An Atypical Cytochromebin the Colorless AlgaPolytomellaspp.: The High PotentialbHHeme Exhibits a Double Transition in the α-Peak of Its Absorption Spectrum

Polytomellaspp. is a colorless alga of the family Chlamydomonadaceae that lacks chloroplasts and cell wall. A highly active ubiquinol-cytochromecoxidoreductase (bc1complex), sensitive to antimycin and myxothiazol, has been purified and characterized from this alga (Gutiérrez-Cirloset al.,1994,J. Biol. Chem.269, 9147–9154). Both in mitochondrial membranes and in the isolated complex, the visible spectrum of cytochromebfromPolytomellaspp. exhibits an atypical α-band with a maximum at 567 nm. This maximum is shifted 3–4 nm to the red when compared withb-type cytochromes from other organisms. Analysis of thebhemes of thebc1complex by high performance liquid chromatography revealed no differences in the retention time and in the absorption spectra of theb-type hemes fromPolytomellaspp. and hemin, indicating that the prosthetic group in this alga is protoheme and thus ruling out the possibility that the red-shift could be due to different chemical substitutions in the porphyrin rings of thebLorbHhemes. The twobhemes were characterized by electrochemical redox titration; at pH 7.8–8.0, the midpoint potential forbLwas −143 mV and forbH+25 mV. The spectra of the twob-type hemes were recorded in the presence of different reductants, at selected electrochemical potentials, and in the presence of antimycin A, to distinguish between the contribution ofbLandbHto the visible spectrum. Both hemesbLandbHof the algal cytochromebcontribute to the observed bathochromic absorption maximum in the α-band of the spectrum. The data also show that the low potentialbLheme fromPolytomellaspp. is spectroscopically similar to that of other organisms, with two transitions in the α-peak at 558.7 and 568.4 nm. The high-potential hemebHalso exhibits a spectrum with two transitions at 557.2 and 568.9 nm, which surprisingly differs from the spectra of cytochromebHof mammals, plants, yeasts, and bacteria, which all exhibit a single transition centered around 560 nm.