Thioredoxin-thioredoxin reductase - a system that has come of age

‹12 000 protein having a redoxactive disulfide; thioredoxin was the name assigned to thisprotein [4]. It was shown that thioredoxin was reduced bythioredoxin reductase in a NADPH-dependent reaction and thatin its dithiol form, thioredoxin served as the reductant ofribonucleotides via a ribonucleotide reductase. It was suggestedthat thioredoxin was equivalent to enzyme II in the methioninesulfoxide-reducing system and to fraction C in the sulfate-reducing system [4]. Thioredoxin reductase was shown to be adimeric flavoenzyme containing a redox active disulfide and aFAD in each subunit [3]. The intense study of the variousphysiological functions of thioredoxin and thioredoxin reduc-tase is the subject of the first Minireview [5].The thioredoxin–thioredoxin reductase system is verybroadly distributed and the two proteins have been isolatedfrom many species. Thioredoxins are similar to one another instructure and the conformation of the single domain is referredto as the thioredoxin fold with the redox-active disulfide form-ing a protrusion about 35 residues from the N-terminus.Thioredoxin reductases, on the other hand, fall into two classesas first noticed by Holmgren and his colleagues [6]. The low M