CHEM 313 Lecture Notes - Lecture 3: Pyridoxal Phosphate, Polar Effect, Epimerase And Racemase
Document Summary
Before you start ps: (cid:6)(cid:7)(cid:1) reactions involving three molecules in concerted fashion are unusual and entropically unfavorable in solution under standard condition. Which one is most likely to occur depends on the enzyme and the type of reaction. In the literature, the pyridine nitrogen n%1 has traditionally been assumed to be protonated in enzyme active sites, with the protonated pyridine ring providing resonance stabilization of carbanionic intermediates and good electron sink. This assumption is certainly correct for some plp enzymes, but the structures of other active sites are incompatible with protonation of n%1, and, consequently, these enzymes are expected to use plp in the n%1 unprotonated form. However, it is unlikely that you have both the protonated n%1 pyridine and the protonated imine/enamine with the neutral o%3" phenol in the majority of enzyme active sites. It is possible that, in many enzyme, the o%3" phenol of the plp is involved in the tautomerization of the imine/enamine.
