Effects of l, N6-ethenoadenylates on the regulation of photosynthetic activities by adenylates; A study of the nucleotide binding sites on chloroplast coupling factor 11

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Abstract
Effects of l, N6-ethenoadenylates (e-adenylates) were tested on phosphorylation, and electron transport under phosphorylation, arsenylation and quasi-arsenylation (stimulation of electron transport in the presence of ATP, AMP and arsenate) conditions in isolated spinach chloroplasts. ε-ATP as well as ATP partially inhibited ferricyanide reduction through binding to the chloroplast coupling factor 1 with an apparent dissociation constant (KDapp) of around 5μM, which was remarkably larger than that for ATP (ca. 2μM). e-ATP at below 500 μM had no effect on phosphorylation but inhibited quasi-arsenylation in competition with ATP with an apparent inhibition constant (K1app) of around 60 μM. ε-ADP as well as ADP partially inhibited ferricyanide reduction with a KDapp value close to that for ε-ATP. ε-ADP was phosphorylated (the apparent Michaelis constant, Kmapp=80μM) accompanying stimulation of ferricyanide reduction to the magnitude predicted (P/Δe=l). ε-ADP-arsenylation was also detected by stimulation of ferricyanide reduction. ε-AMP alone caused little inhibition of ferricyanide reduction as AMP, but competitively depressed the electron transport inhibition by ADP and ATP with a K1app value of around 200 μM. ε-AMP was not effective for ADP phosphorylation but inhibited stimulation due to quasi-arsenylation coupling in competition with AMP K1app=150 μM Among the possible combinations of adenylates and ε-adenylates for quasiarsenylation, only [ATP+AMP] could couple with the energy transduction mechanism. Based on the specificity of binding sites to adenylates and ε-adenylates, an attempt was made to distinguish at least four (two pairs) kinds of binding sites (at least six sites in toto) on the chloroplast coupling factor 1 for photosynthetic energy transduction. When one pair of sites is occupied by the designated adenylates or ε-adenylates (allosteric effectors), the coupling factor is thought to be in a conformation for coupling with the energy transduction mechanism in the presence of phosphate or arsenate.

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