Evidence for two types of electron transfer processes through Photosystem II

Inhibition of electron flow from H₂O to methylviologen by 3-(34 dichlorophenyl)-1,1 dimethyl urea (DCMU), yields a biphasic curve — an initial high sensitivity phase and a subsequent low sensitivity phase. The two phases of electron flow have a different pH dependence and differ in the light intensity required for saturation.Preincubation of chloroplasts with ferricyanide causes an inhibition of the high sensitivity phase, but has no effect on the low sensitivity phase. The extent of inhibition increases as the redox potential during preincubation becomes more positive. Tris-treatment, contrary to preincubation with ferricyanide, affects, to a much greater extent, the low sensitivity phase.Trypsin digestion of chloroplasts is known to block electron flow between Q_A and Q_B, allowing electron flow to ferricyanide, in a DCMU insensitive reaction. We have found that in trypsinated chloroplasts, electron flow becomes progressively inhibited by DCMU with increase in pH, and that DCMU acts as a competitive inhibitor with respect to [H⁺]. The sensitivity to DCMU rises when a more negative redox potential is maintained during trypsin treatment. Under these conditions, only the high sensitivity, but not the low sensitivity phase is inhibited by DCMU.The above results indicate the existence of two types of electron transport chains. One type, in which electron flow is more sensitive to DCMU contains, presumably Fe in a Q_A Fe complex and is affected by its oxidation state, i.e., when Fe is reduced, it allows electron flow to Q_B in a DCMU sensitive step; and a second type, in which electron transport is less sensitive to DCMU, where Fe is either absent or, if present in its oxidized state, is inaccessible to reducing agents.Abbreviations DCMU 3-(34 dichlorophenyl)-1, 1 Dimethyl urea - MV methyl viologen - PS II Photosystem II - Tris tris (hydroxymethyl)aminomethane