Glutamate Racemase
Racemases are enzymes that cause a racemization of the chirality center of a molecule. Glutamate racemase (MurI) was selected to represent this category of isomerases in its ability to catalyze the interconversion of L-glutamate to D-glutamate. D-glutamate is important in the protection of the peptidoglycan layer of bacterial cell walls from cellular proteases. For this reason MurI has been a target for antibacterial drugs.
It seems that MurI deprotonates and then reprotonates the alpha carbon of the glutamate substrate by a general acid-base reaction. Two cysteine residues play the roles of base and conjugate acid in this racemase mechanism. The two cysteine residue mechanism is conserved throughout cofactor independent racemases.
MurI forms a dimer with each monomer consisting of two alpha/beta fold domains. Like TIM, MurI is fully functionally in the absense of any cofactor.
The Aspargine residue holds a nitrogen of glutamate as the oxygen is passed from Cys178 to Cys70 in D-glutamate to L-glutatmate. The bonding view shows D-glutamate bonded to Cys178 with Cys70 waiting to stablize the chirality change.