Mononuclear active sites of molybdoenzymes: chemical approaches to structure and reactivity
One of two general classes of molybdoenzymes contains mono— nuclear catalytic sites and effects oxidation or reduction of substrate x/x0 by oxygen atom transfer: X + (0)XO. The reaction LMoO2 + X —3LMo0 + X0 is well established in synthetic systems, and is frequently accompanied by dimerization of Mo(VI,IV) complexes to [LMo(V)0320. A general kinetic analysis allowing determination of oxotransfer rate constants in systems with p—oxo dimer formation is out— lined. EXAFS results for several enzymes indicate the minimal coordi— nation spheres Mo(VI)02(SR)2 and Mo(IV)0(SR)3 for oxidized and fully reduced forms, respecti'vely. In a synthetic approach to these sites, the ligand pyridine—2,6—bis(1 , 1—diphenylethanethiol) (L-N(SH)2) was prepared. Reaction with MoO 2(acac)2 afforded the 5-coordinate trigonal bipyramidal complex Mo02(L-NS2), which was converted to MoO(L-NS 2 (DMF) with Ph3 P in DMF • In this and other oxo-transfer reactions of these complexes, the gem—diphenyl groups sterically suppress dimerization to a OMo(V)-0-Mo(V)0 complex. This reaction is blocked in enzymes by protein structural constraints. MoO(L—NS2)(DMF) and Me2SO react to produce Mo02(L-NS2) and Me25. Substrate saturation kinetics are observed at sufficient Me2SO concentrations. The two reactions were coupled to generate a catalytic sulfoxide reduction/phosphine oxidation cycle. In a related reaction, d—biotin—d—sulfoxide is reduced by MoO(L-NS2)(DMF) to d-biotin. Inasmuch as d-biotin-d-sulfoxide reductase is a Mo cofactor—dependent enzyme, this reaction provides a meaningful model for an enzymatic oxo—transfer reaction • Kinetic data for various oxo—transfer reactions are presented and their relevance to enzymatic processes is discussed. INTRODUCTION Molybdenum—containing enzymes are of two types: nitrogenases (1—6), which catalyze the reduction of dinitrogen to ammonia, and oxo—transfer enzymes (2,3,7,8), which catalyze what is in effect the transfer of oxygen atoms from or to the substrates X/XO. The latter transformations may be written most simply as reaction [1] or, to emphasize the two—electron nature of the overall processes, as the half—reaction [2]. Representative molybdoenzymes of X+(O) xO [1] X+H20 ± XO+2H++2e [2] the oxo-transfer type are listed in Table 1 • For sulfite oxidase and xanthine oxidase/ dehydrogenase in particular, abundant evidence has demonstrated that a mononuclear Mo coordination unit is the catalytic site. In addition to this site, every enzyme that has been at least partially characterized contains one or more prosthetic groups (heme, flavin, Fe—S cluster) capable of electron storage and transfer. Both nitrogenase and oxo-transfer molybdoenzymes contain dissociable cofactors that are obligatory for catalytic activity but which, except for containing all the molybdenum in a given enzyme, are chemically unrelated. The cofactor of nitrogenase is a Mo—Fe—S cluster of incompletely defined composition and structure (9, 10). The cofactor (Mo—co) of oxo—transfer enzymes (11) is not yet well characterized but has been shown to contain a pterin nucleus carrying a potentially coordinating side chain. Certain leading results of the Duke group (12-14) bearing on the structure of Mo-co from several enzymes are schematically illustrated in Fig. 1. Denaturation of sulfite oxidase and xanthine dehydrogenase from chicken liver and Chlorella nitrate reductase with 6 M guanidine HC1 in the presence of KI and 12 led to dissociation of the same fluorescent species (form A). When denaturation was performed in boiling pH 2.5 solution, a different fluorescent species (form B) was released. The permanganate