Crystal Structure of Aminomethyltransferase in Complex with Dihydrolipoyl-H-Protein of the Glycine Cleavage System: IMPLICATIONS FOR RECOGNITION OF LIPOYL PROTEIN SUBSTRATE,DISEASE-RELATED MUTATIONS,AND REACTION MECHANISM* |
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Authors: | Kazuko Okamura-Ikeda Harumi Hosaka Nobuo Maita Kazuko Fujiwara Akiyasu C Yoshizawa Atsushi Nakagawa Hisaaki Taniguchi |
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Institution: | From the ‡Institute for Enzyme Research, the University of Tokushima, Tokushima 770-8503 and ;the §Institute for Protein Research, Osaka University, Suita 565-0871, Japan |
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Abstract: | Aminomethyltransferase, a component of the glycine cleavage system termed T-protein, reversibly catalyzes the degradation of the aminomethyl moiety of glycine attached to the lipoate cofactor of H-protein, resulting in the production of ammonia, 5,10-methylenetetrahydrofolate, and dihydrolipoate-bearing H-protein in the presence of tetrahydrofolate. Several mutations in the human T-protein gene are known to cause nonketotic hyperglycinemia. Here, we report the crystal structure of Escherichia coli T-protein in complex with dihydrolipoate-bearing H-protein and 5-methyltetrahydrofolate, a complex mimicking the ternary complex in the reverse reaction. The structure of the complex shows a highly interacting intermolecular interface limited to a small area and the protein-bound dihydrolipoyllysine arm inserted into the active site cavity of the T-protein. Invariant Arg292 of the T-protein is essential for complex assembly. The structure also provides novel insights in understanding the disease-causing mutations, in addition to the disease-related impairment in the cofactor-enzyme interactions reported previously. Furthermore, structural and mutational analyses suggest that the reversible transfer of the methylene group between the lipoate and tetrahydrofolate should proceed through the electron relay-assisted iminium intermediate formation. |
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Keywords: | Enzyme Catalysis Enzyme Structure Folate Protein Assembly X-ray Crystallography Aminomethyltransferase Glycine Cleavage System H-protein Nonketotic Hyperglycinemia |
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