Heme A of Cytochrome c Oxidase

Heme A, isolated from bovine heart muscle by procedures which include extractions into pyridine/ chloroform and two-phase, liquid-liquid chromatography on Celite, has been converted to several derivatives. Examination of the proton nuclear magnetic resonance (PMR) spectra and other properties of these derivatives reveals heme A to be the iron complex of %formyl-6,7-bis(2”-hydroxycarbonylethyl)-2-(l’-hydroxy-5’,9’,13’-trimethyl-4’,8’,12’-truns,truns-tetradecatrienyl)-1,3,5-trimethyl-4-vinylporphin. Substituents at the 2, 4, and 8 positions are replaced by hydrogen in a resorcinol melt to give cytodeuteroporphin (&demethyldeuteroporphyrin IX). Treatment of heme A with ethanol/sulfuric acid can yield a monoethyl ether, diethyl ester. In methanol/sulfuric acid, dimethyl esters of monomethyl and dimethyl ethers and a dimethyl acetal have been obtained. The ethers can be introduced first at the 1’ and second at the 13’ positions of the 2-alkyl group without modification of the double bonds at 4’ and 8’ and 4-vinyl. Acidic conditions, with or without ether formation at 2-l’, can lead to an optically inactive product as expected if racemization at the 1’-carbon occurred through loss of the l’-OH upon formation of a carbonium ion. Treatment of heme A with pyridine/acetic anhydride under mild conditions gives the 2-1’-acetoxy derivative. Upon chromatography of heme A esters on alumina, antiferromagnetic hemin A dimers with FeOFe linkages are formed. Diamagnetic solutions of iron(I1) species suitable for high resolution PMR and electronic spectral observations are conveniently obtained by reduction of hemins in dry pyridine with stannous chloride. Such reductions appear first order in hemin with rates faster for ~-0x0 dimers than for chloromonomers and for hemin A than for protohemin (hemin B). Hemes B, C, and S and structural analogs have also been prepared for comparison with heme A derivatives. These comparisons provide a basis for the evaluation of the effects of structural differences on the properties of the natural hemes. Heme A differs from heme B in having formyl and long alkyl groups in place of methyl and vinyl groups, respectively. The fact that the formyl is much more strongly electron-withdrawing than is the vinyl significantly alters redox and ligand binding reactions as well as interactions of the prophyrin ?r system (e.g. with protein, lipid, or copper). Although the long alkyl group may only serve as a lipophilic anchor to cytochrome c oxidase, it may well participate in electron transfer or energy conservation processes, with or without interactions with copper. The farnesylethyl group can assume conformations that permit facile electron transfer over long distances via overlap of x electrons of double bonds and porphyrin; subtle changes in conformation could turn such conduction on or off. Similarly unsaturated structures, e.g. the polyisoprenoid chains of ubiquinones, provide a generally useful means for conformationally controlled electron transfer over long distances in mitochondria, chloroplasts, and other membranous systems.