Glycogen Synthesis Flashcards
What evidence convinced researchers that glycogen synthesis and glycogen breakdown occur via separate pathways?
McArdle’s Disease: Pts lack glycogen phosphorylase and cannot break down glycogen (so have muscle pain/cramping on exertion); however, their muscles contain high amounts of normal glycogen. So glycogen synthesis does not require glycogen phosphorylase.
Which step in glycogen synthesis pulls the entire reaction to completion?
UDP-Glucose Pyrophosphorylase (the first step)
What reaction is coupled with UDP-Glucose Phosphorylase’s conversion of G1P to UDPG and drives forward this otherwise unfavorable reaction.
The hydrolysis of PPi (pyrophosphate).
What does PPi stand for?
Pyrophosphate; it is an unstable molecule that reacts easily with H20 to release a lot of free energy.
What prevents the reverse reaction of UDPG to G1P in the first step of glycogen synthesis?
PPi is totally eliminated, preventing the reaction from moving in reverse
What is the energetic cost of making glycogen?
1 molecule of UTP per molecule of glycogen synthesized
How do the energies of UTP, ATP, GTP, and CTP compare?
They are all about the same.
Which enzyme in glycogen synthesis transfers a glucose from UDP-glucose to glycogen?
Glycogen synthase
Which enzyme in glycogen synthesis creates alpha (1-6) branch linkages?
Glycogen branching enzyme
Which enzyme in glycogen synthesis forms alpha(1-4) linkages on non-reducing ends of glycogen?
Glycogen synthase
True or false: UTP is equivalent to ATP.
True
Which form of glycogen synthase is active?
Form a (dephosphorylated form)
Which form of glycogen synthase is inactive?
Form b (phosphorylated form)
How is the dephosphorylated form (form a) of glucose synthase activated?
By G6P
How do forms a and b of Glycogen synthase compare to forms a and by of Glycogen phophorylase?
In glycogen synthase, form a is dephosphorylated and the active form; in glycogen phosphorylase, phosphorylase a is phosphorylated.
In glycogen synthase, form b is phosphorylated; in glycogen phosphorylase, phosporylase b is dephosphorylated.
What controls glycogen synthase activity?
[G6P] and the amount in dephosphorylated form a.
True or false: glycogen synthesis can be accomplished by simply joining two glucose molecules together.
False; can only extend a pre-existing chain through alpha (1-4 linkages)
What acts as a base structure for the synthesis of glycogen when there is no glycogen present?
Glycogenin
What is glycogenin?
The base of every glucose molecule.
How many molecules of glycogenin and glycogen synthase does each glucose molecule contain?
one of each
How many residues away from another branched point must a branch of glucose be?
At least 4 residues away.
How is a branch created by glycogen branching enzyme?
A 7-residue segment from an 11 reside chain to a C6-OH on the same or a different chain.
