The Sulfide Oxidation Unit (SOU), and limiting steps for sulfide oxidation. Complexes of the mitochondrial respiratory chain are numbered with roman numerals. Three essential blocks of reactions important for the control of sulfide oxidation ‘a, b, c’ are boxed: ‘a’ is the SOU, which oxidizes two molecules of sulfide, uses one dioxygen molecule, releases thiosulfate (H2S2O3) and reduces quinone ‘Q’. ‘b’ accepts electrons from quinone and drives them through Complexes III and IV to oxygen. The sequential intervention of ‘a’ and ‘b’ is necessary for sulfide oxidation to proceed. 1.5 molecules of dioxygen and two molecules of H2S are used: the stoichiometry is thus 1.5/2 = 0.75. Complex IV is the target of sulfide inhibition, which becomes significant at 10 μM and above, while the SOU could operate well below this 10 μM concentration. Block ‘c’ is constituted by the other enzymatic reactions, reducing quinone and is a competitor of SOU. Mitochondrial Complex V (not shown in the scheme) imposes collectively to a, b, c, the constraint of respiratory control matching the flux through the redox reactions (a, b, c) to values imposed by the ATP turnover rate.