STRUCTURE OF A CEPHALOSPORIN SYNTHASE

Penicillins and cephalosporins are among the most widely used therapeu tic agents. These antibiotics are produced from fermentation-derived m aterials as their chemical synthesis is not commercially viable. Uncon ventional steps in their biosynthesis are catalysed by Fe(II)-dependen t oxidases/oxygenases; isopenicillin N synthase (IPNS)(1,2) creates in one step the bicyclic nucleus of penicillins, and deacetoxycephalospo rin C synthase (DAOCS) catalyses the expansion of the penicillin nucle us into the nucleus of cephalosporins. Both enzymes use dioxygen-deriv ed ferryl intermediates in catalysis but, in contrast to IPNS, the fer ryl form of DAOCS is produced by the oxidative splitting of a cosubstr ate, 2-oxoglutarate (alpha-ketoglutarate). This route of controlled fe rryl formation and reaction is common to many mononuclear ferrous enzy mes(3), which participate in a broader range of reactions than their w ell-characterized counterparts, the haem enzymes, Here we report the f irst crystal structure of a 2-oxoacid-dependent oxygenase, High-resolu tion structures for apo-DAOCS, the enzyme complexed with Fe(II), and w ith Fe(II) and 2-oxoglutarate, were obtained from merohedrally twinned crystals. Using a model based on these structures, we propose a mecha nism for ferryl formation.