Oxidative phosphorylation driving force

This potential, or protonmotive force as it is also called, in turn drives a number of energy-requiring functions which include the synthesis of ATP, the coupling of oxidative processes to phosphorylation, a metabohc sequence called oxidative phosphorylation and the transport and concentration in the cell of metabolites such as sugars and amino acids. This, in a few simple words, is the basis of the chemiosmotic theory linking metabolism to energy-requiring processes. 

Allelopathic inhibition of mineral uptake results from alteration of cellular membrane functions in plant roots. Evidence that allelochemicals alter mineral absorption comes from studies showing changes in mineral concentration in plants that were grown in association with other plants, with debris from other plants, with leachates from other plants, or with specific allelochemicals. More conclusive experiments have shown that specific allelochemicals (phenolic acids and flavonoids) inhibit mineral absorption by excised plant roots. The physiological mechanism of action of these allelochemicals involves the disruption of normal membrane functions in plant cells. These allelochemicals can depolarize the electrical potential difference across membranes, a primary driving force for active absorption of mineral ions. Allelochemicals can also decrease the ATP content of cells by inhibiting electron transport and oxidative phosphorylation, which are two functions of mitochondrial membranes. In addition, allelochemicals can alter the permeability of membranes to mineral ions. Thus, lipophilic allelochemicals can alter mineral absorption by several mechanisms as the chemicals partition into or move through cellular membranes. Which mechanism predominates may depend upon the particular allelochemical, its concentration, and environmental conditions (especially pH). 

The driving force of oxidative phosphorylation is the difference between the electron transfer potential of NADH or FADH2 relative to that of 02. For the redox couple... 

How is a concentration gradient of protons transformed into ATP We have seen that electron transfer releases, and the proton-motive force conserves, more than enough free energy (about 200 lcJ) per mole of electron pairs to drive the formation of a mole of ATP, which requires about 50 kJ (see Box 13-1). Mitochondrial oxidative phosphorylation therefore poses no thermodynamic problem. But what is the chemical mechanism that couples proton flux with phosphorylation ... 

Although the primary role of the proton gradient in mitochondria is to furnish energy for the synthesis of ATP, the proton-motive force also drives several transport processes essential to oxidative phosphorylation. The inner mitochondrial membrane is generally impermeable to charged species, but two specific systems transport ADP and Pj into the matrix and ATP out to the cytosol (Fig. 19-26). 

How Many Protons in a Mitochondrion Electron transfer translocates protons from the mitochondrial matrix to the external medium, establishing a pH gradient across the inner membrane (outside more acidic than inside). The tendency of protons to diffuse back into the matrix is the driving force for ATP synthesis by ATP synthase. During oxidative phosphorylation by a suspension of mitochondria in a medium of pH 7.4, the pH of the matrix has been measured as 7.7. 

A quantitative description of oxidative phosphorylation within the cellular environment can be obtained on the basis of nonequilibrium thermodynamics. For this we consider the simple and purely phenomenological scheme depicted in Fig. 1. The input potential X0 applied to the converter is the redox potential of the respiratory substrates produced in intermediary metabolism. The input flow J0 conjugate to the input force X0 is the net rate of oxygen consumption. The input potential is converted into the output potential Xp which is the phosphate potential Xp = -[AG hoS -I- RT ln(ATP/ADP P,)]. The output flow Jp conjugate to the output force Xp is the net rate of ATP synthesis. The ATP produced by the converter is used to drive the ATP-utilizing reactions in the cell which are summarized by the load conductance L,. Since the net flows of ATP are large in comparison to the total adenine nucleotide pool to be turned over in the cell, the flow Jp is essentially conservative. 

Gain ratio 17 r can be calculated at a reference force ratio, such as xopt, which is a natural steady-state force ratio of oxidative phosphorylation. This is seen as a result of the adaptation of oxidative phosphorylation to various metabolic conditions and also as a result of the thermodynamic buffering of reactions catalyzed by enzymes. The experimentally observed linearity of several energy converters operating far from equilibrium may be due to enzymatic feedback regulations with an evolutionary drive towards higher efficiency. 

There is some indications that mitochondria possess a mechanism of self-elimination. This function was ascribed to the so-called permeability transition pore (PTP). The PTP is a rather large nonspecific channel located in the inner mitochondrial membrane. The PTP is permeable for compounds of molecular mass <1.5 kDa. The PTP is usually closed. A current point of view is that PTP opening results from some modification and conformation change of the ATP/ADP antiporter. Oxidation of Cys56 in the antiporter seems to convert it to the PTP in a way that is catalyzed by another mitochondrial protein, cyclophilin. When opened, the PTP makes impossible the performance of the main mitochondrial function, i.e., coupling of respiration with ATP synthesis. This is due to the collapse of the membrane potential and pH gradient across the inner mitochondrial membrane that mediate respiratory phosphorylation. Membrane potential is also a driving force for import of... 

Most of the O2 consumed by aerobic organisms is used to produce energy in a process referred to as oxidative phosphorylation, a series of reactions in which electron transport is coupled to the synthesis of ATP and in which the driving force for the reaction is provided by the four-electron oxidizing power of O2 (Reaction 5.1). (This subject is described in any standard text on biochemistry and will not be discussed in detail here.) The next to the last step in the electron-transport chain produces reduced cytochrome c, a water-soluble electron-transfer protein. Cytochrome c then transfers electrons to cytochrome c oxidase, where they are ultimately transferred to O2. (Electron-transfer reactions are discussed in Chapter 6.)... 

The thylakoid lumen is the part of the chloroplast enclosed by the thylakoid membrane (Figure 17.15). During photosynthesis, electrons are pumped into the thylakoid lumen from the stroma, forming a proton gradient. Movement of the protons out of the thylakoid lumen through the CFO-CFl complex back to the stroma provides the driving force for photophosphorylation, the process of making ATP in photosynthesis. A similar mechanism is responsible for ATP synthesis in oxidative phosphorylation. 

Problem 22.61. What is the driving force behind oxidative phosphorylation ... 

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