Rethinking the P-type ATPase problem

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Abstract

There are very good reasons to stop thinking about the molecular mechanism of the P-type ion-translocating ATPases in terms of the traditional E1E2 model and to start thinking about it in more progressive ways. This makes it possible to see the ion-transport cycle as a rational series of discrete steps with well defined driving forces, including the crucial energy transduction step, where the chemical energy of ATP hydrolysis is exchanged for the osmotic energy of an ion gradient. Importantly, although major enzyme conformational changes accompany each of these steps, none of them drive the energy coupling reaction. Thus, neither the E1E2 model nor conformational energy coupling, the cornerstones of traditional thinking about the P-type ATPases, are reliable paradigms for future efforts to understand how these transporters work. Alternatives must be seriously considered.

Section snippets

The E1E2 model

A description of the E1E2 model for the P-type ATPase reaction cycle, or a reference to it, can be found in virtually all of the articles that have been published about the P-type ATPases since it was formalized in the oft-cited review of DeMeis and Vianna [9]. The model emerged around the same time as the purely conformational two-state energy coupling model of Jardetzky [10], and has many features in common with it. Briefly, it purports the existence of two, and only two, conformational

A more useful model

If the E1E2 dogma can be mentally put aside, it becomes possible to think about the P-type ATPase reaction cycle in a more rational way that is based upon established principles of enzyme catalysis and therefore of more certain heuristic value. This has the added distinct advantage of explicit stipulation of the driving forces for the individual steps, which is essential for a true understanding of any mechanism. A model arising from such considerations is shown in Figure 1. The reaction cycle

Ramifications of the new model

The scheme presented in Figure 1 thus comprises a straightforward cytoplasmic ion-binding reaction for which the atomic structures of both reactants and product are known, and a few additional steps that follow as expected from established principles of enzyme catalysis. It is simple, completely consistent with all of the experimental facts, and almost certainly correct. It should therefore be valuable for the interpretation of future experimental results and reinterpretation of past ones. It

Molecular mechanism of energy coupling

The preceding analysis reduces the quest for an understanding of the molecular mechanism of the P-type ATPases to a single critical step in the catalytic cycle: the E∼PCa2 to TSII reaction. In this reaction, the phosphoryl group of the covalent phosphoenzyme intermediate becomes more tightly bound, and the resulting decrease in chemical potential of the bound phosphoryl group is used to increase the chemical potential of the bound ions, thereby affecting their release to the far side of the

Concluding remarks

The traditional E1E2 model for the reaction cycle of the P-type ATPases has outlived its usefulness and should be replaced. The revised model presented here is more accurate, is based upon established principles of enzyme catalysis, stipulates the forces that drive the individual reaction steps and sheds new light on the nature of the energy coupling reaction. It should therefore be more useful for the interpretation of experimental results and as a guide toward eventual molecular understanding

Acknowledgements

This work was supported by USPHS NIH grant #GM24784.

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  • Conformational changes produced by ATP binding to the plasma membrane calcium pump

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