Glycogenesis

“Let your conversation be always full of grace, seasoned with salt, so that you may know how to answer everyone.” Colossians 4:6

Why must glycogen synthesis occur via a different pathway than glycogen breakdown?

Glycogen breakdown is favorable, meaning spontaneous. DeltaG = C - S

What three enzymse are required for glycogenesis?

1. UDP-glucose phosphorylase

2. Glycogen synthase a. primer: glycogenin

3. Glycogen branching enzyme

What is the first step of glycogen synthesis?

UDP-Glucose phosphorylase combines G1P with UTP to create activated UDP-glucose (UDPG)

What is the second step of glycogen synthesis?

Glycogen synthase transfers the glucose from UDPG to the non-reducing end of a glycogen chain to form a alpha1>4 glycosidic bond.

This step requires the primer glycogenin.

What does glycogen synthase do?

• Spontaneous reaction adding glucose onto the non-reducing end of glycogen from UDPG

• forms alpha1>4

• Oxonium ion intermediate

• UTP used last step, UDP released this step

• ATP used to regenerate UTP, so energetically costly

• worth it to store energy tho

What does glycogenin do?

Glycogenin is the primer for the second enzyme of glycogenesis, glycogen synthase.

• Generates a glycogen primer up to 12 glucose residues

• Glucose is transferred from UDPG to Tyr

• Shields Mn2+ -Glycogen synthase can then take over

• Glycogenin remains bound to the single reducing end of glycogen buried within glycogen molecule

What is the third enzyme of glycolysis?

Glycogen branching enzyme transfers a fragment of 6-7 residues from the end of a main chain to C6-OH group of glucose on a motor chain.

What happens with glycogen branching enzyme?

Residue from the end of main chain of C6-OH group on glucose on a motor chain

• Transferred segment comes from a chain with at least 11 residues

• branch points must be at least 4 residues away