General description of our research
Last updated 05 Sep 2000

MECHANISM OF ENERGY COUPLING: PRESENT STATE OF THE PROBLEM

The question about the mechanism of energy coupling of ATP-synthesis with electron transfer in biomembranes redox-chains is one of the most fundamental issues of Bioenergetics. For over thirty years the chemiosmotic hypothesis [Mitchell, 1966], confirmed by a great number of experimental data, occupies the dominant place whenever questions related to the coupling systems are considered [Nicholls, Ferguson, 1992]. However, during the long period of the existence of the hypothesis, it has been accumulated a considerate number of facts which cannot be adequately explained in terms of delocalized chemiosmosis, [Ferguson, 1985, Kell, 1992, Krulwich et al., 1996]. As mentioned in Paul D. Boyer's review "the possibilities of some type of localized coupling still merit consideration" [Boyer, 1997].

Among the examples of experimental dependencies contradicting to the orthodox variant of the chemiosmotic hypothesis there are following:
1) Mitochondria respiration rate activates to the different extent at the reduction of membrane potential (Dm_H+) for the same value but in different ways - by an uncoupler or by ADP [Azzone et al., 1984];
2) The dependence between ATP-synthesis rate and the magnitude of membrane potential varies at different ways of Dm_H+ reduction - by means of adding of an respiratory inhibitor or a uncoupler [Zoratti, Petronili, 1985]
3) The ratio of the phosphate potential to the membrane one grows at the Dm_H+ reduction under the influence of an uncoupler [Westerhoff et al., 1981].

Other important data inconsistent with delocalised chemiosmosis principle are related to: ATP synthesis under low Dm_H+ in extremely alkaliphilic bacteria [Guffanti, Krulwich, 1994; Krulwich, Guffanti, 1992] and yeast mitochondria [Castrejon et al., 1997], double uncoupler-inhibitor titrations of energy coupling system [Hitchens, Kell,1983; Herweijer et al., 1985; Herweijer et al., 1986; Kell, 1992], decoupler action mechanism research [Rottenberg, 1990], and a number of other data that Ferguson, 1985]). (In the near future a set of such the most important facts with article abstracts and brief comments will be placed on this site).

The necessity to explain such experimental data stimulated appearance of a number of modifications of the chemiosmotic hypothesis [Mitchell, 1985, De Kouchkovsky et al., 1984, Ferguson, 1995, Westerhoff et al., 1984]. Some attempts were also made in order to explain existing experimental data in terms of other hypotheses offering more localized ways of energy transfer between electron transport and ATP synthesis [Slater et al., 1985, Rottenberg et al., 1985, Gupte et al., 1991]. Up to the present moment there appear various hypotheses describing the energy coupling mechanism [Menendez, 1996, Schole, Schole, 1994, Dmitriev, 1995].

However, the detailed and grounded explanation most of experimental data from the unified point of view has not been proposed. Even when different hypotheses explain a number of experimental facts, they, as a rule, do not provide quantitative estimations of theoretical dependencies between coupling system parameters. This circumstance complicates the comparison to each other of different hypotheses in respect of reliability of the coupling system description. However, detailed description of the hypothetical system and its analysis can give forcible arguments permitting to question certain mechanism or to show its theoretical possibility.

At the present time the structure and mechanism of functioning of ATP-synthase macrocomplex is intensively investigated [Boyer, 1997]. Theoretical examination of the work of ATP-synthase is an actual addition to the experimental data. The development of molecular biology and methods of work with the genetic material stimulated numerous present investigations of the connection between inherited mitochondrial and nuclear DNA damages and energy coupling system defects [Muller-Hocker et al., 1998]. But, obviously, considerable progress in solving of these issues can be made only with understanding of the energy coupling mechanism and normal work of the respiratory chain.

Clarification of the membrane coupling mechanism also has a number of important practical applications. Urgent problems in this field are: looking for ways of reduction the rate with which respiratory chain generates free radicals influencing on aging processes, therapeutic effect at the ischemic tissue damage and at a number of other pathologies and disease [Hardy et al., 1990, Olson et al., 1988, Schagger, Ohm, 1995, Muller-Hocker et al, 1997].

Thus, the urgency of the study of the energy coupling mechanism is determined by numerous present research directions, both of fundamental and clinical nature.

RESEARCH OBJECTIVE AND TASK

The objective of our research is to create theoretical model of coupling process, which could explain the most of accumulated experimental data, including those, which are in contradiction to delocalised chemiosmotic principle.

In this order we decide to determine theoretical compatibility with existing experimental data for two fundamentally different energy coupling mechanisms - for the delocalized chemiosmotic mechanism [Mitchell, 1985] and for the proton-chemical coupling mechanism [Lemeshko, 1988]. The latter combines both the local chemical and the delocalized chemiosmotic ways of energy transfer. According to preliminary investigations this mechanism can explain a number of experimental results, both contradicting to delocalised coupling and compatible with it.

To perform this task we use mathematical modeling approach with subsequent comparing of model behavior with set of experimental data available from literature. At first stage we have created models by method of equivalent electric circuits. This qualitative approximation suits to fast and easy creation and analysis a number of different models of coupling system. We propose new modification of this method, which allows more adequate representation of coupling system with some nonlinear elements and multiple flows coupling. 16 models for different variants of delocalised chemiosmotic and proton-chemical coupling mechanisms were created and analyzed. More detailed description of model analysis can be found here in our publications.

Basing on the results of analysis we have started more detailed investigation of mechanisms, which are in best correlation with experimental data. For this we use mathematical models created in terms of chemical kinetics.

POSSIBLE PRACTICAL APPLICATIONS OF COUPLING SYSTEM MODELS

The obtained results can have broad theoretical and practical application. One of the possibilities of using the developed models is carrying out of a computer experiment allowing to study the influence of different factors on the ATP-synthesis system. This can serve as an instrument of coupling hypothesis testing and as a quick way of system behavior prognoses.

The development of quantitatively adequate models of energy coupling based on the proposed qualitative ones can be eventually used in medical research. The examples of this are: the prognosis of the behavior of energy coupling system of heart mitochondria under is-chemic damage; modeling of the effect of different drugs with side uncoupling or inhibiting action on ATP-synthesis; the prognosis of mitochondrial coupling system functioning at a certain genetic defect of the respiratory chain.

BASIC RESULTS OF EQUIVALENT ELECTRICAL MODELS ANALYSIS

1. The proposed modification of the equivalent electric circuits method allows to create models which reflect qualitatively basic characteristics of membrane energy coupling systems [Lemeshko, Anishkin, 1996; Lemeshko, Anishkin, 1998];
2. Models based on the proton-chemical mechanism of energy coupling show that the theoretical correlation between the ATP-synthesis rate and the membrane potential depends on the way of Dm_H+ changes. But the chemiosmotic model does not show such a dependence [Lemeshko, Anishkin, 1998];
3. Models of different variants of the proton-chemical coupling give the ratio between the respiration rate and the membrane potential, which depends on the protonmotive force change. Orthodox chemiosmotic models do not show such a dependence [Anishkin, 1998];
4. Different proton-chemical models show that the ratio of the phosphate potential to the membrane one could increase at the reduction of Dm_H⁺ by means of an uncoupler [Anishkin,  Lemeshko, 1998]. Delocalized chemiosmotic models show only the constant or decreased ratio at protonmotive force reduction;
5. Combined models of proton-chemical coupling points with chemiosmotic proton pumps act like pure proton-chemical models and qualitatively correlate with experimental data [Anishkin, 1997].

More detailed description of modeling method and analysis results you can find here: