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，在高等生物中没有作用。然而，近年来，d -氨基酸也在包括哺乳动物在内的真核生物中被发现，其生理作用已被阐明。d -丝氨酸存在于哺乳动物大脑中，作为n -甲基- d -天冬氨酸（NMDA）谷氨酸受体的内在凝聚剂，调节大脑功能。d -丝氨酸被认为是由哺乳动物型丝氨酸消旋酶合成，并被d -氨基酸氧化酶降解，但具体的代谢途径尚不清楚。在本研究项目中，我们研究了融合酵母、Schoizosaccharomyces pombe、细胞黏菌、Dyctiostelium disideum和家蚕（Bombyx mori）中d -氨基酸的代谢途径。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。S. pombe和D. disideum丝氨酸外消旋酶表现出与哺乳动物酶相似的特性，例如它们催化d -丝氨酸和l -丝氨酸的脱水以及外消旋。为了了解详细的反应机理，我们对酵母丝氨酸消旋酶进行了x射线晶体学研究。我们对蚕丝氨酸消旋酶进行了部分纯化，并证明该酶主要催化d -丝氨酸和l -丝氨酸的消旋而不脱水。在这种情况下，蚕丝氨酸消旋酶是真正的消旋酶。在对梨d -氨基酸氧化酶的研究中，我们发现该酶对酸性氨基酸具有独特的作用。我们还发现，出芽酵母（Saccharomyces cerevisiae）含有吡哆醛磷酸依赖的d -丝氨酸脱水酶，其初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.