Electron transfer between pyridine nucleotides and disulfide compounds is catalyzed by three flavoproteins which are well characterized. Lipoamide dehydrogenase reoxidizes reduced lipoamide (lip-(SH)2) by NAD+. Glutathione reductase catalyzes reduction of glutathione (GSSG) by NADPH. Thioredoxin reductase catalyzes the reduction of oxidized thioredoxin (thioredoxin-S2) by NADPH. These three enzymes resemble each other structurally and mechanistically. They have, in addition to FAD, a redox active disulfide which is functional in catalysis. The flavoproteins consist of two identical or near identical polypeptide chains, each with a functional cystine residue, and a molecule of FAD which is noncovalently linked.
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Arne Holmgren (1980) studied this question.