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Cys-AMP and Hcy-AMP, but not Met-AMP, can degrade via cyclization. (A). When deprotonated, both Cys-AMP and Hcy-AMP can cyclize to their respective thiolactones. The five-membered ring formed by homocysteine is favored over the four-membered ring formed by cysteine [48]. (B) The methionine side chain is not nucleophilic under physiological conditions and so a similar cyclization reaction will not occur.
