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Regulation of energy transduction and electron transfer in cytochrome c oxidase by adenine nucleotides
B Kadenbach1, J Napiwotzki, V Frank
1Fachbereich Chemie, Philipps-Universität, Marburg, Germany.
Insights
Bovine heart cytochrome c oxidase has multiple ATP/ADP binding sites that modulate enzyme activity and proton stoichiometry. This suggests a role for specific subunits in mammalian thermogenesis, differing from bacterial and fish enzymes.
Area of Science:
- Biochemistry
- Enzyme kinetics
- Mitochondrial respiration
Background:
- Cytochrome c oxidase (CcO) is a key enzyme in the electron transport chain, crucial for cellular respiration and ATP synthesis.
- The enzyme's activity and proton stoichiometry (H+/e-) are influenced by regulatory molecules like ATP and ADP.
- Variations in CcO structure and function across species are linked to different metabolic demands, including thermogenesis.
Purpose of the Study:
- To investigate the role of ATP/ADP binding sites in bovine heart cytochrome c oxidase.
- To elucidate the functional significance of different CcO subunit isoforms, particularly subunit VIa, in regulating enzyme activity and proton stoichiometry.
- To explore the potential link between CcO subunit composition and thermogenesis in mammals.
Main Methods:
- Biochemical characterization of bovine heart cytochrome c oxidase.
- Analysis of ATP/ADP binding sites and their impact on enzyme kinetics.
- Comparative analysis of CcO structure and function across different species (mammals, bacteria, fish).
Main Results:
- Bovine heart CcO exhibits seven high-affinity ATP/ADP binding sites and three ADP-specific sites.
- A specific ATP/ADP binding site on subunit VIaH modulates H+/e- stoichiometry from 0.5 to 1.0 upon ATP to ADP exchange.
- Other binding sites on subunit IV influence cytochrome c affinity and respiratory activity at high ATP/ADP ratios.
Conclusions:
- The distinct H+/e- stoichiometry of mammalian CcO, potentially involving subunit VIaL (0.5 H+/e-), may be linked to thermogenesis, contrasting with bacterial and ectotherm enzymes (1.0 H+/e-).
- The absence of subunit VIa isoforms in fish CcO supports the hypothesis of specialized roles in mammalian thermoregulation.
- ATP/ADP binding sites on CcO subunits play critical regulatory roles in cellular respiration and energy metabolism.
Abstract:
Cytochrome c oxidase from bovine heart contains seven high-affinity binding sites for ATP or ADP and three additional only for ADP. One binding site for ATP or ADP, located at the matrix-oriented domain of the heart-type subunit VIaH, increases the H+/e- stoichiometry of the enzyme from heart or skeletal muscle from 0.5 to 1.0 when bound ATP is exchanged by ADP. Two further binding sites for ATP or ADP, located at the cytosolic and the matrix domain of subunit IV, increases the K(M) for cytochrome c and inhibit the respiratory activity at high ATP/ADP ratios, respectively. We propose that thermogenesis in mammals is related to subunit VIaL of cytochrome c oxidase with a H+/e- stoichiometry of 0.5 compared to 1.0 in the enzyme from bacteria or ectotherm animals. This hypothesis is supported by the lack of subunit VIa isoforms in cytochrome c oxidase from fish.