Screening of microorganisms producing α-methylserine hydroxymethyltransferase, purification of the enzyme, gene cloning, and application to the enzymatic synthesis of α-methyl-L-serine

Abstract Through the screening of microorganisms capable of utilizing α-methylserine, three representative strains belonging to the bacterial genera Paracoccus , Aminobacter , and Ensifer were selected as potent producers of α-methylserine hydroxymethyltransferase, an enzyme that catalyzes the interconversion between α-methyl- l -serine and d -alanine via tetrahydrofolate. Among these strains, Paracoccus sp. AJ110402 was selected as the strain exhibiting the highest α-methylserine hydroxymethyltransferase activity. The enzyme was purified to homogeneity from a cell-free extract of this strain. The native enzyme is a homodimer with apparent molecular mass of 85 kDa and contains 1 mol of pyridoxal-5′-phosphate per mol of the subunit. The K m for α-methyl- l -serine and tetrahydrofolate was 0.54 mM and 73 μM, respectively. The gene from Paracoccus sp. AJ110402 encoding α-methylserine hydroxymethyltransferase was cloned and expressed in Escherichia coli . Sequence analysis revealed an open reading frame of 1278 bp, encoding a polypeptide with a calculated molecular mass of 46.0 kDa. Using E. coli cells as whole-cell catalysts, 9.7 mmol of α-methyl- l -serine was stereoselectively obtained from 15 mmol of d -alanine and 13.2 mmol of formaldehyde.