Paradigm Shift for Radical S-Adenosyl-l-methionine Reactions: The Organometallic Intermediate ? Is Central to Catalysis.

ABSTRACT: Radical S-adenosyl-l-methionine (SAM) enzymes comprise a vast superfamily catalyzing diverse reactions essential to all life through homolytic SAM cleavage to liberate the highly reactive 5'-deoxyadenosyl radical (5'-dAdo·). Our recent observation of a catalytically competent organometallic intermediate ? that forms during reaction of the radical SAM (RS) enzyme pyruvate formate-lyase activating-enzyme (PFL-AE) was therefore quite surprising, and led to the question of its broad relevance in the superfamily. We now show that ? in PFL-AE forms as an intermediate under a variety of mixing order conditions, suggesting it is central to catalysis in this enzyme. We further demonstrate that ? forms in a suite of RS enzymes chosen to span the totality of superfamily reaction types, implicating ? as essential in catalysis across the RS superfamily. Finally, EPR and electron nuclear double resonance spectroscopy establish that ? involves an Fe-C5' bond between 5'-dAdo· and the [4Fe-4S] cluster. An analogous organometallic bond is found in the well-known adenosylcobalamin (coenzyme B₁₂) cofactor used to initiate radical reactions via a 5'-dAdo· intermediate. Liberation of a reactive 5'-dAdo· intermediate via homolytic metal-carbon bond cleavage thus appears to be similar for ? and coenzyme B₁₂. However, coenzyme B₁₂ is involved in enzymes catalyzing only a small number (?12) of distinct reactions, whereas the RS superfamily has more than 100?000 distinct sequences and over 80 reaction types characterized to date. The appearance of ? across the RS superfamily therefore dramatically enlarges the sphere of bio-organometallic chemistry in Nature.

SUBMITTER: Byer AS 

PROVIDER: S-EPMC6053644 | biostudies-literature | 2018 Jul

REPOSITORIES: biostudies-literature