SPECIFIC ARRANGEMENT OF 3 AMINO-ACID-RESIDUES FOR FLAVIN-BINDING BARREL STRUCTURES IN NADH-CYTOCHROME-B(5) REDUCTASE AND THE OTHER FLAVIN-DEPENDENT REDUCTASES

The structure of NADH-cytochrome b(5) reductase from pig liver microso mes has been refined to a crystallographic R factor of 0.223 at 2.4 An gstrom resolution. A structural comparison between the flavin-binding beta barrel domain of NADH-cytochrome b(5) reductase and those of the other flavin-dependent reductases, ferredoxin-NADP(+) reductase, phtha late dioxygenase reductase and nitrate reductase, indicated that the o verall barrel foldings are similar to each other and that the specific arrangement of three amino acid residues (Arg, Tyr and Ser/Thr) is us ually necessary for flavin-binding. These conserved residues overlap e ach other in their three-dimensional structures and stabilize the flav in-binding site in the four flavin-dependent reductases.