of electron flow in the CPR/P450 system is: {| border="0" width="60%" cellpadding="3" cellspacing="1" style="text-align:center;" |----- | align="right" | NADPH || → | bgcolor="orange" | FAD | → | bgcolor="yellow" | FMN | → | bgcolor="lightblue" | P450 | → || O2 |} CBR/b5/P450 systems The ubiquitous electron-transport protein cytochrome b5 can serve as an effector (activator or inhibitor) of P450s. It was hypothesized that cytochrome b5 is involved in the transfer of the second electron to P450, either from CPR or from NADH:cytochrome b5 reductase (CBR; ): {| border="0" width="60%" cellpadding="3" cellspacing="1" style="text-align:center;" |----- | align="right" | NADPH || → | CPR || → | cyt b5 | → || P450 | → || O2 |----- | align="right" | NADH || → | CBR || → | cyt b5 | → || P450 | → || O2 |} The ability of the CBR/cytochrome b5 system to support P450 catalysis has been demonstrated in vitro using purified CBR and cytochrome b5 from Saccharomyces cerevisiae and CYP51 enzyme from Candida albicans. In this system, both the first and second electrons are donated by CBR. {| border="0" width="60%" cellpadding="3" cellspacing="1" style="text-align:center;" |----- | align="right" | NAD(P)H | → | bgcolor="orange" |FAD | → | bgcolor="lightpink" | b5 | → | bgcolor="lightblue" | P450 | → | O2 |} FMN/Fd/P450 systems An unusual one-component P450 system was originally found in Rhodococcus sp. NCIMB 9784 (CYP116B2). In this system, the N-terminal P450 domain is fused to the reductase domain that shows sequence similarity to phthalate dioxygenase reductase and consists, in its turn, of FMN-binding domain and C-terminal plant-type ferredoxin domain. Similar systems have been identified in the heavy-metal-tolerant bacterium Ralstonia metallidurans (CYP116A1) and in several species of Burkolderia. The general scheme of electron flow in this system appears to be: {| border="0" width="60%" cellpadding="3" cellspacing="1" style="text-align:center;" |----- | align="right" | NADH || → | bgcolor="yellow" | FMN | → | bgcolor="lightgreen" | Fe2S2 | → | bgcolor="lightblue" | P450 | → || O2 |} P450-only systems Nitric oxide reductase (P450nor) is a P450 enzyme involved in denitrification in several fungal species. The best-characterized P450nor is CYP55A1 from Fusarium oxysporum. This enzyme does not have monooxygenase activity but is able to reduce nitric oxide (NO·) to form nitrous oxide (N2O) directly using NAD(P)H as electron donor: {| border="0" width="40%" cellpadding="3" cellspacing="1" style="text-align:center;" |----- | align="right" | NAD(P)H || → | bgcolor="lightblue" | P450 | → ||