Biochemistry And Biophysics Of Mitochondrial Membranes

The mitochondrion is known as the powerhouse of all living cells. Its role is indispensable to life, and its malfunction can lead to serious diseases. On the basis of experiments with fluorescent probes that distribute primarily in the inner mitochondrial membrane and the matrix-side of the inner membrane, it has recently been suggested that the mitochondrion is likely to be operating at much higher temperatures than physiological temperature of the body (~ 50°C). This discovery, if true, would necessitate the re-evaluation of the thermodynamic efficiency and all of the thermodynamic cycles, based on the Mitchell equation central to chemiosmotic theory. It is this aspect of the bio-energetics of mitochondria that is at the core of the present project. In chapter 2, doubts and criticisms regarding the suggestion of a hotter mitochondrion have been raised and are briefly discussed. The possible implications and repercussion of such a claim - if true - on some aspects of mitochondrial biochemistry and biophysics are reviewed. A hotter mitochondrion - as claimed - would imply a 3% correction in the chemical gradient term. Further, if this claim is true and to the extent claimed (10°C), this may imply some heat-engine character for mitochondrial thermodynamic operation albeit this may only represent 4% at most. A "hot mitochondrion" calls for a closer examination of the energy balance that endows it with these claimed elevated temperatures. In chapters 3 and 4, as a first step in this effort, we present a semi-quantitative bookkeeping whereby, in one stroke, a formula is proposed that yields the rate of heat production in a typical mitochondrion and a formula for estimating the number of "active" ATP synthase molecules per mitochondrion. One-hundred thirty heterotrophic protozoa cell types are considered in this study. The studied cells, selected to cover a wide range of sizes (volumes) from ca. 25 μm3 to 125 million μm3, are estimated to exhibit a power per mitochondrion ranging from ca. 2×100 pW to 4×10−4 pW. In these cells, the corresponding number of active ATP synthases per mitochondrion ranges from 37,000 to just about ten. In chapter 5, the origin of disagreement between the experimental measurement and the theoretical estimates is investigated. Since the steady-state theoretical estimates place the temperature difference between the mitochondrion and its surrounding at a maximum of 10−5 °C, this million-fold disagreement is called the "hot mitochondrion paradox". It is suggested that every proton translocated via ATP synthase sparks a picosecond temperature-difference spike of the order of magnitude measured by Chrétien et al. Time-averaging of these spikes recovers the theoretical value. Further, a temporal and spatial superposition of the fluorescence intensity of a very large number of molecular thermometer molecules in the sample can give the appearance of a steady signal. The inner mitochondrial membrane appears to be flanked by temperature differences fluctuating in time and along the membrane's surface, with "hot" and "cold" spots as ultrashort temperature spikes. In chapter 6, it is shown that the ATP synthase's intrinsic molecular electrostatic potential (MESP) adds constructively to, and hence reinforces, the chemiosmotic voltage. This ATP synthase voltage represents a new free energy term that appears to have been overlooked. This term, at least in ATP synthase molecules derived from five different organisms, is roughly equal in order of magnitude and opposite in sign to the energy needed to be dissipated as a Maxwell's demon (Landauer principle). The electransic potential across the inner mitochondrial membrane must be maintained within certain bounds for the proper functioning of the cell. In chapter 7, a qualitative mechanism for the homeostasis of this membrane potential is proposed whereby an increase in the electric field slows-down the rate-limiting steps of the electron transport chain (ETC). An increase in voltage thus limits the rate of pumping of the protons in the inter-membrane gap by slowing the ETC directly but also through short-circuiting the ETC by electroporation of the membrane leading to depolarization of the membrane. The thesis ends with a chapter on the possible future directions of this research and the main conclusions that arise from the work presented therein.

