Identification and characterization of a serine racemase in the silkworm Bombyx mori

Abstract

The pupae of lepidopterans contain high concentrations of endogenous d-serine. In the silkworm Bombyx mori, d-serine is negligible during the larval stage but increases markedly during the pupal stage, reaching 50% of the total free serine. However, the physiological function of d-serine and the enzyme responsible for its production are unknown. Herein, we identified a new type of pyridoxal 5′-phosphate (PLP)-dependent serine racemase (SR) that catalyzes the racemization of l-serine to d-serine in B. mori. This silkworm SR (BmSR) has an N-terminal PLP-binding domain that is homologous to mammalian SR and a C-terminal putative ligand-binding regulatory-like domain (ACT-like domain) that is absent in mammalian SR. Similar to mammalian SRs, BmSR catalyzes the racemization and dehydration of both serine isomers. However, BmSR is different from mammalian SRs as evidenced by its insensitivity to Mg2+/Ca2+ and Mg-ATP—which are required for activation of mammalian SRs—and high d-serine dehydration activity. At the pupal stage, the SR activity was predominantly detected in the fat body, which was consistent with the timing and localization of BmSR expression. The results are an important first step in elucidating the physiological significance of d-serine in lepidopterans.

d‐amino acid auxotrophic Escherichia coli strain for in vivo functional cloning of novel d‐amino acid synthetic enzyme

“…However, the physiological functions and biosynthetic pathways of other d ‐amino acids remain largely unexplored. High concentrations of endogenous d ‐Ser have been previously detected in insects [26,27]. In the silkworm Bombyx mori , d ‐Ser levels are negligible during the larval stage but increase markedly during the pupal stage, to reach ~ 50% of the total Ser, suggesting that d ‐Ser may be involved in metamorphosis.…”