Electron transfer in nitric-oxide synthase

Nitric oxide (NO) has many diverse biological functions as an important signaling and cytotoxic molecule in the cardiovascular, nervous, and immune systems. NO is generated from L-Arg via formation of N-G-hydroxyl-L-Arg as an intermediate by a family of enzymes termed nitric-oxide synthases (NOSs). NOS consists of two functional domains: one is an amino-terminal oxygenase domain that has a cytochrome-P450-like heme active site, and the other is a carboxy-terminal reductase domain that is similar to NADPH-cytochrome P450 reductase. In contrast to the well-known P450/NADPH-P450-reductase fusion protein, cytochrome P450BM3, however, all NOS isoforms are only active as homodimers and require the presence of both Ca2+/calmodulin and (6R)-5,6,7,8-tetrahydrobiopterin for electron transfer and catalysis. We summarize recent results focusing on electron transfer reaction within this novel enzyme and demonstrate differences between the NOS and P450 systems. (C) 2002 Elsevier Science B.V. All rights reserved.