Computational modeling of mitochondrial K+- and H+-driven ATP synthesis
Introduction
ATP synthesis in E. coli, mitochondria, and chloroplasts is catalyzed by F1Fo ATP synthase that transduces phosphorylation and redox potentials into electrochemical (ion gradients) and chemical (ATP) energy [1,2]. Foundationally, bioenergetics rests on the principle of F1Fo ATP synthase harnessing the energy stored in H+ gradients to generate ATP.
Recently, our laboratory has described that a large part of the ATP synthesis flux is driven by K+ [3,4]. In spite of the large F1Fo selectivity for H+ over K+ (or Na+) (~106-fold with regard to both alkali metal ions), the large abundance of cytoplasmic K+ over H+ (>106-fold excess) in mammalian cells, reveals that, in principle, a non-trivial flux of K+ could actually be driving the F1Fo ATP synthase in its normal operation. The portion of the H+ gradient not directly dissipated by the ATP synthase activity would be available to drive the efflux of K+ through the K+/H+ exchanger (KHE) restoring the osmotic balance. In this way, the K+- and H+- transporting function of ATP synthase remains fully compatible with Mitchell's chemiosmotic mechanism [1,5].
The K+ transport mechanism allows mitochondrial volume regulation of respiration since, unlike H+, K+ is, functionally, osmotically active owing to its 6 orders-of-magnitude greater concentration than H+. Purified F1Fo ATP synthase reconstituted into proteoliposomes can harness energy from K+ flux to generate ATP, when subjected to a transmembrane K+ gradient [3,4]. Pure K+ current-driven ATP synthase activity was accompanied by ATP generation recorded as an increase in the photon rate over background that, in turn, was significantly inhibited by specific Fo blockers venturicidin/oligomycin [3,4].
Isolated rat heart mitochondria show that, compared to absence of K+, at constant osmolality (260 mOsm), physiological pH and K+ concentration drive significantly enhanced fluxes of ATP synthesis and respiration [3,6]. Under the same conditions, and using radioactive tracers, we quantified the mitochondrial PMF, its individual components, ΔΨm and ΔpH, and volume, under both states 4 and 3 respiration, in the presence or absence of K+ [3,6].
In the conventional view of cation flux cycles in mitochondria, a H+ gradient is driving solely H+ through F1Fo to make ATP whereas K+ only enters the matrix through an “ordinary” channel, dissipating the energy change of this K+ movement as heat (see Fig. 1A). In the new mechanism studied herein, in addition to that of H+, a significant fraction of the energy of ΔΨm-driven charge moves as K+ whose flux is directly harnessed by F1Fo to make ATP (see Fig. 1B) rather than being wasted as heat in an ordinary channel, while the remainder of the H+ gradient energy is utilized to remove K+ through KHEmito. The bioenergetic advantage in this mechanism over the conventional one is that more ATP is produced for the same input energy by not wasting some of that energy on maintaining what was originally thought to be a separate K+ cycle that would generate heat but not ATP.
The mechanism of K+- and H+-driven ATP synthesis remains fully compatible with Mitchell's chemiosmotic mechanism (reviewed in [5]) and the present work computationally tests some of its major mechanistic and bioenergetic implications. We utilize a computational modeling approach that accounts for the mechanism of K+- and H+-driven ATP synthesis to address its feasibility to predict integrated biological outcomes. We explore the model's ability to simulate experimental data obtained with isolated mitochondria that, in turn, had validated the proteoliposomal data [3]. We also seek to analyze the bioenergetic impact produced by K+-and H+-driven ATP synthesis as well as put forward testable predictions emerging from the model results. Herein, the mechanistic role(s) of K+ are highlighted. Overall, the results obtained support the experimental evidence available on ATP synthesis driven by K+ and H+ transport through the F1Fo ATP synthase [3,4,6].
Section snippets
A minimal model that describes the core mechanism of K+- and H+-driven ATP synthesis
To investigate the potential bioenergetic advantage of a mechanism in which both K+ and H+ drive ADP phosphorylation through mitochondrial F1Fo ATP synthase, we formulated an updated minimal model of OxPhos comprising the following inner membrane processes: H+ pumping by respiration coupled to NADH oxidation, adenine nucleotide translocator (ANT), phosphate carrier (Pi carrier), ATP synthase transport of H+ and K+, and the K+/H+ exchanger (KHEmito) (Fig. 1B Updated Minimal Model). The behavior
Simulation of the core mechanism of K+ - and H+-driven ATP synthesis
First, we tested the essential mechanistic components able to sustain K+- and H+-driven ATP synthesis through F1Fo ATP synthase in an updated minimal model of OxPhos (Fig. 1B) compared to a similar, but conventional model in which K+ is transported via a channel with a Goldman-Hodgkin-Katz (GHK) mechanism not linked to OxPhos (conventional GHK model) (Fig. 1A).
Compared to the conventional K+-transport described by the GHK model, the updated minimal model shows that, upon transition from state 4
Discussion
Using a computational modeling approach to assess the mechanism of K+- and H+-driven ATP synthesis through F1Fo ATP synthase, the comparison of simulations with experimental data obtained in isolated mitochondria, and its feasibility to predict integrated biological outcomes, the present work reports the following main findings: (i) the experimentally-demonstrated natural capacity of F1Fo ATP synthase to transport K+ in addition to H+ to generate ATP bestows mitochondria with a significant
Declaration of Competing Interest
None.
Acknowledgement
This work was supported entirely by the Intramural Research Program, National Institute on Aging, NIH.
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