Glycogen phosphorylase (GP) Flashcards
structure
two identical subunits with two domains (N and C)
N-terminal domain has crevice = glycogen storage cite
N terminal crevice
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…
Where is actve site
between the two domains (N and C) has a required PLP vit B6 cofactor. This cofactor/coenzyme is also a prosthetic group.
PLP in this enzyme
not used as an e- sink as it normally is,,,
Two GP structures
- phsphorylate A with a Pi bound to ser-14 at both subunits
phosphrylate B, does not have serine that is phosphorylated. - In A and B, Have two conformers, T and R.
MULTILAYER REGULATION
Why is GP regulation complex?
protecting a precious resource of glycogen, want to make sure you really want to use it
T state
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)
mechanistic phosphate
different from regulatory phosphate at ser-14.
T to R conf change
- 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.