Pressure modulation of cytochrome-to-cytochrome electron-transfer. Models and enzyme reactions

Effects of hydrostatic pressure on the kinetics of electron transfer in an isolated system of chloroplast cytochrome bf complex, plastocyanin and P700

The effects of pressure on the kinetics of redox reactions in and around the chloroplast cytochrome bf complex were studied using a reconstituted system consisting of Photosystem I (PS I) particles, cytochrome bf complex and plastocyanin (PC), all derived from pea chloroplasts. There were no significant permanent effects of pressure in the range 0.1-191 MPa on the reaction kinetics, or on the shape of the absorption spectra of components studied. Discernable effects on rate-coefficients of increasing pressure were observed on the reduction of P700(+) by PC(I), on the reduction of PC(II) by ascorbate, and on the oxidation of decyl plastoquinol by the bf complex. The volumes of activation ΔV(#) were determined from the dependence of the rate-coefficient on pressure using: [Formula: see text] The volume of activation is the difference in partial molar volume between the activated state and the reactants for the redox reaction. Such data was sought to help define in detail those redox reactions and the corresponding activated states. For the reduction of P700(+) by PC(I) and the oxidation of decyl plastoquinol by the bf complex, the rate coefficient decreased with increase in pressure, whilst for the reduction of PC(II) by ascorbate it increased. The corresponding volumes of activation were 9.6±0.6×10(-6) m(3) mol(-1), 18±2×10(-6) m(3) mol(-1) and -14±1×10(-6) m(3) mol(-1), respectively. Much of the pressure-dependence of PC(II) reduction by ascorbate was ascribed to an increase in ascorbate ionisation with increase in pressure. There was little effect of pressure on the kinetics of oxidation of ferrocytochrome f by PC(II), or on the equilibrium constant of the redox pair ferrocytochrome f/ferricytochrome f: PC(II)/PC(I). Possible physical bases for these activation volumes are discussed, and they are compared with literature values.

Hydrostatic and osmotic pressure as tools to study macromolecular recognition

Clearly, hydrostatic and osmotic pressure techniques offer unique potential in the study of fundamental problems of molecular recognition in biological systems. With the recent advances in technology such investigations are rapidly becoming commonplace. We look forward to further advances and their report in succeeding compendiums such as this volume.

Proteins under pressure. The influence of high hydrostatic pressure on structure, function and assembly of proteins and protein complexes

Oceans not only cover the major part of the earth's surface but also reach into depths exceeding the height of the Mt Everest. They are populated down to the deepest levels (approximately 11,800 m), which means that a significant proportion of the global biosphere is exposed to pressures of up to 120 MPa. Although this fact has been known for more than a century, the ecology of the 'abyss' is still in its infancy. Only recently, barophilic adaptation, i.e. the requirement of elevated pressure for viability, has been firmly established. In non-adapted organisms, increased pressure leads to morphological anomalies or growth inhibition, and ultimately to cell death. The detailed molecular mechanism of the underlying 'metabolic dislocation' is unresolved. Effects of pressure as a variable in microbiology, biochemistry and biotechnology allow the structure/function relationship of proteins conjugates to be analyzed. In this context, stabilization by cofactors or accessory proteins has been observed. High-pressure equipment available today allows the comprehensive characterization of the behaviour of proteins under pressure. Single-chain proteins undergo pressure-induced denaturation in the 100-MPa range, which, in the case of oligomeric proteins or protein assemblies, is preceded by dissociation at lower pressure. The effects may be ascribed to the positive reaction volumes connected with the formation of hydrophobic and ionic interactions. In addition, the possibility of conformational effects exerted by moderate, non-denaturing pressures, and related to the intrinsic compressibility of proteins, is discussed. Crystallization may serve as a model reaction of protein self-organization. Kinetic aspects of its pressure-induced inhibition can be described by a model based on the Oosawa theory of molecular association. Barosensitivity is known to be correlated with the pressure-induced inhibition of protein biosynthesis. Attempts to track down the ultimate cause in the dissociation of ribosomes have revealed remarkable stabilization of functional complexes under pseudo-physiological conditions, with the post-translational complex as the most pressure-sensitive species. Apart from the key issue of barosensitivity and barophilic adaptation, high-pressure biochemistry may provide means to develop new approaches to nonthermic industrial processes, especially in the field of food technology.