converts it to glycogen synthase b
- many different residues can be phosphorylated
- casein kinase II (CKII) does a single phosphorylation which primes it for GSK3
- glycogen synthase kinase 3(GSK3) phosphorylates three serine residues near the carboxyl terminus
- PKA mediates the action of glucagon (liver) and epinephrine (muscle) and phosphorylates glycogen synthase a to inactivate the enzyme by covalent modification

Question 25

dephosphorylation of glycogen synthase B

Answer 25

converts it to glycogen synthase a
- promoted by PPI

Insulin stimulates glycogen synthesis by activation PPI