A comparative study of the terminal stages of chlorophyll biosynthesis before and after heavy water (D2O) introduction into greening plant leaves

By spectral methods, the final stages of chlorophyll formation from protochlorophyllide proceeding in intact greening maize leaves were studied before and after the introduction of heavy water (D2O) into etiolated leaves. Three effects of D2O introduction were observed: 1) a complete inhibition of the reaction pathway leading to pheophytin biosynthesis and formation of pheophytin/chlorophyll-containing complexes (presumably, direct precursors of Photosystem II reaction centers): 2) 5-fold inhibition of the reaction of the Shibata shift; 3) appearance of a new dark reaction of the primary chlorophyllide native form Chld 684/676--> 'Chld 690/680'. It was shown that the intermediate Chld 684/676 presents the point of a triple branching of chlorophyllide transformation; activities of these three parallel pathways of Chld 684/676 transformation can be regulated by light intensity as well as by temperature. [GRAPHICS] es of Chl. aurantiacus were adsorbed onto a phospholipid and menaquinone-impregnated collodion him. We found that after the second excitation flash, but not after the first one, the photoelectric response included, in addition to the fast kinetic components reflecting the charge separation between the tetraheme cytochrome c and MQA, a slower kinetic component with a rise time of 3 mu s (pH = 8.3) and a relative amplitude of about 10% of the charge separation phase in the RC, We attributed this reaction to the electrogenic proton transfer which accompanied the transfer of the second electron during the MQ(A)(-)MQ(B)(-) --> MQ(A)MQ(B)H(2) transition. The rise time of the same reaction was reported to be almost three orders of magnitude slower in the isolated, proteoliposome-incorporated RC from this bacterium. The possible reasons of the faster turnover rates observed in the chlorosome-carrying native membrane preparations from Chl. aurantiacus are discussed.