Electrons deriving from intermediary metabolism are funneled to oxygen through the respiratory chain in a process coupled to H +ejection on the redox H + pumps. It is today universally accepted that the initial event of energy conservation is charge separation at the inner mitochondrial membrane. The 1960s and early 1970s witnessed a heated debate over chemiosmotic energy coupling ( 226), which was largely centered over the very existence of a mitochondrial membrane potential. It will now be useful to summarise the basis of the chemiosmotic coupling hypothesis in the form of four essential postulates for, these postulates can be used, on the one hand, for the further development of the theory of chemiosmotic coupling, and on the other hand, as the target for critical experiments designed to show that the chemiosmotic hypothesis may be untenable.ġ) The membrane-located ATPase systems of mitochondria and chloroplasts are hydro-dehydration systems with terminal specificities for water and ATP and their normal function is to couple reversibly the translocation of protons across the membrane to the flow of anhydro-bond equivalents between water and the couple ATP/(ADP − P i).Ģ) The membrane-located oxido-reduction chain systems of mitochondria and chloroplasts catalyse the flow of reducing equivalents, such as hydrogen groups and electron pairs, between substrates of different oxido-reduction potential and their normal function is to couple reversibly the translocation of protons across the membrane to the flow of reducing equivalents during oxido-reduction.ģ) There are present in the membrane of mitochondria and chloroplasts substrate-specific exchange-diffusion carrier systems that permit the effective reversible transmembrane exchange of anions against OH − and of cations against H + and the normal function of these systems is to regulate the pH and osmotic differential across the membrane, and to permit entry and exit of essential metabolites (e.g., substrates and phosphate acceptor) without collapse of the membrane potential.Ĥ) The systems of postulates 1, 2, and 3 are located in a specialised coupling membrane which has a low permeability to protons and to anions and cations generally. In his “Summary of the basic postulates” Mitchell stated ( 225) The history of bioenergetics came to a turning point when the late Peter Mitchell proposed his chemiosmotic hypothesis of energy conservation ( 223, 225). Mechanism of Energy Conservation and Cation Transport Topics that are not treated in the review (mitochondrial Ca 2+-binding proteins, Mg 2+transport, electrophysiology, and mitochondrial involvement in cell death) are briefly covered in section v, where the interested reader can find essential bibliographic indications.Ī. This includes the mitochondrial permeability transition and its potential role in Ca 2+ homeostasis. I then cover in more detail specific transport pathways for cations and discuss open problems about their nature and physiological function. This should provide the general reader with the basic elements needed to understand earlier mitochondrial literature and current problems associated with mitochondrial transport of cations. In this review, which is limited to mammalian mitochondria, I provide a selective history of how studies of mitochondrial cation transport (K +, Na +, and Ca 2+) developed in relation to the major themes of energy conservation. Given the view, prevailing well into the 1980s, that mitochondria did not possess cation channels, it is not too surprising that research in this area did not yield results comparable to those obtained in the general field of membrane transport. As a result, research on mitochondrial cation transport was shaped by the debate on chemiosmosis. The more the debate on energy conservation became focused on the chemiosmotic hypothesis, the more studies of mitochondrial ion transport tended to become tests of the predictions of this theory, in particular about the existence of a membrane potential across the inner membrane. Studies of ion transport were mostly carried out in the same laboratories involved in clarifying the mechanisms of energy conservation in mitochondria and chloroplasts, and the two fields evolved in parallel. The explanation for this state of affairs is largely historical. Despite an enormous amount of literature and the importance of the problem, very little information is available about the structural features of mitochondrial cation channels and exchangers, whereas a vast amount of information is available about the functional properties of these mitochondrial transport systems.
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