Why Nature Uses Radical SAM Enzymes so Widely: Electron Nuclear Double Resonance Studies of Lysine 2,3-Aminomutase Show the 5鈥?dAdo鈥?鈥淔ree Radical鈥?Is Never Free

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• 作者：Masaki Horitani ; Amanda S. Byer ; Krista A. Shisler ; Tilak Chandra ; Joan B. Broderick ; Brian M. Hoffman
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：June 10, 2015
• 年：2015
• 卷：137
• 期：22
• 页码：7111-7121
• 全文大小：572K
• ISSN：1520-5126

文摘

Lysine 2,3-aminomutase (LAM) is a radical S-adenosyl-l-methionine (SAM) enzyme and, like other members of this superfamily, LAM utilizes radical-generating machinery comprising SAM anchored to the unique Fe of a [4Fe-4S] cluster via a classical five-membered N,O chelate ring. Catalysis is initiated by reductive cleavage of the SAM S鈥揅5鈥?bond, which creates the highly reactive 5鈥?deoxyadenosyl radical (5鈥?dAdo鈥?, the same radical generated by homolytic Co鈥揅 bond cleavage in B₁₂ radical enzymes. The SAM surrogate S-3鈥?4鈥?anhydroadenosyl-l-methionine (anSAM) can replace SAM as a cofactor in the isomerization of l-伪-lysine to l-尾-lysine by LAM, via the stable allylic anhydroadenosyl radical (anAdo鈥?. Here electron nuclear double resonance (ENDOR) spectroscopy of the anAdo鈥?radical in the presence of ¹³C, ²H, and ¹⁵N-labeled lysine completes the picture of how the active site of LAM from Clostridium subterminale SB4 鈥渢ames鈥?the 5鈥?dAdo鈥?radical, preventing it from carrying out harmful side reactions: this 鈥渇ree radical鈥?in LAM is never free. The low steric demands of the radical-generating [4Fe-4S]/SAM construct allow the substrate target to bind adjacent to the S鈥揅5鈥?bond, thereby enabling the 5鈥?dAdo鈥?radical created by cleavage of this bond to react with its partners by undergoing small motions, 鈭?.6 脜 toward the target and 鈭?.5 脜 overall, that are controlled by tight van der Waals contact with its partners. We suggest that the accessibility to substrate and ready control of the reactive C5鈥?radical, with 鈥渧an der Waals control鈥?of small motions throughout the catalytic cycle, is common within the radical SAM enzyme superfamily and is a major reason why these enzymes are the preferred means of initiating radical reactions in nature.