Vitamin C, also known as ascorbate, is required in numerous essential metabolic reactions in eukaryotes. The eukaryotic ascorbate-dependent oxidoreductase cytochrome b 561 (Cyt b 561 ), a family of highly conserved transmembrane enzymes, plays an important role in ascorbate recycling and iron absorption. Although Cyt b 561 was identified four decades ago, its atomic structure and functional mechanism remain largely unknown. Here, we report the high-resolution crystal structures of cytochrome b 561 from Arabidopsis thaliana in both substrate-free and substratebound states. Cyt b 561 forms a homodimer, with each protomer consisting of six transmembrane helices and two heme groups. The negatively charged substrate ascorbate, or monodehydroascorbate, is enclosed in a positively charged pocket on either side of the membrane. Two highly conserved amino acids, Lys 81 and His
106, play an essential role in substrate recognition and catalysis. Our structural and biochemical analyses allow the proposition of a general electron transfer mechanism for members of the Cyt b 561 family. V itamin C, an essential nutrient for humans (1), is important for the synthesis of collagen (2), carnitine (3), and the neurotransmitter norepinephrine (4). Vitamin C also plays an important role in protection against oxidative stress (5). Oxidation of ascorbate results in sequential production of monodehydroascorbate and dehydroascorbate through loss of one and two electrons, respectively (6). Ascorbate serves as an electron donor for various enzymes, such as prolyl and lysyl hydroxylase, dopamine β-hydroxylase, ascorbate peroxidase, and cytochrome b 561 (Cyt b 561 ). Cyt b 561 , initially identified in the chromaffin granules of bovine adrenal medullae about 40 y ago (7,8), is a transmembrane ascorbate-dependent oxidoreductase (9-13) that plays a key role in ascorbate recycling and other physiological processes, such as iron absorption (14). To our knowledge, Cyt b 561 is the only membrane-embedded oxidoreductase that relies on ascorbate as the electron donor.Homologs resides in the chromaffin vesicle membrane and transfers electrons from cytoplasmic ascorbate to the intravesicular monodehydroascorbate radical for the regeneration of ascorbate (9, 13, 16), which is a substrate of dopamine β-hydroxylase for the synthesis of neurotransmitter norepinephrine. DCyt b 561 is present in the duodenal mucosa cell membrane, where it relays electrons from cytoplasmic ascorbate to ferric-chelate in the intestinal lumen, yielding soluble ferrous ion for absorption via a Fe 2+ -transporter (17-20). Expression levels of DCyt b 561 in the duodenal mucosa cell membrane are closely associated with iron metabolism disorders, such as chronic anemia and iron-deficiency anemia (19). Similar to CGCyt b 561 , ZmCyt b 561 uses ascorbate and the monodehydroascorbate radical as the physiological electron donor and acceptor, respectively (11,21,22).Despite rigorous investigation, there is no detailed structural information for any member of the Cyt b 561 family. Cons...