Fig. 2.

Reaction mechanism for serine racemization (a) and α,β-elimination (b) by SR [25]. PLP bound to SR through an internal aldimine with Lys56 reacts with l-serine yielding an external aldimine; α-proton abstraction from this intermediate gives a resonance-stabilized carbanion. This intermediate can follow two pathways: for racemization reaction (a), the reprotonation of the intermediate on the opposite face of the planar carbanion generates the external aldimine which releases d-serine via transimination with Lys56; for α,β-elimination reaction (b), the elimination of the β-OH group from the carbanionic intermediate leads to the formation of the aminoacrylate-PLP intermediate; subsequent transimination releases iminopyruvate product, which non-enzymatically hydrolyzes to pyruvate and ammonia