section 3: Part 2 - glycogen synthesis | Quizlet

what are the enzymes of glycogen synthesis

UDP-glucose pyrophosphorylase

- activates glucosyl units

glycogen synthase

- extends glycogen chains

glycogen branching enzyme

- transfers 7- residue glycogen segments

glycogen synth. from G1P is thermodynamically...

unfavorable

Describe the actions of UDP- glucose pyrophosphorylase

activates glucosyl units

- uridine diphosphate glucose (UDPG)

UDPG is an activated compound

- can donate a glucosyl unit to a growing glycogen

chain

formation of UDPG has ′ΔG°′≈0

Subsequent exergonic hydrolysis of PPi by inorganic pyrophosphatase makes the overall reaction exergonic

what is the net ΔG°from UDP-Glucose Pyrophosphorylase action

-19.2

G1P +UTP <-> UDPG + PPi ~ 0

H2O + PPi -> 2Pi ~ -19.2

what is PPi

inorganic pyrophosphate

Describe glycogen synthase (GS)

extends glycogen chains by:

transfer of glucosyl unit of UDPG to C4-OH group on one glycogen's non-reducing ends to from an α(1→4) glyosidic bond

ΔG° = -13.4 kJ-mol

spontaneous

UDPG + glycogen -> UDP + glycogen

How is UTP replenished

using ATP as a substrate through a phosphoryl-transfer reaction

mediated by nucleoside diphosphate kinase

is UTP consumption energetically equivalent to ATP consumption

yes

can GS simply link 2 glucose residues together

no, it can only extend using already existing α(1→4)-linked glucan chain

in the first step of GS what is at work

glycogenin working as a glycosyltransferase

What does glycogenin do

attaches glucose residue donated by UDPG to OH on its tyr 194

extends glucose chain by up to 7 addition UDPH donated glucose residues

forms a glycogen primer, allowing GS to start by extending the primer

each glycogen molecule is associated with

1 molc. glycogenin

1 molc. GS

what does GS generate

transfers only α(1→4)linkages to yield α-amylose

what is the glycogen branching enzyme called

amylo-(1,4 ->1,6) - transglycosylase

distinct from glycogen debranching enzyme

glycogen branching enzyme does what

creates branches by transferring a 7-residue segment from chain end to C6-OH group of a glucose residue on same or other glycogen chain

each transferred segments must come from a chain of at least 11 residues

each branch point must be at least 4 residues from other branch points

why is the branching pattern of glycogen important

its optimized by evolution for the efficient storage and mobilization of glucose