Catalytic Mechanism of Glycine N-Methyltransferase
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- 作者：Yoshimi Takata ; Yafei Huang ; Junichi Komoto ; Taro Yamada ; Kiyoshi Konishi ; Hirofumi Ogawa ; Tomoharu Gomi ; Motoji Fujioka ; and Fusao Takusagawa
- 刊名：Biochemistry
- 出版年：2003
- 出版时间：July 22, 2003
- 年：2003
- 卷：42
- 期：28
- 页码：8394 - 8402
- 全文大小：500K
- 年卷期：v.42,no.28(July 22, 2003)
- ISSN：1520-4995
文摘
Methyltransfer reactions are some of the most important reactions in biological systems. GlycineN-methyltransferase (GNMT) catalyzes the S-adenosyl-L-methionine- (SAM-) dependent methylation ofglycine to form sarcosine. Unlike most SAM-dependent methyltransferases, GNMT has a relatively high
value and is weakly inhibited by the product S-adenosyl-L-homocysteine (SAH). The major role ofGNMT is believed to be the regulation of the cellular SAM/SAH ratio, which is thought to play a keyrole in SAM-dependent methyltransfer reactions. Crystal structures of GNMT complexed with SAM andacetate (a potent competitive inhibitor of Gly) and the R175K mutated enzyme complexed with SAMwere determined at 2.8 and 3.0 Å resolutions, respectively. With these crystal structures and the previouslydetermined structures of substrate-free enzyme, a catalytic mechanism has been proposed. Structural changesoccur in the transitions from the substrate-free to the binary complex and from the binary to the ternarycomplex. In the ternary complex stage, an 
-helix in the N-terminus undergoes a major conformationalchange. As a result, the bound SAM is firmly connected to protein and a "Gly pocket" is created near thebound SAM. The second substrate Gly binds to Arg175 and is brought into the Gly pocket. Five hydrogenbonds connect the Gly in the proximity of the bound SAM and orient the lone pair orbital on the aminonitrogen (N) of Gly toward the donor methyl group (CE) of SAM. Thermal motion of the enzyme leadsto a collision of the N and CE so that a SN2 methyltransfer reaction occurs. The proposed mechanism issupported by mutagenesis studies.
value and is weakly inhibited by the product S-adenosyl-L-homocysteine (SAH). The major role ofGNMT is believed to be the regulation of the cellular SAM/SAH ratio, which is thought to play a keyrole in SAM-dependent methyltransfer reactions. Crystal structures of GNMT complexed with SAM andacetate (a potent competitive inhibitor of Gly) and the R175K mutated enzyme complexed with SAMwere determined at 2.8 and 3.0 Å resolutions, respectively. With these crystal structures and the previouslydetermined structures of substrate-free enzyme, a catalytic mechanism has been proposed. Structural changesoccur in the transitions from the substrate-free to the binary complex and from the binary to the ternarycomplex. In the ternary complex stage, an -helix in the N-terminus undergoes a major conformationalchange. As a result, the bound SAM is firmly connected to protein and a "Gly pocket" is created near thebound SAM. The second substrate Gly binds to Arg175 and is brought into the Gly pocket. Five hydrogenbonds connect the Gly in the proximity of the bound SAM and orient the lone pair orbital on the aminonitrogen (N) of Gly toward the donor methyl group (CE) of SAM. Thermal motion of the enzyme leadsto a collision of the N and CE so that a SN2 methyltransfer reaction occurs. The proposed mechanism issupported by mutagenesis studies.