PhK Flashcards
key points
1: using PhK, book uses PK.
2: confusion pt 6… there is an active site on these subunits which is similar yet different to PKA. so DONT CONFUSE THESEEE
structure of phK
large, four nonidentical SU, with four copies of these for a total of 16 SU.
X shaped, four arms, each subunit repeated on each arm.
These SU are alpha, beta, gamma, delta (CaM: Ca2+ modulator, responsive to Ca2+ in cell)
PhK activation
activated by Ca2+(which can be low concentrations) concentrations, and by covalent modification by PKA phosphorylating it and can be dephospho at some point too
alpha and beta subunits
can be phosphorylated, which is the cov mod by PKA (regulative)
when +Pi phK is active at lowest possible concentration that its responsive to.
when -Pi PhK CAN be active but the Ca2+ needs to be higher.
gamma subunit
has a catalytic site, where +Pi is being done by it to its substrates.
also a C-terminal reg domain, which is referred to as an Autoinhibitor, inhibiting itself. Need to move it out of the way to be active.
PKA’s C-subunit vs phk gamma
PKA: Permanently +pi threonine, NOTHING TO DO WITH REGULATION, gamma of phk has glutamic acid instead.
common trend between thr +pi and glu: negatively charged.
So the permanently phosphorylated thr makes it negative. Glu doesnt have to worry about it, bc already negative, as a result glu wil ion pair with arg.
This ion pair will help arg to interact with catalytic Asp that will -H+ of GP. GP is a substrate of PhK.
To get max activity of gamma
gamma is where cat site and AI are… need to remove AI and have Ca2+ present.
B subunit also have AI, which crowds gamma space… so we need to get rid of this too… So the +Pi of B subunit causes conf change, causes AI to be unable to inhibit gamma. this explains why phosphorylation and calcium are synergistic in their approach to affecting PhK:
CaM-Ca2+ removes gamma’s AI and +Pi of beta removes another AI.
delta subunit (CaM) structurre
small protein, and can be part of a subunit or a subunit in a complex.
Can bind up to 4 calcium, has an important central alpha helix that allows for flexibility. Two globular domains, gives PhK Ca2+ sensitivity…
CaM binding Ca2+
has octahedral coordination, which is its shape and binding to 4 things but 2 mmore on top and bottom.
Binds 4 Ca2+ to peptide loop O’s… oriented to EF hand. E helix and F helix are right angles. in the junction is where Ca2+ will bind…. so this spot is referred to as an EF hand.
The central a-helix is flexible and is important bc allows things to be bound, such as PhK’s gamma SU AI.
When Ca2+ bound to either globular end, causes conf change, exposes MET rich hydrophobic patch. This exposure enables CaM to bind to gamma subunits CaM binding site so it undergoes conf change to remove AI and Gamma will now be active
GP from b (less active) to a (more active) can occur why
because there is +Pi by PhK, and now removed all AI and PhK can phosphorylate GP, increasing glycogenolysis and which causes glycogen synthesis rate to decrease.
WHy do we want a decrease? because we want to breakdown glycogen, and does so by +Pi of GP and GS so synthe decrease and glygenolysis increase.
Why is Ca2+ used?
In muscle: Ca2+ is a trigger, because muscle contraction requires transient calcium increase. WHich means rate of glycogen breakdown (GP) will be linked to that so there is fuel to make ATP, as wel need atp to move muscle
Liver: Ca2+ linked to epinephrine hormonal control…. and phosphoinositide pathway
flexible helix
allows you to wrap ca-cam around its binding site of gamma domains AI
