Glutamate racemase
[3] Glutamate racemase (MurI) serves two distinct metabolic functions: primarily, it is a critical enzyme in cell wall biosynthesis,[2] but also plays a role in gyrase inhibition.
The enzyme glutamate racemase (MurI) is an example of a moonlighting protein, functioning both in bacterial cell wall biosynthesis as well as in gyrase inhibition.
In some species, the active site also incorporates sulfate ions to undergo hydrogen bonding on the amide backbone and the side chains.
[6] This process, in which MurI helps catalyze the interconversion of glutamate enantiomers, like L-Glutamate, into the essential D-glutamate, is also cofactor independent.
As such it can proceed without needing an additional source, which would bind to an allosteric site, altering the enzyme shape to assist in catalyzing the reaction.
Those domains mentioned earlier are symmetric and this symmetry suggests that this racemase activity of the protein may have evolved from gene duplication.
[5] Due to this main function of biosynthesis of bacterial cell walls MurI has been targeted as an antibacterial in drug discovery.
[10] The presence of glutamate racemase in the process inhibits gyrase from effectively binding to DNA by deforming the shape of the enzyme's active site.
Conversely, the effects of cell wall production on gyrase inhibition were discovered by varying the concentration of the racemization substrate.
Glutamate racemase is known to use its active site to undergo racemization and participate in the cell wall biosynthesis pathway of bacteria.
[7] Surprisingly however, substituting either of the two residues with serine did not appreciable change the rate of the reaction significantly; the kcat value remained within .3% to 3% compared to the wild-type enzyme.
This explains how glutamate racemase in certain bacteria, such as Glr from B. subtilis, do not inhibit gyrase;[12] if one active site is involved with both functions, this independence would not be possible.
[13] Glutamate racemase has emerged as a potential antibacterial target since the product of this enzyme, D-glutamate, is an essential component of bacterial walls.
Inhibiting the enzyme will prevent bacterial wall formation and ultimately result in lysis of the bacteria cell by osmotic pressure.
[5] Possible inhibitors to MurI includes aziridino-glutamate that would alkylate the catalytic cysteines; N-hydroxy glutamate that by mimicking Wat2 (the bound water molecule that interacts with glutamate amino group) would prevent binding of the substrate;[5] or 4-substituted D-glutamic acid analogs bearing aryl-, heteroaryl-, cinnamyl-, or biaryl-methyl substituents that would also prevent binding of substrate.