Mitochondrial transport systems are essential to mitochondrial function and therefore to energy homeostasis within the cell. The book contains studies utilizing the techniques of biochemistry, physiology, molecular biology and genetics to reveal the structure and function of mitochondrial transport systems. It is divided into the following six sections: - Proton Translocation: The Uncoupling Protein and the ATPase; - Carriers and Transporters; - Mitochondrial Ion Channels; Structure of the Outer Mitochondrial Membrane Channel, VDAC; - VDAC, Peripheral Kinases and Energy Utilization; - Mitochondrial Channels in Humans and Relationship to Disease.

On February 14, 1977, a symposium entitled "The Molecular Biology of Membranes" was held in New Orleans in honor of Professor David E. Green, whose many contributions in mitochondrial structure and metabolism have influenced and guided research in this important area of biochemistry for many years. The symposium was attended by many former and present-day colleagues, friends, and interested scientists. The contents of this volume represent papers that were delivered at the symposium and other contributions from individuals who have been associates of Professor Green. We wish to thank Plenum Press for their help in making the symposium and publication of this book possible. Sidney Fleischer Youssef Hatefi David MacLennan Alexander Tzagoloff vii Contents Impressions of David E. Green by His Colleagues . . . . . . . . . . . . . . 1 Enzyme Institute Days 13 F. M. Huennekens Perspective for the Mitochondrion 17 David E. Green Chapter 1 Organization of Protein and Lipid Components in Membranes 29 Garret Vanderkooz Chapter 2 Structure of the Mitochondrion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Robert A. Haworth and Douglas R. Hunter Chapter 3 Permeability of Membrane as a Factor Determining the Rate of Mitochondrial Respiration: Role of Ultrastructure . . . . . . . . . . . . . . 75 Jerzy Popinigis Chapter 4 Chemistry of Intrinsic Membrane Protein Complexes: Studies on Complex III and Cytochrome c Oxidase . . . . . . . . . . . . . . . . . . . . . . . 103 Roderick A. Capaldi ix x Contents Chapter 5 Reversible Mitochondrial ATPase as a Model for the Resolution of a 121 Membrane Complex A. E.

In order to provide a functional organisation of the various biochemical processes, the intracellular environment is subdivided into compartments formed by organelles. Mitochondria serve to restore energy-rich compounds, while energy is mainly consumed in the cytosolic space. Molecules involved in these metabolic cycles have to cross the mitochondrial membrane via specialized carriers embedded in the inner mitochondrial membrane. All relevant structural and functional aspects of the Anion Carriers of Mitochondrial Membranes as well as isolation procedures, reconstitution experiments and biogenesis of the carriers, kinetics of transport as well as the metabolic regulation mechanisms are treated in this volume.

This book is indispensable to researchers in fields as diverse as Molecular Biology and Biophysics. It covers the important role that mitochondria play in a variety of biochemical spheres. It analyses how mitochondria affect metabolic pathways, how they are active in the regulation of cytosolic constituents, and their role in initiating signal pathways. Also covered are the way mitochondria help to regulate apoptosis, and how they modulate cellular hypertrophy and proliferation. It gives an overview of the emergence of mitochondria as an important regulator of cell signaling, with a particular focus on their pathophysiology.

This book covers recent advances in the study of structure, function, and regulation of metabolite, protein and ion translocating channels, and transporters in mitochondria. A wide array of cutting-edge methods are covered, ranging from electrophysiology and cell biology to bioinformatics, as well as structural, systems, and computational biology. At last, the molecular identity of two important channels in the mitochondrial inner membrane, the mitochondrial calcium uniporter and the mitochondrial permeability transition pore have been established. After years of work on the physiology and structure of VDAC channels in the mitochondrial outer membrane, there have been multiple discoveries on VDAC permeation and regulation by cytosolic proteins. Recent breakthroughs in structural studies of the mitochondrial cholesterol translocator reveal a set of novel unexpected features and provide essential clues for defining therapeutic strategies. Molecular Basis for Mitochondrial Signaling covers these and many more recent studies of mitochondria function, their communication with other organelles, and their critical roles in development, aging, and in a plethora of stressful or degenerative events. Authored by leading researchers in the field, this volume will be an indispensable reference resource for graduate students and academics working in related areas of biophysics and cell biology as well as for professionals within industry.