Photophosphorylation in chloroplasts

Factors regulating the slow electrogenic phase in green algae and higher plants

The electrochromic rise in the millisecond range, corrected for the subsequent decay, has been studied in Chlorella cells and spinach chloroplasts. The half-time of the electrochromic rise in the millisecond range is independent of the redox states of the components in the electrogenic loop. It is also independent of pH in the absence of a pH gradient, but it is dependent upon the transmembrane electric field and the pH gradient. The limiting step of the overall process is thus the electrogenic reaction itself. The amplitude of the electrochromic rise in the millisecond range is controlled by two classes of factors: the redox state of its reactants and the free energy available in the overall process including the electrogenic reaction. This free energy depends upon delta pH and transmembrane electric field. A fast reduction of cytochrome f+ (and thus probably of the Rieske protein) by Photosystem II is still possible when the electrogenic reaction is impeded for energetic reasons. The kinetic behavior of the electrogenic reaction could be interpreted by the following reaction: UH2int + FeS+ + X2 + Hext in equilibrium U + 2H+int + FeS + X2H where FeS is the Rieske protein; X2 would be a new protein (observed in Bouges-Bocquet, B. (1980) FEBS Lett. 117, 54-58); UH2 which is likely a special quinone, would release its two protons inside the thylakoids and X2 would use protons from the external medium.

The mechanism of proton translocation driven by the respiratory nitrate reductase complex of Escherichia coli

Low concentrations (1-50mum) of ubiquinol(1) were rapidly oxidized by spheroplasts of Escherichia coli derepressed for synthesis of nitrate reductase using either nitrate or oxygen as electron acceptor. Oxidation of ubiquinol(1) drove an outward translocation of protons with a corrected -->H(+)/2e(-) stoichiometry [Scholes & Mitchell (1970) J. Bioenerg.1, 309-323] of 1.49 when nitrate was the acceptor and 2.28 when oxygen was the acceptor. Proton translocation driven by the oxidation of added ubiquinol(1) was also observed in spheroplasts from a double quinone-deficient mutant strain AN384 (ubiA(-)menA(-)), whereas a haem-deficient mutant, strain A1004a, did not oxidize ubiquinol(1). Proton translocation was not observed if either the protonophore carbonyl cyanide m-chlorophenylhydrazone or the respiratory inhibitor 2-n-heptyl-4-hydroxyquinoline N-oxide was present. When spheroplasts oxidized Diquat radical (DQ(+)) to the oxidized species (DQ(++)) with nitrate as acceptor, nitrate was reduced to nitrite according to the reaction: [Formula: see text] and nitrite was further reduced in the reaction: [Formula: see text] Nitrite reductase activity (2) was inhibited by CO, leaving nitrate reductase activity (1) unaffected. Benzyl Viologen radical (BV(+)) is able to cross the cytoplasmic membrane and is oxidized directly by nitrate reductase to the divalent cation, BV(++). In the presence of CO, this reaction consumes two protons: [Formula: see text] The consumption of these protons could not be detected by a pH electrode in the extra-cellular bulk phase of a suspension of spheroplasts unless the cytoplasmic membrane was made permeable to protons by the addition of nigericin or tetrachlorosalicylanilide. It is concluded that the protons of eqn. (3) are consumed at the cytoplasmic aspect of the cytoplasmic membrane. Diquat radical, reduced N-methylphenazonium methosulphate and its sulphonated analogue N-methylphenazonium-3-sulphonate (PMSH) and ubiquinol(1) are all oxidized by nitrate reductase via a haem-dependent, endogenous quinone-independent, 2-n-heptyl-4-hydroxyquinoline N-oxide-sensitive pathway. Approximate-->H(+)/2e(-) stoichiometries were zero with Diquat radical, an electron donor, 1.0 with reduced N-methylphenazonium methosulphate or its sulphonated analogue, both hydride donors, and 2.0 with ubiquinol(1) (QH(2)), a hydrogen donor. It is concluded that the protons appearing in the medium are derived from the reductant and the observed-->H(+)/2e(-) stoichiometries are accounted for by the following reactions occurring at the periplasmic aspect of the cytoplasmic membrane.: [Formula: see text]

The Ninth Sir Hans Krebs Lecture. Compartmentation and communication in living systems. Ligand conduction: a general catalytic principle in chemical, osmotic and chemiosmotic reaction systems

Chemical reactions, like osmotic reactions, are transport processes when looked at in detail. Chemical catalysis by enzymes or catalytic carriers, and osmotic catalysis by porters, may be conceived as occurring by specific ligand-conduction mechanisms. In chemiosmotic reaction systems, the pathways of specific ligand conduction are spatially orientated through anisotropic enzyme and catalytic carrier complexes in which the reactions of chemical group transfer occur as vectorial diffusion processes of group translocation down gradients of group potential that represent real spatially-directed fields of chemical force. Thus, it is easier to explain biochemistry in terms of transport than it is to explain transport in terms of biochemistry.