ISOLATION AND CHARACTERIZATION OF CYTOCHROME-C550 FROM THE METHYLAMINE-OXIDIZING ELECTRON-TRANSPORT CHAIN OF THIOBACILLUS-VERSUTUS

The isolation and purification of cytochrome C550 from the methylamine-oxidizing electron-transport chain in Thiobacillus versutus is reported. The cytochrome is a single-heme-containing type I cytochrome c with a relative molecular mass of 16 +/- 1 kDa, an isoelectric point of 4.6 +/- 0.1, a midpoint potential of 272 +/- 3 mV at pH < 4 and 255 +/- 5 mV at pH = 7.0, and an axial coordination of the Fe by a methionine and a histidine. The midpoint potential decreases with increasing pH due to the deprotonation of a group tentatively identified as a propionate (pK(a) = 6.5 +/- 0.1 and 6.7 +/- 0.1 in the oxidized and reduced protein, respectively) and a change in the Fe coordination at pH > 10. The electron-self-exchange rate appears to depend strongly on the ionic strength of the solution and is relatively insensitive to changes in pH. At 313 K and pH 5.2 the electron-exchange rate amounts to 0.7 x 10(2) M-1 S-1 and 5.3 x 10(2) M-1 S-1 at I = 40mM and I = 200 mM, respectively. Amino acid composition and molar absorption coefficients at various wavelengths are reported. Resonances of heme protons and the epsilon-H3 group of the ligand methionine of the Fe have been identified in the H-1-NMR spectrum of the reduced as well as the oxidized cytochrome.