The X-ray structure of thioredox (Fig. 23-11) reveals that its redox-active disulfide
group is located on a molecular protrusion, making the ptotein the only known
example of a “male” enzyme.
Oxidized thioredoxin is, in turn, reduced in a reaction mediated by
thioredoxin reductase, which contains redox-active thiol groups and an FAD
prosthetic group. This enzyme is a homolog of glutathione reductase (Box 15-4)
and lipoamide dehydrogenase (Section 17-2B) and catalyzes a similar reaction:
the NADPH-mediated reduction of a substrate disulfide bond. NADPH therefore
serves as the terminal reducing agent in the ribonucleotide reductase-catalyzes
reduction of NDPs to dNDPs(Fig. 23-12).