Schematic representation of the heme biosynthetic pathway and its connection with the electron transport chain of the inner mitochondrial membrane. (1) 5-Aminolevulinic acid synthase; (2) 5-aminolevulinic acid dehydratase; (3) porphobilinogen deaminase; (4) uroporphyrinogen III synthase; (5) uroporphyrinogen III decarboxylase; (6) coproporphyrinogen III oxidase; (7) protoporphyrinogen III oxidase. Complex I, NADH dehydrogenase; complex II, succinate dehydrogenase; complex III, b/c1-complex = ubiquinol cytochrome c reductase; complex IV, cytochrome c oxidase; Q, coenzyme Q = ubiquinone; c, cytochrome c. The broken line (grey) represents the outer mitochondrial membrane. In accordance with the point mutations identified in mtDNA-encoded subunit I of cytochrome c oxidase, the reduction of Fe³⁺ to Fe²⁺ has been placed into complex IV of the respiratory chain.