Enzymatic Characterization of 5-Methylthioribose 1-Phosphate Isomerase from Bacillus subtilis

Abstract

The product of the mtnA gene of Bacillus subtilis catalyzes the isomerization of 5-methylthioribose 1-phosphate (MTR-1-P) to 5-methylthioribulose 1-phosphate (MTRu-1-P). The catalysis of MtnA is a novel isomerization of an aldose phosphate harboring a phosphate group on the hemiacetal group. This enzyme is distributed widely among bacteria through higher eukaryotes. The isomerase reaction analyzed using the recombinant B. subtilis enzyme showed a Michaelis constant for MTR-1-P of 138 μM, and showed that the maximum velocity of the reaction was 20.4 μmol min⁻¹ (mg protein)⁻¹. The optimum reaction temperature and reaction pH were 35 °C and 8.1. The activation energy of the reaction was calculated to be 68.7 kJ mol⁻¹. The enzyme, with a molecular mass of 76 kDa, was composed of two subunits. The equilibrium constant in the reversible isomerase reaction [MTRu-1-P]/[MTR-1-P] was 6. We discuss the possible reaction mechanism.