Functional characterization of a novel ArgA from Mycobacterium tuberculosis

The Mycobacterium tuberculosis gene Rv2747 encodes a novel 19-kDa ArgA that catalyzes the initial step in L-arginine biosynthesis, namely the conversion of L-glutamate to α-N-acetyl-L-glutamate. Initial velocity studies reveal that Rv2747 proceeds through a sequential kinetic mechanism, with K_m values of 280 mM for L-glutamine and 150 μM for acetyl-coenzyme A and with a k_cat value of 200 min^-1. Initial velocity studies with L-glutamate showed that even at concentrations of 600 mM, saturation was not observed. Therefore, only a k_cat/K_m value of 125 M ^-1 min^-1 can be calculated. Inhibition studies reveal that the enzyme is strongly regulated by L-arginine, the end product of the pathway (50% inhibitory concentration, 26 μM). The enzyme was completely inhibited by 500 μM arginine, with a Hill coefficient of 0.60, indicating negatively cooperative binding of L-arginine.