The effect of various energy-conversion states of chloroplasts on proton and electron transport

Abstract

The stoichiometry of H + and electron transport in spinach chloroplasts was very sensitive to the presence or absence of the phosphate-acceptor system and energy-transfer inhibitors. With methyl viologen as the electron acceptor, the H + e − ratio (based on initial rates) in the absence of the complete coupling system was about 2H + per electron. With all of the cofactors for phosphorylation present (or with arsenate replacing phosphate) the ratio drops to about 1 to 1.3 H + per electron. The steady-state ratio in both cases was about 0.5 to 0.7 H + per electron. Synthalin and DIO-9, energy-transfer inhibitors, caused an increase in the H + e − ratio from about 1 up to about 3, while inhibiting phosphorylation and the coupled electron flow. Synthalin and DIO-9 also stimulate the H + e − ratio in the absence of ADP and phosphate. In this case, the H + uptake rate was stimulated almost to the same rate as found with the complete acceptor system present and with these inhibitors added. Poly- l-lysine stimulated both the initial and the steady-state proton pump rates and inhibited ATP formation. If the complete phosphate-acceptor system was present, electron flow was little affected; in the absence of ADP, electron flow was increased to the coupled rate. In both cases, the H + e − ratio was increased to about 4–6 H + per electron, the same values obtained at pH 6 without added inhibitors. Quantum yield experiments supported the H + e − ratio data, showing that in the presence of polylysine at pH 7.8, 3–4 protons were transported per quantum absorbed by Photosystem I. The data are discussed in terms of two possible mechanisms: (A) than an H + ion carrier capable of transporting a variable number of protons is linked to the electron-transport chain but on a side path from direct energy flow to ATP synthesis. In this scheme, energy-transfer inhibition or the absence of the complete coupling system would lead to greater H + ion transport activity due to the alternative use of the highenergy state by the ion-transport mechanism; (B) the proton-transport carrier could be linked to one of two electron transport chains. In this scheme, inhibition of phosphorylation by withholding ADP or by energy-transfer inhibitors would lead to higher H + per electron ratios due to a greater proportion of the total electron flow going through the H +-linked arm of the redox chain. This hypothesis is discussed in view of recent work implicating the involvement of ion gradients in the energy-conversion mechanism of chloroplasts.