Any enzyme system that includes cytochrome P450 protein or domain can be called a P450-containing system. P450 enzymes usually function as a terminal oxidase in multicomponent electron-transfer chains, called P450-containing monooxygenase systems, although self-sufficient, non-monooxygenase P450s have been also described. All known P450-containing monooxygenase systems share common structural and functional domain architecture. Apart from the cytochrome itself, these systems contain one or more fundamental redox domains: FAD-containing flavoprotein or domain, FMN domain, ferredoxin and cytochrome b5. These ubiquitous redox domains, in various combinations, are widely distributed in biological systems. FMN domain, ferredoxin or cytochrome b5 transfer electrons between the flavin reductase (protein or domain) and P450. While P450-containing systems are found throughout all kingdoms of life, some organisms lack one or more of these redox domains. FR/Fd/P450 systems Mitochondrial and some bacterial P450 systems employ soluble Fe2S2 ferredoxins (Fd) that act as single electron carriers between FAD-containing ferredoxin reductase (FR) and P450. In mitochondrial monooxygenase systems, adrenodoxin functions as a soluble electron carrier between NADPH:adrenodoxin reductase and several membrane-bound P450s (CYP11A, CYP11B, CYP27). In bacteria, putidaredoxin, terpredoxin, and rhodocoxin serve as electron carriers between corresponding NADH-dependent ferredoxin reductases and soluble P450s (CYP101, CYP108, CYP116). {| border="0" width="60%" cellpadding="3" cellspacing="1" style="text-align:center;" |----- | align="right" | NADH || → | putidaredoxin reductase | → | putidaredoxin | → | CYP101 | → || O2 |----- | align="right" | NADH || → | terpredoxin reductase | → | terpredoxin | → | CYP108 | → || O2 |----- | align="right" | NADH || → | rhodocoxin reductase | → | rhodocoxin | → | CYP116 | → || O2 |----- | align="right" | NADPH || → | Adrenodoxin reductase | → | Adrenodoxin | → | CYP11A1 | → || O2 |} The general scheme of electron flow in the P450 systems containing adrenodoxin-type ferredoxins is: {| border="0" width="60%" cellpadding="3" cellspacing="1" style="text-align:center;" |----- | align="right" | NAD(P)H || → | bgcolor="orange" | FAD | → | bgcolor="lightgreen" | Fe2S2 | → | bgcolor="lightblue" | P450 | → || O2 |} CPR/P450 systems Eukaryotic microsomal P450 enzymes and some bacterial P450s receive electrons from a FAD- and FMN-containing enzyme known as cytochrome P450 reductase (CPR; ). Microsomal CPR is membrane-bound protein that interacts with different P450s. In Bacillus megaterium and Bacillus subtilis, CPR is a C-terminal domain of CYP102, a single polypeptide self-sufficient soluble P450 system (P450 is an N-terminal domain). The general scheme