Proton gradient ApH

Fig. 14 (B) shows ATP synthesis in chloroplasts, without the aid oflight, measured as afunction oftime at three different diffusion potentials with a constant proton gradient [ ApH] of 3.2 imposed, as reported by Graber, Junesch and Schatz". The authors used a rapid-mixing, rapid-quenching apparatus to allow ApH and A F to be established very rapidly (in 2 ms), and measurements completed in 100 ms, over which ApH and A P remain virtually the same as their original values.

As electrons pass through the ETC, protons are transported from the matrix and released into the intermembrane space. As a result, an electrical potential F and a proton gradient ApH arise across the inner membrane. The electrochemical proton gradient is sometimes referred to as the protonmotive force Ap. 

Lipophilic tertiary amines (t-amines) are effective inhibitors of photophosphorylation (1-3) Bound up with the inhibition, light-induced uptake of (l C)-methylamine and quenching of 9 afflinoacridine (9 AA) fluorescence disappear. This indicates that inhibition of ATP synthesis may be related to a decline of the transthylakoid proton gradient, ApH (4). IntriguingTy, a strong pH-dependent control of electron flow was maintained in the presence of t-amines (3 5) despite of the low ApH Therefore, t-amines were termed selective uncouplers (1). 

Photosynthetic electron transport is under control of the intrathyla-koid proton potential, pHj. (1,2). When pH. increases in the light, with the build-up of a transthylakoid proton gradient (ApH), electron flow is decelerated. Two major sites of pH. -dependent feed-back control have been discussed the reduction of the primary photosystem (PS) II acceptor (2) and the oxidation of plastohydroquinone at the cytochrome... 

A proton-motive force of approximately —250 mV is needed to achieve ATP synthesis. This proton-motive force, A, is composed of a membrane potential, A P, and a pH gradient, ApH (Chapter 21). The proton-motive force is defined as the free energy difference, AG, divided by S, Paraday s constant ... 

In the short term (minutes), energy-dependent quenching reflects the buildup of the trans-thylakoid proton gradient. When the ApH is high, there is an increase in the orderly dissipation of energy that protects... 

The potential free energy associated with the formation of the proton gradient is called the proton motive force (PMF), the counterpart of the electro-motive force (EMF), and represented by the symbol ApH - PMF consists of two components, one of which is associated with the difference between the proton concentrations on the inside (i) and outside (o) of the membrane, i.e., ([H ]j - [H ]q). As proton concentration can be expressed in terms of pH, the proton-concentration difference can be written as ApH, i. e., -log,([H ] / [H o), and the potential difference in volts written as 2.3(RT/c ApH where< is the Faraday constant. The second component of PMF arises from the fact that protons are positively charged, and so there is an electro-static transmembrane potential, designated as AT. When PMF, Apn% is expressed as the sum of the proton-gradient term and the membrane-potential term, we have... 

Studies on sugar and amino acid uptake by L. donovani promastigotes revealed that addition of D-glucose or L-proline caused a rapid influx of protons into these cells, indicating that both substrates are co-transported with protons (52). This active transport system involves a proton-motive force (pmf)-driven mechanism which requires the maintenance of a proton electrochemical gradient. Such a gradient is composed of the chemical gradient (ApH) and the membrane potential (Ai/ ) (52). 

Fig. 19.8. Overview of energy transformations in oxidative phosphorylation. The electrochemical potential gradient across the mitochondrial membrane is represented by ApH, the proton gradient, and A F, the membrane potential. The role of the electrochemical potential in oxidative phosphorylation is discussed in more depth in Chapter 21.

Fig. 21.2. Proton motive force (electrochemical gradient) across the inner mitochondrial membrane. The proton motive force consists of a membrane potential, AvJ/, and a proton gradient, denoted by ApH for the difference in pH across the membrane. The electrochemical potential is called the proton motive force because it represents the potential energy driving protons to return to the more negatively charged alkaline matrix.

ATP synthesis involves the transfer of electrons from the intermembrane space, through the inner membrane, back to the matrix. The transfer of electrons from the matrix to the intermembrane space leads to a substantial pH difference between the two sides of the membrane (about 1.4 pH units). Mitchell recognized that this represents a large energy difference, because the chemiosmotic potential is actually composed of two components. One component is the difference in hydrogen ion concentration (pHom - pHm), symbolized by the term ApH. The other component follows from the fact that protons are positively charged, so there is a difference in electrical potential symbolized by the term AT. The proton gradient results in a state where the intermembrane space is positive and acidic relative to the matrix. 

If the iimer membrane is so impermeable, and ATP is made in the matrix side of the membrane, how does it get out into the cell where it s needed Specific transport systems use either the electrical (AT) or acid/base (ApH) components of the proton gradient to move substrates in and out of the matrix. (See Figure 11-7.)... 

Using the protocol of titrations of qE against the ionophoric uncoupler nigericin in p>ea and spinach chloroplasts, and using 9-aminoacridine fluorescence as a monitor for the proton gradient, we have investigated some relationships between ApH, "redox state", and zeaxanthin content, in their effects up>on the magnitude of qE. 

The total amount of protons stored in the chromatophore vesicles after a period of illumination is related to the ApH and to the buffering capacity of the vesicle interior. The decay kinetics in the dark of the established proton gradient depend on the membrane permeability for protons and on the concentration and proton affinity of the internal buffering groups. A model based on these assumptions, developed by J. Whitmarsh (1), has been applied to the experimental results in order to obtain a quantitative evaluation of these parameters. 

Mainly based on the comparison of how light (H influx) and nigericin (H efflux) affect (10), it has been suggested that the redox chain directly interacts with ATPase (2,10). In fact, these experiments were made at unknown and uncontrolled proton gradients. We have established, using the hexylamine method, that actually no significant differences exist (unpublished). This was made possible because hexylamine not only stabilizes but also delocalizes ApH (11). 

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