Sulfur participation, external

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

By bacterial disproportionation H S and are produced concurrently without participation of an external electron acceptor or donor (Bak and Pfennig 1987 Thamdrup et al. 1993). The biogeo-chemical transformations of sulfur in marine sediments are closely coupled to the cycles of iron and manganese. Sulfate, iron oxides, and manganese oxides all serve as electron acceptors in the respiratory degradation of organic matter. As there are also non-enzymatic reactions between iron, manganese and H S within the sediment, the quantification of dissimilatoiy, heterotrophic Fe and Mn reduction is particularly difficult. 

In summary, zinc is most suited to participate in formation of the sulfurating complex. Coordination of external ligands (ROO—,R 2NH ) of the zinc atom causes the bonding between XS—Sjc... and. .. Sy—SX groups to weaken, thereby increasing the contribution of the polar canonical form ... 

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