[The Role of Plastocyanin and Cytochrome F in Photosynthetic Electron Transport]

The dependence of photosynthetic NADP reduction on plastocyanin in three different fragmented systems from spinach chloroplasts was investigated. 1. In sonicated chloroplasts oxygen evolution and NADP reduction is restored by the addition of 3 mμmoles of plastocyanin obtained from spinach. Thirty mμmoles of cytochrome552 from Euglena replaces plastocyanin at pH 7.4 to about 75% and at pH 8.0 to only about 30%. NADP reduction at the expense of an artificial donor system by the same sonicated chloroplast preparation is, however, restored by plastocyanin and cytochrome552 equally well. 2. It is already well documented that in digitonin fragmented chloroplasts NADP reduction at the expense of an artificial donor system is stimulated by the addition of plastocyanin. Cytochrome552 from Euglena is as effective as plastocyanin in this system. 3. Heptane treatment of chloroplasts followed by water extraction also leads to the liberation of plastocyanin. NADP reduction in heptane treated chloroplasts at the expense of an artificial donor system is stimulated either by the addition of plastocyanin or of cytochrome552. These results show that in three different types of particles from spinach chloroplasts both plastocyanin (spinach) and cytochrome552 (Euglena) are equally effective als electron donors for pigment system I of photosynthesis, coupled to NADP reduction. This conclusion follows from the fact that both are equally effective in stimulating NADP reduction at the expense of an artificial electron donor system. In sonicated chloroplasts plastocyanin seems to be the better electron acceptor for electrons coming from the photooxidation of water by light reaction II, since addition of plastocyanin to a system depending on oxygen evolution yields better rates than addition of cytochrome552.In order to explain the result that there are two possible electron donors for pigment system I it is suggested that there are two-perhaps spatially separated-pigment systems I in photosynthesis which are participating in a non-cyclic or a cyclic electron transport system and which are either coupled to plastocyanin or to cytochrome f. The difference in rates mentioned above may indicate that plastocyanin is a component of non-cyclic and cytochrome f of cyclic electron flow. The cyclic system can be converted into a non-cyclic system by the addition of an artificial electron donor and NADP.