Cloning and structural analysis of Mycobacterium leprae serine hydroxymethyltransferase

Abstract

Serine hydroxymethyltransferase (SHMT) plays a key role in cell physiology as it participates in the different interconversion pathway of folate coenzymes, provides almost exclusively folate one carbon fragments for the biosynthesis of a variety of end products. For the first time, Mycobacterium leprae glyA gene, encodes the enzyme serine hydroxymethyltransferase, has been cloned in Escherichia coli, over-expressed and purified the protein product (mlSHMT) for folding and stability studies under various denaturating conditions. The recombinant mlSHMT exists as homo-dimer of molecular mass about 90 kDa under physiological conditions . The studies on catalytic properties of mlSHMT show that the enzyme catalyzes the H₄-folate dependent retro-aldol cleavage of l-serine, however, D-alanine dependent transaminase activity was absent in the enzyme. Further analysis of the enzyme kinetics for hydroxymethyltransferase reaction for mlSHMT demonstrates a comparable K_m value for l-serine to SHMTs from other sources but significantly lower catalytic efficiency (k_cat/K_m). The mlSHMT is resistant to alkaline denaturation and exist as apo-dimer up to pH 10.5. Urea and guanidinium chloride induces dissociation of mlSHMT dimer into monomer at low denaturant concentrations, and leads to loss of enzymatic activity.