Glycogen phosphorylase (GP)

Question 1

structure

Answer 1

two identical subunits with two domains (N and C)

N-terminal domain has crevice = glycogen storage cite

Question 2

N terminal crevice

Answer 2

only hold 4-5 units of linear portion of glycogen, where there is NO branching…. If there is a branch, cant fit in storage site.
SO WHy have a crevice?
a number of the breakdown steps, wont have to dissociate/reassociate the glycogen…. why is this good: this is more efficient…

Question 3

Where is actve site

Answer 3

between the two domains (N and C) has a required PLP vit B6 cofactor. This cofactor/coenzyme is also a prosthetic group.

Question 4

PLP in this enzyme

Answer 4

not used as an e- sink as it normally is,,,

Question 5

Two GP structures

Answer 5

1. phsphorylate A with a Pi bound to ser-14 at both subunits
   phosphrylate B, does not have serine that is phosphorylated.
2. In A and B, Have two conformers, T and R.

MULTILAYER REGULATION

Question 6

Why is GP regulation complex?

Answer 6

protecting a precious resource of glycogen, want to make sure you really want to use it

Question 7

T state

Answer 7

has buried active site, cant get S in, so have low affinity.

R has AS accessible, so has high affinity, including mechanistic phosphate. (enzyme a little more open here)

Question 8

mechanistic phosphate

Answer 8

different from regulatory phosphate at ser-14.

Question 9

T to R conf change

Answer 9

• When going from T to R, the tower helices will change position to give more favorable packing. This conf change keeps overall symmetry which is the MWC model

Ser-14 containing regulatory Pi strongly stabilizes the R state.

T to R also displaces and disorders the 280s Loop so that it no longer covers AS, which means AS is open and S can get in.

Also causes a rotation in Arg side chain, which increases the mechanistic Pi affinity (substrate affinity for mechanistic Pi) due to ion pairing.