Flavoprotein disulfide reductases: advances in chemistry and function.

The flavoprotein disulfide reductases represent a family of enzymes that show high sequence and structural homology. They catalyze the pyridine-nucleotide-dependent reduction of a variety of substrates, including disulfide-bonded substrates (lipoamide dehydrogenase, glutathione reductase and functional homologues, thioredoxin reductase, and alkylhydroperoxide reductase), mercuric ion (mercuric ion reductase), hydrogen peroxide (NADH peroxidase), molecular oxygen (NADH oxidase), and the reductive cleavage of a carbonyl-activated carbon-sulfur bond followed by carboxylation (2-ketopropyl-coenzyme-M carboxylase?oxidoreductase). They use at least one nonflavin redox center to transfer electrons from reduced pyridine nucleotide to their substrate through flavin adenine dinucleotide. The nature of the nonflavin redox center located adjacent to the flavin varies and three types have been identified: an enzymic disulfide (most commonly), an enzymic cysteine sulfenic acid (NADH peroxidase and NADH oxidase), and a mixed Cys-S-S-CoA disulfide (coenzyme A disulfide reductase). Selection of the particular nonflavin redox center and utilization of a second, or even a third, nonflavin redox center in some cases presumably represents the most efficient strategy for reduction of the individual substrate.