Alkaline State of the Domain-Swapped Dimer of Human Cytochrome : A Conformational Switch for Apoptotic Peroxidase Activity.

A 2.08 Å structure of an alkaline conformer of the domain-swapped dimer of K72A human cytochrome (Cyt) crystallized at pH 9.9 is presented. In the structure, Lys79 is ligated to the heme. All other domain-swapped dimer structures of Cyt have water bound to this coordination site. Part of Ω-loop D (residues 70-85) forms a flexible linker between the subunits in other Cyt domain-swapped dimer structures but instead converts to a helix in the alkaline conformer of the dimer combining with the C-terminal helix to form two 26-residue helices that bracket both sides of the dimer. The alkaline transition of the K72A human dimer monitored at both 625 nm (high spin heme) and 695 nm (Met80 ligation) yields midpoint pH values of 6.6 and 7.6, respectively, showing that the Met80 → Lys79 and high spin to low spin transitions are distinct. The dimer peroxidase activity increases rapidly below pH 7, suggesting that population of the high spin form of the heme is what promotes peroxidase activity. Comparison of the structures of the alkaline dimer and the neutral pH dimer shows that the neutral pH conformer has a better electrostatic surface for binding to a cardiolipin-containing membrane and provides better access for small molecules to the heme iron. Given that the pH of mitochondrial cristae ranges from 6.9 to 7.2, the alkaline transition of the Cyt dimer could provide a conformational switch to tune the peroxidase activity of Cyt that oxygenates cardiolipin in the early stages of apoptosis.