EFFECTS OF PH ON THE KINETICS OF REDOX REACTIONS IN AND AROUND THE CYTOCHROME BF COMPLEX IN AN ISOLATED SYSTEM

Rate-coefficients describing the electron transfer reactions between P 700 and plastocyanin, between cytochrome f in cytochrome bf complexes and plastocyanin, and between decyl plastoquinol and cytochrome bf com plexes were determined as a function of pH in the range 4-10 from flas h-induced absorbancy changes at four wavelengths. The reactions betwee n P700 and plastocyanin, and between cytochrome f and plastocyanin wer e optimised when there was electrostatic interaction between ionised a cidic groups in plastocyanin with a pK(a) of 4.3-4.7 and ionised basic constituents in P700 (assumed to be in the PSI-F subunit) and in cyto chrome f, with a pK(b) of 8.9-9.4. The basic groups are thought to be lysine rather than arginine. This mechanism agrees with that inferred from effects of ionic strength changes on rate-coefficients. The relat ion between the second-order rate-coefficient for decyl plastoquinol o xidation by the bf complex and pH was characterised by a pK(a) of 6.1. This is interpreted as showing that the anion radical form of that qu inol, which has a pK(a) of 6, and which becomes pro,oressively protona ted when pH is changed from 7 to 5, is essential to reduce cytochrome b-563 (low potential) during quinol oxidation. Above pH 9, permanent e ffects were observed on this rate-coefficient, which were absent in th e reactions between P700, plastocyanin and cytochrome f.