Sulfite oxidase, electron-transfer reaction

Redox potentials of the molybdenum centers in several of the enzymes have been obtained by potentiometric titration (Table 3a). Although the substrate reaction chemistry requires the metal center to participate in net two-electron redox reactions, the simple electron-transfer reactions of the active sites occur in one-electron steps involving the MoVI/Mov and Mov/MoIV couples. Several of the molybdenum enzymes studied have MoVI/Mov and Mov/MoIV couples that differ by less than 40 mV. However, in sulfite oxidase the Movl/Mov (38 mV) and Mov/Molv (-239 mV) couples are separated by roughly 275 mV [88], In formate dehydrogenase (D. desulfuricans) the MoVI/Mov (-160 mV) and Mov/MoIV (-330 mV) couples are separated by 170 mV [89], Both the MoVI/Mov and... 

Molybdenum. Molybdenum is a component of the metaHoen2ymes xanthine oxidase, aldehyde oxidase, and sulfite oxidase in mammals (130). Two other molybdenum metaHoen2ymes present in nitrifying bacteria have been characteri2ed nitrogenase and nitrate reductase (131). The molybdenum in the oxidases, is involved in redox reactions. The heme iron in sulfite oxidase also is involved in electron transfer (132). 

The oxo-transfer chemistry of molybdenum in sulfite oxidase is probably the best characterized, in terms of synthetic models, structural and mechanistic data, of all the elements we have described up till now. The reaction cycle (Figure 17.5) involves binding of sulfite to the oxidized MoVI, two-electron reduction of the Mo centre and release of sulfate. The Movl centre is restored by successive one-electron transfers from a cytochrome (bs in mammals). The primary oxo-transfer reaction ... 

The redox properties of Mo also make it useful in enzymes that catalyze reactions involving two-electron or oxygen-atom transfer (Frausto da Silva and Williams 2001). Such enzymes include nitrate reductase, sulfite oxidase, formate dehydrogenase and aldehyde oxidase (Hille 1996 Stiefel 1997 Kroneck and Abt 2002). Hence, while Mo is rarely a terminal electron... 

Shown in Figures 5-7 are the redox pathways for xanthine oxidase, sulfite oxidase, and nitrate reductase (assimilatory and respiratory), respectively. These schemes address the electron and proton (hydron) flows. The action of the molyb-doenzymes is conceptually similar to that of electrochemical cells in which half reactions occur at different electrodes. In the enzymes, the half reactions occur at different prosthetic groups and intraprotein (internal) electron transfer allows the reactions to be coupled (i.e., the circuit to be completed). In essence, this is the modus operandi of these enzymes, which must be determined before intimate mechanistic considerations are seriously addressed. 

These kinetics data are consistent with a preequilibrium dissociation of dmf from the molybdenum center to form a reactive five-coordinate species that rapidly reduces the Fe(III) center via an inner sphere (halogen transfer) reaction. Other one-electron atom transfer reactions are known in oxo-molybdenum chemistry (262). An innersphere (atom transfer) mechanism is not a viable model for intramolecular transfer in sulfite oxidase because in the enzyme the Mo and Fe centers are almost certainly held too far apart by the protein framework. Moreover, the 65-type heme center of sulfite oxidase is six-coordinate with axial histidine ligands from the protein and hence cannot participate in atom transfer reactions. 

Fe(III) states of sulfite oxidase itself. Complexes 53 and 54 are the primitive precursors to incorporating the chemistry of Scheme 7 (Section V.E) with electron transfer to an Fe(III) porphyrin center as a model for the overall reaction of sulfite oxidase shown in Fig. 16. 

The second category includes enzymes that typically catalyze proper oxygen atom transfer reactions to or from an available electron lone pair of a substrate, and can be further subdivided into two families. The first family includes sulfite oxidase and assimilatory nitrate reductase, the physiological functions of which are to reduce nitrate to nitrite in the first stage of its reduction to ammonia for use by the plant cell. The second family comprises bacterial enzymes such as dimethylsulfoxide... 

Finally, and perhaps most importantly, for nitrate reductase and sulfite oxidase, we have to inquire about the number of electrons transferred from the enzyme to the substrate, or vice versa. This brings us to the question of the valence states between which molybdenum in the enzymes cycles in the turnover processes. For both of these enzymes, the overall reaction is a two-electron process, as is the xanthine oxidase reaction. For xanthine oxidase, the evidence (Olson et al, 1974) favors xanthine reducing Mo(VI) directly to... 

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