The causes of reduced proton-pumping efficiency in type B and C respiratory heme-copper oxidases,and in some mutated variants of type A |
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Authors: | Virve Rauhamäki Mårten Wikström |
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Institution: | Institute of Biotechnology, University of Helsinki, Viikinkaari 1, 00014 Helsinki, Finland |
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Abstract: | The heme-copper oxidases may be divided into three categories, A, B, and C, which include cytochrome c and quinol-oxidising enzymes. All three types are known to be proton pumps and are found in prokaryotes, whereas eukaryotes only contain A-type cytochrome c oxidase in their inner mitochondrial membrane. However, the bacterial B- and C-type enzymes have often been reported to pump protons with an H+/e- ratio of only one half of the unit stoichiometry in the A-type enzyme. We will show here that these observations are likely to be the result of difficulties with the measuring technique together with a higher sensitivity of the B- and C-type enzymes to the protonmotive force that opposes pumping. We find that under optimal conditions the H+/e- ratio is close to unity in all the three heme-copper oxidase subfamilies. A higher tendency for proton leak in the B- and C-type enzymes may result from less efficient gating of a proton pump mechanism that we suggest evolved before the so-called D-channel of proton transfer. There is also a discrepancy between results using whole bacterial cells vs. phospholipid vesicles inlaid with oxidase with respect to the observed proton pumping after modification of the D-channel residue asparagine-139 (Rhodobacter sphaeroides numbering) to aspartate in A-type cytochrome c oxidase. This discrepancy might also be explained by a higher sensitivity of proton pumping to protonmotive force in the mutated variant. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference. |
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Keywords: | Heme-copper oxidase Heme-copper oxygen reductase Cellular respiration Proton pumping |
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