Febs Letters Volume 135, Number 1-2./Volume 136, Number 1-2. [antikvár]

Volume 135, number 1 FEBS LETTERS November 1981 Review Letter ON THE FUNCTION OF MULTIPLE SUBUNITS OF CYTOCHROME c OXIDASE FROM HIGHER EUKARYOTES Bernliard KADENBACH and Peter MERLE Biochemie, Fachbereich Chemie der Philipps-Universität, Hans-Meerwein-Straße, S5S0 Marburg/Lahn, FRG Received 7 September 1981 1. Introduction Cytoclirome c oxidase (ferrocytochrome c : oxygen oxidoreductase, EC 1.9.3.1) complex IV of the respiratory chain of mitochondria [1], catalyzes the transfer of 4 electrons from reduced cytochrome c via 4 redox centers (2 heme a and 2 copper atoms) to dioxygen. The reaction occurs without accumulation of free toxic intermediates such as H2O2 or the superoxide radical Oj, and was identified as the third coupling site in mitochondria for the formation of ATP [2]. According to the chemiosmotic hypothesis, the electrochemical gradient of protons is the driving force for ATP formation and for ion and metabolite transport across the mitochondrial membrane [3]. In the original chemiosmotic hypothesis [3] and from later experiments [4] Mitchell denied a proton translocation through cytochrome c oxidase, but depicts it as the electron-carrying arm of the last loop in the respiratory chain. Original experiments with mitochondria by Wikström [5,6] and subsequent work with isolated cytochrome c oxidase, reconstituted in Uposomes in different laboratories [7-10], have now estahUshed that cytochrome c oxidase pumps protons in addition to the formation of a membrane potential and concomitant alkalinisation of the matrix side, due to the transfer of electrons from cytochrome c to oxygen. The data of Wikström [5-7] and of Carafoli and coworkers [10,11] are consistent with an H7e" stoichiometry of 2. The reaction can be summarized asfoUows [10]: 2cytc^^ + 4H,^ + l/2 0^ 2Ho +H20i >2 cytcj The molecular mechanism of electron transfer from cytochrome c to oxygen has been intensively studied by spectroscopic methods (review [12—16]). Although progress has been made, in particular due to EPR studies [16], concerning the electron transfer from cytochrome c via the 4 redox centers to oxygen, very Uttle is known on the arrangement and subunit location of the 4 metal centers within the enzyme complex. In addition almost nothing is known on the mechanism of coupling between electron transport and proton pumping, although some speculations have been presented [15]. This is in particular due to the compUcated structure and multisubunit composition of the enzyme complex. In the following, all protein components wliich are found tightly associated to the enzyme complex are denoted 'subunits' in default of a proven specific function which could be catalyticaUy or regulatory. 2. Subunit composition The protem subunit composition of cytochrome c oxidase from Neurospora crassa and from yeast was thoroughly investigated in connection with the identification of the mitochondrial or cytoplasmic site of synthesis of the various subunits. The enzyme from N. crassa was found to contain 8 [17,18], that from yeast 7 different polypeptide chains, designated 'subunits' [ 19-21 ]. The apparent A/^-values, estimated by SDS gel electrophoresis, were from 4600-40 000 [21]. All protein components seem to occur in the enzyme complex in stoichiometric amounts (1:1) as was deduced fiom the ['H] leucine radioactivity incorporated into the subunits in intact N. crassa cells and corrected for the leucine content of the subunits [17]. Published by Elsevier !North-Holland Biomedical Press 00145793/81/0000-0000/$02.75 ® 1981 Federation of European Biochemical Societies