Redox active disulfide

The thioredoxin domain (see Figure 2.7) has a central (3 sheet surrounded by a helices. The active part of the molecule is a Pa(3 unit comprising p strands 2 and 3 joined by a helix 2. The redox-active disulfide bridge is at the amino end of this a helix and is formed by a Cys-X-X-Cys motif where X is any residue in DsbA, in thioredoxin, and in other members of this family of redox-active proteins. The a-helical domain of DsbA is positioned so that this disulfide bridge is at the center of a relatively extensive hydrophobic protein surface. Since disulfide bonds in proteins are usually buried in a hydrophobic environment, this hydrophobic surface in DsbA could provide an interaction area for exposed hydrophobic patches on partially folded protein substrates. 

Mammalian thioredoxin reductases are a family of selenium-containing pyridine nucleotide-disulfide oxidoreductases. These enzymes catalyze NADPH-dependent reduction of the redox protein thioredoxin (Trx), which contains a redox-active disulfide and dithiol group and by itself may function as an efficient cytosolic antioxidant [77]. One of the functions of Trx/ thioredoxin reductase system is the NADPH-catalyzed reduction of protein disulfide [78] ... 

This EAD-dependent enzyme [EC 1.6.4.2] catalyzes the reaction of NADPH with glutathione disulfide to produce NADP+ and two glutathione molecules. The enzyme activity is dependent on a redox-active disulfide group in each of the active sites. 

Fig. 7.4 Reactions associated with the thioredoxin system. Thioredoxin is a redox-regulating protein with a redox-active disulfide/dithiol within the conserved active site sequence -Cys-Gly-pro-Cys-. Thioredoxin reductase, a 55 kDa flavoprotein that catalyzes the NADPH-dependent reduction of thioredoxin (1) and thioredoxin oxidase (2), a flavin-dependent sulfhydry 1 oxidase that catalyzes the oxidative protein folding with the generation of disulfides...

Both MAO-A and MAO-B contain a redox-active disulfide at the catalytic center. The results imply that MAO may be a novel type of disulfide oxidoreductase and may open the way to characterizing the catalytic and chemical mechanism of the enzyme. 

Thioredoxin reductase (TrxR) acts in the reverse direction and shows a somewhat different mechanism, which is dependent on the protein source. Prokaryotes, plants, and lower eukaryotes contain a 35-kDa TrxR with one redox-active disulfide. Higher eukaryotes produce a 55-kDa TrxR that has either an additional redox-active disulfide or a selenenylsulfide in the flexible C-terminal part of the neighboring subunit (15). In low Mr TrxR, a large conformational change is required to move reducing equivalents from the apolar flavin site to the surface of the protein where the thioredoxin redox partner binds. In high Mr TrxR, this transfer is mediated by the second disulfide or selenylsulfide, and the conformational changes required are comparatively small (17). 

Mercuric ion reductase, the FAD-containing merA gene product, has several pairs of conserved cysteines. From site-specific mutagenesis studies, cysteine residues in the sequence 134-Thr-Cys-Val-Asn-Val-Gly-Cys-140 are known to comprise a redox-active disulfide group in addition, a redox-inactive pair of cysteines near the carboxyl terminus is also required for the selective reduction of Hg(II). Exactly how the enzyme achieves the chemistry shown in Equation... 

Much less is known about the final step of the pathway, specifically the Bi2-dependent conversion of o( into queuosine. In his review of queuosine biosynthesis, Iwata-Reuyl draws a parallel between the reduction of o( and the reaction performed by ribonucleotide reductases and proposes a mechanism involving a thiyl radical and redox-active disulfide (Figure 36). 

Figure 36 Proposed mechanism for oQ reduction to queuosine is based on an analogy drawn to ribonucleotide reductases and involves a thiyi radical and a redox active disulfide. This mechanism was proposed by Iwata-Reuyl. Reproduced from D. Iwata-Reuyl, Bioorg. Chem. 2003, 31, 24-43.

The high M,. TxrRs use a different strategy to accomplish the same chemistry. The enzymes from human and Drosophila melanogaster are the best-studied examples. In addition to the usual enzymatic disulfide near the flavin, these enzymes have a second redox active disulfide or a selenosulfide pair at their C-terminus. In the... 

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