Novel tetrahydrofolate‐dependent d‐serine dehydratase activity of serine hydroxymethyltransferases

Abstract

d‐Serine plays vital physiological roles in the functional regulation of the mammalian brain, where it is produced from l‐serine by serine racemase and degraded by d‐amino acid oxidase. In the present study, we identified a new d‐serine metabolizing activity of serine hydroxymethyltransferase (SHMT) in bacteria as well as mammals. SHMT is known to catalyze the conversion of l‐serine and tetrahydrofolate (THF) to glycine and 5,10‐methylenetetrahydrofolate, respectively. In addition, we found that human and Escherichia coli SHMTs have d‐serine dehydratase activity, which degrades d‐serine to pyruvate and ammonia. We characterized this enzymatic activity along with canonical SHMT activity. Intriguingly, SHMT required THF to catalyze d‐serine dehydration and did not exhibit dehydratase activity toward l‐serine. Furthermore, SHMT did not use d‐serine as a substrate in the canonical hydroxymethyltransferase reaction. The d‐serine dehydratase activities of two isozymes of human SHMT were inhibited in the presence of a high concentration of THF, whereas that of E. coli SHMT was increased. The pH and temperature profiles of d‐serine dehydratase and serine hydroxymethyltransferase activities of these three SHMTs were partially distinct. The catalytic efficiency (kcat/Km) of dehydratase activity was lower than that of hydroxymethyltransferase activity. Nevertheless, the d‐serine dehydratase activity of SHMT was physiologically important because d‐serine inhibited the growth of an SHMT deletion mutant of E. coli, ∆glyA, more than that of the wild‐type strain. Collectively, these results suggest that SHMT is involved not only in l‐ but also in d‐serine metabolism through the degradation of d‐serine.