Studies on well-coupled Photosystem-I-enriched subchloroplast vesicles. Electron transfer by b- and c-type cytochromes in relation to the origin of the ‘slow’ electric potential component

Abstract

Cytochrome redox changes and electric potential generation are kinetically compared during cyclic electron transfer in Photosystem-I-enriched and Photosystem-II-depleted subchloroplast vesicles (i.e., stroma lamellae membrane vesicles) supplemented with ferredoxin using a suitable electron donating system. In response to a single-turnover flash, the sequence of events is: (1) fast reduction of cytochrome b-563 ( t 0.5 ≈ 0.5 ms) (2) oxidation of cytochrome c-554 ( t 0.5 ≈ 2 ms), (3) slower reduction of cytochrome b-563 ( t 0.5 ≈ 4 ms), (4) generation of the ‘slow’ electric potential component ( t 0.5 ≈ 15–20 ms), (5) re-reduction of cytochrome c-554 ( t 0.5 ≈ 30 ms) and (6) reoxidation of cytochrome b-563 t 0.5 ≈ 90 ms). Per flash two cytochrome b-563 species turn over for one cytochrome c-554. These b-563 cytochromes are reduced with different kinetics via different pathways. The fast reductive pathway proceeds probably via ferredoxin, is insensitive to DNP-INT, DBMIB and HQNO and is independent on the dark redox state of the electron transfer chain. In contrast, the slow reductive pathway is sensitive to DNP-INT and DBMIB, is strongly delayed at suboptimal redox poising (i.e., low NADPH NADP + ratio) and is possibly coupled to the reduction of cytochrome c-554. Each reductive pathway seems obligatory for the generation of about 50% of the slow electric potential component. Also cytochrome c-559 LP (LP, low potential) is involved in Photosystem-I-associated cyclic electron flow, but its flash-induced turnover is only observed at low preestablished electron pressure on the electron-transfer chain. Data suggest that cyclic electron flow around Photosystem I only proceeds if cytochrome b-559 LP is in the reduced state before the flash, and a tentative model is presented for electron transfer through the cyclic system.