Figure 6.7 Inhibition by transition-state analogs. (A) The isomerization of l-proline to d-proline by proline racemase, a bacterial enzyme, proceeds through a planar transition state in which the α-carbon atom is trigonal rather than tetrahedral. (B) Pyrrole 2-carboxylate, a transition-state analog because of its trigonal geometry, is a potent inhibitor of proline racemase.