Febs Letters Volume 111, Number 1-2./Volume 112, Number 1-2. [antikvár]

Volume 107, number 636 FEBS LETTERS February 1980 Hypothesis A MODEL FOR THE CYTOCHROME b DIMER OF THE UBIQUINOL : CYTOCHROME c OXIDOREDUCTASE AS A PROTON TRANSLOCATOR G. VON JAGOW and W. D. ENGEL Institut für Physikalische Biochemie der Universität München, Goethestrasse 33, 8000 München 2, FR G Received 20 December 1979 1. Introduction Complex III of the respiratory chain is a unique multiprotein complex consisting of 9 polypeptide subunits [1]. Four of them possess electron-transferring redox centers: the two cytochromes 6, the Rieske Fe/S-protein, and cytochrome c^ [2], Electrons are transferred from ubiquinol to cytochrome c, associated with a proton translocation across the mitochondrial inner membrane [3]. This segment of the electron-transfer chain is also termed the energy conservation site II. The mechanism of the electrogenic H* transport is stiU under investigation. As a hypothetic mechanism (a) a ubiquinone-mediated hydrogen transport via a so-called protonmotive redox loop is proposed [4,5]; as an alternative (b) a proton transport by certain, as yet undefined protein components by means of a redox-Iinked electrogenic proton pump is suggested [6,7]. The cytochrome b dimer proton translocator to be described follows mechanism (b) [8]. At site II, 4 h72 e", but only 2 charges/2 e", are transferred across the membrane [9,10]. The hydrogen carrier ubiquinone, in its classical arrangement, releases 2 h72 e" to the outside upon oxidation; the remaining 2 H'^ and 2 charges may be translocated by a proton pump. The existence of a heme-linked proton pump seems to be established at site III [11]. An analogous mechanism may function at site II. A partial model for such a mechanism has already been proposed [12]. show Aij. 30 000 in SDS gel electrophoresis, but they form a dimer withTl/j 60 000 when mild detergents are used for their solubihzation [13,14]. Attempts to find a difference between the two monomers have failed so far [15]. Protein—chemical studies, for instance, partial amino acid sequence analysis, gave no evidence for the existence of two heterogeneous monomers [14]. Accordingly, genetic studies revealed the existence of only one single mosaic structural gene coding for a cytochrome b apoprotein of -30 000 [16,17]. In contrast to these findings, the existence of two functionally different cytochromes b in complex III has generally been postulated on the basis of spectro-photometric, kinetic and potentiometric studies carried out with cytochrome integrated in the mitochondrial membrane and on the isolated cytochrome 6c 1 complex [2]. About half of the cytochrome b dimer has an a-absorbance band with a 562 nm, and a half-reduction potential oiE^-j = +50 mV; the other half has an a-absorbance band with a 566 nm, and a half-reduction potential of L'„, 7 = -50 mV. Cytochrome 6-566 can be reduced by succinate only if a high electrochemical proton gradient is adjusted. The phenomenon has to be attributed to the numerous energy-linked reduction reactions of cytochrome 6 [18]. 3. Protonationrdeprotonation reactions of cytochromes 6 2. Types of cytochrome 6 The two cytochromes 6 of complex III are hydrophobic, integral membrane proteins, both of which An analysis of the pubHshed redox titration experiments reveals the sequence of protonation and reduction of cytochromes 6-562 and 6-566. The two possible sequences, first protonation and then reduc- Elsevier !North-Holland Biomedical Press 1