Fig. 5.

Proposed mechanism for the interconversion of ll-DAP and dl-meso-DAP by DAP epimerase. The transition state is a planar carbanion, with the six atoms, NH₃⁺, C^β, C^α, C, and the two carboxyl oxygens forming the plane. This transition state can be approached from either direction with equal frequency. The two catalytic cysteine residues Cys-73 and Cys-217 are poised to repeatedly deprotonate and reprotonate the C^α of the bound substrate from opposite faces until it is released.