Figure 4. Production of O₂^•− by complex I.

The cartoon of complex I is a chimaera modelled on the hydrophobic arm of Yarrowia lipolytica obtained by electron microscopy [132] and the crystal structure of the hydrophilic arm from Thermus thermophilus [82]. The location of the FMN and the FeS centres in the water-soluble arm are indicated, along with the putative CoQ-binding site. In mode 1, there is extensive O₂^•− production from the FMN in response to a reduced NADH pool. In mode 2, a high Δp and a reduced CoQ pool lead to RET and a high flux of O₂^•− from the complex. The site of this O₂^•− production is uncertain, hence the question mark, but may be associated with the CoQ-binding site(s).