Phase I and phase II mechanisms of PC Flashcards
What does Phase I mech require?
PC requires Lysine covalently bound to biotin prosthetic group to carry CO2 from one active subsite to another.
phase I
AKA SN2-like reaction.
Form a very reactive intermediate (carboxy phosphate), then going to lose phosphate you added that was added as you load CO2 onto biotin arm.
Adding CO2 to more accessible Amide of di-amide bond.
Form Carboxybiotinyl-enzyme. Its on an arm due to the linker, and the Co2 is going to be lost so need a good bond cleavage (large -∆G)
Phase 2
occurs on a second subsite. Ussing biotinyl arm to move from one subsite to another.
Now Pyruvate is bound, and when it is pressent and the carboxybiotinyl-enzyme has swung the CO2 end over to nearby, the CO2 gets lost and is added to enolate.
???
This occurs on a second subsite. Pyruvate will bind in the active site and destabilize N-C bond on carboxybiotinyl-E. PYRUVATE MUST BE PRESENT when CO2 is released. This destabilizing of N-C is what allows this transfer to occur (CO2 being pulled off), caused by the urido pull.
Pyruvate loses an electron to Biontyl- E, and then attacks CO2, forming OA,
Phase I and II occur…
on different active site (subsites).
These are on different subunits as well…. need a dimeric species or something with more than one subunit present…
Biotinyl arm
moves biotin ring between different active sites of PC (diff subunits(
Timing
Important timing…
CO2 is a gas, hard to capture. Must release CO2 when pyruvate is present.
Pyruvate binding to AS phase 2 will destabilize the N-C bond on the carboxybiotinyl-enzyme.
This will enable the CO2 transfer. Must be timed correctly!!!!
Pull on urido oxygen
If di amide were broken, would have urea…. so urido group derived from urea.
Urido group O must have a pull, which is a zinc ion. This pull will help to pull electrons to destabilize and release CO2
