ABSTRACT
METALLOPROTEINS: EMERGING THEMES or the isomerizations that involve the intermediate formation of the adenosyl free radical. Before the appearance of B12, and even now in its absence, the adenosyl radical was formed by the reversible one-electron reduction of S-adenosylmethionine, while Me transfer proceeded directly from the methyl donor to a Zn-coordinated thiolate acceptor at a slower rate (Chapter 13). The difficulty in achieving the lowspin state of Fe with only amino acids is emphasized by the fact that the only mono nuclear nonheme enzyme known to possess an LS Fe(III) ion is nitrile hydratase; in this enzyme the Fe is surrounded by three Cys residues and two amide N atoms (but it is not clear whether deprotonated or not) and the sixth site may be vacant or occupied by a ligand from the solvent [67]. The only nonheme enzymes presently known to possess an LS Fe(II) ion are the hydrogenases (Sec. 1 .2 and [10]).
