Contribution of the Collective Excitations to the Coupled Proton and Energy Transport along Mitochondrial Cristae Membrane in Oxidative Phosphorylation System

A protet-based model that can account for energy coupling in oxidative and photosynthetic phosphorylation

Two-stage (e.g. light-dark) phosphorylation experiments showed that there is a stored 'high-energy' intermediate linking electron transport and phosphorylation. Large, artificial electrochemical proton gradients (protonmotive forces or pmfs) can also drive phosphorylation, a fact seen as strongly supportive of the chemiosmotic coupling hypothesis that a pmf is the 'high-energy' intermediate. However, in such experiments there is an experimental threshold (pmf >170 mV, equivalent to ΔpH ∼2.8) below which no phosphorylation is in fact observed, and 220 mV are required to recreate in vivo rates. This leads to the correct question, which is then whether those values of the pmf generated by electron transport are large enough. Even the lower ones as required for any phosphorylation (leave alone those required to explain in vivo rates) are below the threshold [1, 2], whether measured directly with microelectrodes or via the use of membrane-permeant ions and/or acids/bases (which are always transporter substrates [3], so all such measurements are in fact artefactual). The single case that seemed large enough (220 mV) is now admitted to be a diffusion potential artefact [4]. Many other observables (inadequate bulk H+ in 'O2-pulse'-type experiments, alkaliphilic bacteria, dual-inhibitor titrations, uncoupler-binding proteins, etc.) are consistent with the view that values of the pmf, and especially of Δψ, are actually very low. A protet-based charge separation model [2], a protonic version analogous to how energy may be stored in devices called electrets, provides a high-energy intermediate that can explain the entire literature, including the very striking demonstration [5] that close proximity is required between electron transport and ATP synthase complexes for energy coupling between them to allow phosphorylation to occur. A chief purpose of this article is thus to summarise the extensive and self-consistent literature, much of which is of some antiquity and rarely considered by modern researchers, despite its clear message of the inadequacy of chemiosmotic coupling to explain these phenomena.

What causes cardiac mitochondrial failure at high environmental temperatures?

Although a mechanism accounting for hyperthermic death at critical temperatures remains elusive, the mitochondria of crucial active excitable tissues (i.e. heart and brain) may well be key to this process. Mitochondria produce ∼90% of the ATP required by cells to maintain cellular integrity and function. They also integrate into biosynthetic pathways that support metabolism as a whole, allow communication within the cell, and regulate cellular health and death pathways. We have previously shown that cardiac and brain mitochondria demonstrate decreases in the efficiency of, and absolute capacity for ATP synthesis as temperatures rise, until ultimately there is too little ATP to support cellular demands, and organ failure follows. Importantly, substantial decreases in ATP synthesis occur at temperatures immediately below the temperature of heart failure, and this suggests a causal role of mitochondria in hyperthermic death. However, what causes mitochondria to fail? Here, we consider the answers to this question. Mitochondrial dysfunction at high temperature has classically been attributed to elevated leak respiration suspected to result from increased movement of protons (H+) through the inner mitochondrial membrane (IMM), thereby bypassing the ATP synthases. In this Commentary, we introduce some alternative explanations for elevated leak respiration. We first consider respiratory complex I and then propose that a loss of IMM structure occurs as temperatures rise. The loss of the cristae folds of the IMM may affect the efficiency of H+ transport, increasing H+ conductance either through the IMM or into the bulk water phases of mitochondria. In either case, O2 consumption increases while ATP synthesis decreases.

Safari with an Electron Gun: Visualization of Protein and Membrane Interactions in Mitochondria in Natural Environment

This paper presents new structural data about mitochondria using correlative light and electron microscopy (CLEM) and cryo-electron tomography. These state-of-the-art structural biology methods allow studying biological objects at nanometer scales under natural conditions. Non-invasiveness of these methods makes them comparable to observing animals in their natural environment on a safari. The paper highlights two areas of research that can only be accomplished using these methods. The study visualized location of the Aβ42 amyloid aggregates in relation to mitochondria to test a hypothesis of development of mitochondrial dysfunction in Alzheimer's disease. The results showed that the Aβ42 aggregates do not interact with mitochondria, although some of them are closely located. Therefore, the study demonstrated that mitochondrial dysfunction is not directly associated with the effects of aggregates on mitochondrial structure. Other processes should be considered as sources of mitochondrial dysfunction. Second unique area presented in this work is high-resolution visualization of the mitochondrial membranes and proteins in them. Analysis of the cryo-ET data reveals toroidal holes in the lamellar structures of cardiac mitochondrial cristae, where ATP synthases are located. The study proposes a new mechanism for sorting and clustering protein complexes in the membrane based on topology. According to this suggestion, position of the OXPHOS system proteins in the membrane is determined by its curvature. High-resolution tomography expands and complements existing ideas about the structural and functional organization of mitochondria. This makes it possible to study the previously inaccessible structural interactions of proteins with each other and with membranes in vivo.

Directed proton transfer from Fo to F1 extends the multifaceted proton functions in ATP synthase

The role of protons in ATP synthase is typically considered to be energy storage in the form of an electrochemical potential, as well as an operating element proving rotation. However, this review emphasizes that protons also act as activators of conformational changes in F1 and as direct participants in phosphorylation reaction. The protons transferred through Fo do not immediately leave to the bulk aqueous phase, but instead provide for the formation of a pH gradient between acidifying Fo and alkalizing F1. It facilitates a directed inter-subunit proton transfer to F1, where they are used in the ATP synthesis reaction. This ensures that the enzyme activity is not limited by a lack of protons in the alkaline mitochondrial matrix or chloroplast stroma. Up to one hundred protons bind to the carboxyl groups of the F1 subunit, altering the electrical interactions between the amino acids of the enzyme. This removes the inhibition of ATP synthase caused by the electrostatic attraction of charged amino acids of the stator and rotor and also makes the enzyme more prone to conformational changes. Protonation occurs during ATP synthesis initiation and during phosphorylation, while deprotonation blocks the rotation inhibiting both synthesis and hydrolysis. Thus, protons participate in the functioning of all main components of ATP synthase molecular machine making it effectively a proton-driven electric machine. The review highlights the key role of protons as a coupling factor in ATP synthase with multifaceted functions, including charge and energy transport, torque generation, facilitation of conformational changes, and participation in the ATP synthesis reaction.