SPLITTING OF D1-PROTEIN AND D2-PROTEIN BY TRYPSIN DOES NOT AFFECT THE PHOTOCHEMICAL ACTIVITY OF PHOTOSYSTEM-2 REACTION CENTER

The effect of trypsin on photosystem (PS) 2 reaction centre (RC) has been studied. The RC were isolated from spinach chloroplasts and at the last step the nonionic detergent Triton X-100 was replaced by dodecyl-beta-D-maltoside. Spectral (absorption spectrum, the 4th derivative of the absorption spectrum) and photochemical characteristics (pheophytin photoreduction, P680 photooxidation etc) were not changed in the PS 2RC after trypsin treatment. A rapid decrease in the rate of photoreduction of silicomolybdate in the presence of diphenylcarbazide was observed in the control and trypsinized PS 2 RC which is ascribed to photoactivation of the PS 2 donor side. The enzyme digested all the proteins in the PS 2 RC (D1/D2 heterodimer; D1-, D2-, 4.8 kDa proteins; alpha and beta-subunits of cytochrome b559). During the electrophoresis under mild conditions the PS 2 RC treated with trypsin migrated by one band. No pigments or peptides of molecular masses higher than 3-4 kDa were found to be separated from it. It is suggested that after the enzyme effect the native spatial structure of the PS 2 RC core is preserved. For comparison we have studied the effect of trypsin on the RC from Rhodopseudomonas viridis. It has been shown that in the bacterial RC the enzyme induced a dramatic decrease in the photochemical activity and considerable changes in the absorption spectrum (destruction of the native forms of bacteriochlorophyll and the appearance of the absorption band of monomeric bacteriopheophytin). The nature of the differences observed under the effect of the enzyme on the RC from the bacteria and PS 2 is discussed.