Glycine cleavage system: reaction mechanism, physiological significance, and hyperglycinemia

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概要

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• The glycine cleavage system catalyzes the following reversible reaction: Glycine + H4folate + NAD+⇄5,10-methylene-H4folate + CO2 + NH3 + NADH + H+The glycine cleavage system is widely distributed in animals, plants and bacteria and consists of three intrinsic and one common components: those are i) P-protein, a pyridoxal phosphate-containing protein, ii) T-protein, a protein required for the tetrahydrofolate-dependent reaction, iii) H-protein, a protein that carries the aminomethyl intermediate and then hydrogen through the prosthetic lipoyl moiety, and iv) L-protein, a common lipoamide dehydrogenase. In animals and plants, the proteins form an enzyme complex loosely associating with the mitochondrial inner membrane. In the enzymatic reaction, H-protein converts P-protein, which is by itself a potential α–amino acid decarboxylase, to an active enzyme, and also forms a complex with T-protein. In both glycine cleavage and synthesis, aminomethyl moiety bound to lipoic acid of H-protein represents the intermediate that is degraded to or can be formed from N5,N10-methylene-H4folate and ammonia by the action of T-protein. N5,N10-Methylene-H4folate is used for the biosynthesis of various cellular substances such as purines, thymidylate and methionine that is the major methyl group donor through S-adenosyl-methionine. This accounts for the physiological importance of the glycine cleavage system as the most prominent pathway in serine and glycine catabolism in various vertebrates including humans. Nonketotic hyperglycinemia, a congenital metabolic disorder in human infants, results from defective glycine cleavage activity. The majority of patients with nonketotic hyperglycinemia had lesions in the P-protein gene, whereas some had mutant T-protein genes. The only patient classified into the degenerative type of nonketotic hyperglycinemia had an H-protein devoid of the prosthetic lipoyl residue. The crystallography of normal T-protein as well as biochemical characterization of recombinants of the normal and mutant T-proteins confirmed why the mutant T-proteins had lost enzyme activity. Putative mechanisms of cellular injuries including those in the central nervous system of patients with nonketotic hyperglycinemia are discussed.(Communicated by Tatsuo SUDA, M.J.A.)

著者

• Hiraga Koichi

  Department Of Biochemistry Graduate School Of Medicine And Pharmaceutical Sciences

• Yoshida Tadashi

  Yamagata University School of Medicine

• KIKUCHI Goro

  Tohoku University

• MOTOKAWA Yutaro

  University of Tokushima

• HIRAGA Koichi

  Department of Biochemistry, University of Toyama Graduate School of Medicine and Pharmaceutical Sciences

• YOSHIDA Tadashi

  Yamagata University

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