Energy Transfer in Solute Carrier Transport
For a discussion of the problem of energy transfer to solute transport through biomembranes, it is useful to divide up this transfer into (a) the internal energy transfer used for the catalytic activation of the translocation process and (b) the external energy transfer required for raising the chemical potential of the solute after translocation. The internal energy transfer is always required for carrier catalysis whereas the external energy transfer is optional and superimposed on the catalytic energy relations. The external energy transfer may strongly influence also the internal energy transfer relations.
In analyzing the energy relationship in carrier-catalyzed transport, it is very useful to compare the carrier with enzyme catalysis. In both cases a protein with specific binding in the recognition sites is involved and the substrate-protein interaction represents a key element in understanding the catalytic function of the protein. The different nature of the catalytic reactions, in enzymes a chemical transformation and in carriers a vectorial translocation, asks for a different utilization of the catalytic energy.
In the following we shall elucidate how, by an activation energy analysis, one arrives at a series of important conclusions which make some particular features of carrier function understandable. These are: firstly, the differentiation between a unidirectional type of transport and counter-exchange transport, secondly, the action of inhibitors on carriers, their membrane-side specificity, and thirdly, the necessary segregation of transport from the chemical reaction, i.e. the improbability of the vectorial group transfer. These ideas have been developed in particular on the basis of our detailed research on the molecular properties of the ADP/ATP exchange carrier from mitochondria and the uncoupling protein from brown adipose tissue mitochondria (1,2). The fact that these two carriers are structurally similar but differ widely in several translocation functions was an incentive to formulate the activation energy diagrams of carrier function.
KeywordsEnergy Transfer Activation Barrier Carrier Transport Uncouple Protein Vectorial Group
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