Structure and mechanism of a eukaryotic transmembrane ascorbate-dependent oxidoreductase

Significance Vitamin C (also known as ascorbate), an essential nutrient for humans, plays an important role in protection against oxidative stress. The ascorbate-dependent oxidoreductase cytochrome b 561 (Cyt b 561 ) is a family of highly conserved, multipass transmembrane enzymes found only in eukaryotes. Cyt b 561 plays a key role in ascorbate recycling and many other important physiological processes, such as iron absorption. The atomic structure and functional mechanism of Cyt b 561 remain unknown. In this study, we report the high-resolution crystal structures of Cyt b 561 in both ascorbate-free and ascorbate-bound states. Our structural and biochemical analyses identify a general functional mechanism for the Cyt b 561 family.