1995 Fiscal Year Final Research Report Summary
Stereochemical Studies of the Structure, Function, and Molecular Evolution of Pyridoxal Enzymes
| Project/Area Number |
06680611
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
Functional biochemistry
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| Research Institution | KYOTO UNIVERSITY |
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Principal Investigator |
YOSHIMURA Tohru : Institute for Chemical Research, Kyoto University Instructor, 化学研究所, 助手 (70182821)
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| Co-Investigator(Kenkyū-buntansha) |
ESAKI Nobuyoshi Institute for Chemical Research, Kyoto University Associate Professor, 化学研究所, 助教授 (50135597)
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| Project Period (FY) |
1994 – 1995
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| Keywords | Pyridoxal phosphate / Transaminase / Racemase / Stereochemistry / Molecular Evolution |
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| Research Abstract |
The reactions of pyridoxal 5'-phosphate (PLP)-dependent enzymes proceed through the formation of an anionic Schiff base intermediate of the substrate and the coenzyme. Three stereochemical possibilities exist for the enzyme reactions : The formation and cleavage of bonds occur stereospecifically on either si-or re-face of the plannar intermediate, and alternatively non-stereospecifically on both faces. The stereospecificities of various PLP enzymes have been studied for the hydrogen transfer between C-4' of the cofactor and substrate moieties which occurs in their transamination reactions including a side-reaction transamination. The stereospecificities reflect the active-site structures of the enzymes, especially the topographical situation of a coenzyme-substrate Schiff base and a catalytic base for the hydrogen transfer. The PLP enzymes so far studied catalyze only the si-face specific hydrogen transfer. This suggests that these PLP enzymes have the similar active-site structure and
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are evolved divergently from a common ancestral protein. We recently established a new method for the determination of stereospecificity for the hydrogen transfer, and found that D-amino acid aminotransferase and branched chain L-amino acid aminotransferase, which show a significant sequence homology, catalyze the reface hydrogen transfer on the intermediate. The X-ray chrystallographical studies of D-amino acid aminotransferase revealed that the relative arrangement of the catalytic base of the enzyme to the C4' of the cofactor is opposite to that of other aminotransferases catalyzing the si-face intermediate. The fold of D-amino acid aminotransferase is different from those of other aminotransferase so far demonstrated. Therefore, the classifications of the aminotransferases based on the primary structure, three dimensional structure, and stereochemistry of the hydrogen transfer coincide with one another. We also found that PLP-dependent amino acid racemases, whose primary structures are different from those of other PLP-enzymes catalyze the non-stereospecific hydrogen transfer on both faces of the planar intermediate. The PLP-dependent amino acid racemases are probably evolved from the ancesrtral protein which differes to those of aminotransferases. Less
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