D-Amino acids in eucaryotic cells: physiological function and its molecular mechanism.

D-氨基酸被认为只存在于低等生物中，而在高等生物中没有任何作用。然而，最近在包括哺乳动物在内的真核生物中也发现了D-氨基酸，其生理作用已被阐明。D-丝氨酸存在于哺乳动物的大脑中，是N-甲基-D-天冬氨酸(NMDA)谷氨酸受体的内在共激剂，调节大脑功能。D-丝氨酸被认为是由哺乳动物类型的丝氨酸外消旋酶合成，并被D-氨基酸氧化酶降解，但其具体的代谢途径尚不清楚。在本研究项目中，我们研究了D-氨基酸在融合酵母菌、酿酒酵母、细胞黏菌、盘状发育不良菌和家蚕中的代谢途径。前两种生物含有哺乳动物型丝氨酸消旋酶和D-氨基酸氧化酶。Pombe和D.discoideum丝氨酸消旋酶具有与哺乳动物酶相似的性质，如催化D-丝氨酸和L-丝氨酸的脱水以及消旋等。为了了解详细的反应机理，我们对酵母丝氨酸外消旋酶进行了X射线结晶学研究。我们对家蚕丝氨酸外消旋酶进行了部分纯化，证明该酶主要催化D-丝氨酸和L-丝氨酸的外消旋反应而不脱水。在这种情况下，家蚕丝氨酸消旋酶是真正的消旋酶。在对D-氨基酸氧化酶的研究中，我们发现该酶在作用于酸性氨基酸方面是独一无二的。我们还发现，芽生酵母酿酒酵母含有依赖磷酸吡哆醛的D-丝氨酸脱水酶，其一级结构与细菌酶完全不同。

D-Amino acids had been believed to occurr only in lower organisms and to have no functions in higher organisms. However, recently, D-amino acids were discovered also in eukaryotes, including mammals and their physiological roles have been clarified. D-Serine occurs in mammalian brains and serves as an intrinsic coagonist of the N-methyl-D-aspartate (NMDA) glutamate receptors and modulates brain functions. D-Serine is considered to be synthesized by mammalian-type serine racemase and degraded by D-amino acid oxidase, however the detailed metabolic pathway remains unclear. In this research project, we studied the metabolic pathways of D-amino acids in fusion yeast, Schoizosaccharomyces pombe, cellular slime mold, Dyctiostelium discoideum, and silkworm, Bombyx mori. The former two organisms contain mammalian-type serine racemase and D-amino acid oxidase. The S. pombe and D. discoideum serine racemases exhibit the similar properties to those of the mammalian enzyme, for example they catalyze the dehydrataion of D-and L-serine as well as the racemization. To understand the detailed reaction mechanism, we carried out the X-ray crystallography of the yeast serine racemase. We partially purified the silkworm serine racemase and demonstrated that the enzyme catalyzes dominantly racemization and no dehydratation of D-and L-serine. The silkworm serine racemase is the real racemase in this context. During the study of the S. pome D-amino acid oxidase, we found that the enzyme is unique in acting on acidic amino acids. We also found that the budding yeast, Saccharomyces cerevisiae, contains pyridoxal phosphate-dependent D-serine dehydratase, of which primary structure is completely different from that of the bacterial enzymes.