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+, Cβ, Cα, 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 Cα of the bound substrate from opposite faces until it is released.