Mechanistic Enzymology of Radical SAM Enzymes in Natural Product Biosynthesis

2021 Fiscal Year Final Research Report

• Project/Area Number: 18H02095
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 37020:Chemistry and chemical methodology of biomolecules-related
• Research Institution: Tokyo Institute of Technology
• Principal Investigator: Eguchi Tadashi 東京工業大学, 理学院, 教授 (60201365)
• Project Period (FY): 2018-04-01 – 2022-03-31
• Keywords: ラジカルSAM酵素 / 天然物 / 生合成 / 反応機構

Outline of Final Research Achievements

In this study, functional analysis of a radical S-adenosyl-L-methionine (SAM) adenosylhopane synthase HpnH and a methylcobalamin-dependent radical SAM methyltransferase Orf29 were performed.
Adenosylhopane is found to be formed by a crosslinking reaction between diploptene and a 5'-deoxyadenosyl radical that is generated by the radical S-adenosyl-L-methionine (SAM) enzyme HpnH. Further, we successfully propose that the thiol of Cys106 stereoselectively reduces the radical intermediate generated at the C22 position by the addition of the 5'-deoxadenosyl radical to diploptene, to complete the reaction.
By in vitro analysis using Orf29, Orf29 was found to catalyze the C-methylation of the L-methionine moiety of SAM. The resulting methylated derivative of SAM was then converted into 1-amino-2-methylcyclopropancarboxylic acid by Orf30. Thus, we demonstrate that C-methylation of SAM occurs prior to cyclopropanation in the biosynthesis of a bacterial 1-amino-2-methylcyclopropancarboxylic acid.

Free Research Field

天然物化学

Academic Significance and Societal Importance of the Research Achievements

本研究では未解明のラジカルSAM酵素の機能および反応機構解析を目的とした。生体内酵素反応は、すべてイオン的な反応メカニズムによって触媒されると考えられてきた。しかし、有機ラジカル中間体が関与する酵素反応の例が明らかになり始め、現在ではラジカル中間体を経由する反応機構が広く受け入れられている。ラジカル機構を利用する酵素は、イオン的なメカニズムによって触媒するのが難しい、あるいは不可能と考えられる反応がラジカル中間体を経て反応を進行させている。本研究で明らかにしたラジカルSAM酵素の反応はラジカルを経て行われる酵素化学の深化および新たな酵素化学の領域を築けたと考えている。