Modulation of arginine decarboxylase activity from Mycobacterium smegmatis. Evidence for pyridoxal-5'-phosphate-mediated conformational changes in the enzyme

Abstract

Arginine decarboxylase (arginine carboxy-lyase, EC 4.1.1.19) from Mycobacterium smegmatis, TMC 1546 has been purified to homogeneity. The enzyme has a molecular mass of 232 kDa and a subunit mass of 58.9 kDa. The enzyme from mycobacteria is totally dependent on pyridoxal 5'-phosphate for its activity at its optimal pH and, unlike that from Escherichia coli, Mg²⁺ does not play an active role in the enzyme conformation. The enzyme is specific for arginine (K_m= 1.6 mM). The holoenzyme is completely resolved in dialysis against hydroxylamine. Reconstitution of the apoenzyme with pyridoxal 5'-phosphate shows sigmoidal binding characteristics at pH 8.4 with a Hill coefficient of 2.77, whereas at pH 6.2 the binding is hyperbolic in nature. The kinetics of reconstitution at pH 8.4 are apparently sigmoidal, indicating the occurrence of two binding types of differing strengths. A lowaffinity (K_d= 22.5 μM) binding to apoenzyme at lower pyridoxal-5'-phosphate concentrations and a high-affinity (K_d= 3.0 μM) binding to apoenzyme at high pyridoxal 5'-phosphate concentrations. The restoration of full activity occurred in parallel with the tight binding (high affinity) of pyridoxal 5'-phosphate to the apoenzyme. Along with these characteristics, spectral analyses of holoenzyme and apoenzyme at pH 8.4 and pH 6.2 indicate a pH-dependent modulation of coenzyme function. Based on the pH-dependent changes in the polarity of the active-site environment, pyridoxal 5'-phosphate forms different Schiff-base tautomers at pH 8.4 and pH 6.2 with absorption maxima at 415 nm and 333 nm, respectively. These separate forms of Schiff-base confer different catalytic efficiencies to the enzyme.