glycogen metabolism Flashcards
glycogen
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
what do plants use instead of glycogen?
starch
what are the two primary storage organs?
muscle and liver
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
muscle
glycogen accounts for about 10% of this organ’s mass
stores glycogen for the whole body
liver
glycogen stores are quickly _______
depleted; only really enough for 24 hours
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.
fats
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 ___________
condensation; hydrolysis
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
reducing end
( everything adds to the nonreducing end)
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
a(1->4)
a(1->6)
branching increases the number of ____________ ends
this is a good thing because it increases ____________ and _______________
non-reducing ends; solubility; accessibility
glycogen breakdown (glycogenolysis) step 1
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)
glycogen breakdown (glycogenolysis) step 2
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
glycogen breakdown (glycogenolysis) step 3
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
glycogen breakdown (glycogenolysis) step 4
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
glycogen synthesis (glycogenesis)
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)
glycogenesis step 1
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
glycogenesis step 2
enzyme: glycogen synthase
UDP-glucose to non-reducing end of glycogen residue
- makes a new a(1->4) glycosidic bond
- UDP is eliminated
glycogenesis step 3
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
regulation of glycogenolysis
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
hormonal control is dependent on energy demand and blood glucose
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)
there are two allosteric control mechanisms
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
regulation of glycogenesis
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
phosphorylation of glycogen synthase A
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
dephosphorylation of glycogen synthase B
converts it to glycogen synthase a
- promoted by PPI
Insulin stimulates glycogen synthesis by activation PPI
