Synthesis of 2′-/3′-O-Acylated Adenine Nucleotide Analogs and Their Interactions in Photophosphorylation

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Abstract
By mono esterification of 3′(2′)-hydroxyl residues of adenine nucleotides with various carboxylic acids a series of nucleotide analogs is available including fluorescent and photoaffinity labels. Their chemical synthesis is described. The equilibrium between 2′ and 3′ esters is determined by NMR spectroscopy, stability of the esters and their tendency of acyl migration is discussed. The interaction of the ADP derivatives with the chloroplast ATP synthesizing system is investigated. Actually, the analogs are typical energy transfer inhibitors, strongly inhibiting photophosphorylation and concomitant coupled electron transport (ci50 values ranging from 0.3 to 85 hm). On the basis of inhibitory activities of analogs bearing varying 3′-(2′)-substituents, structure-activity relationships are discussed. The inhibitory properties of the employed ADP analogs are based on their specific interaction with the catalytic ADP binding site of CF, and their extremely slow phosphorylation on the enzyme (rate 0.25% or less compared to ADP phosphorylation). Inhibition is competitive to ADP but non-competitive with regard to Pi. It is specific for the ADP derivatives, whereas the corresponding ATP analogs are only weak inhibitors in phosphorylation and the AMP derivatives are completely inactive. In light-triggered ATP hydrolysis, however, the ATP analogs exhibit an even stronger competitive inhibition than the ADP derivatives. The results suggest that a conformational change of ATPase takes place when the chloroplasts are transferred from energized to de-energized conditions which greatly affects the properties of the active site with respect to nucleotide binding.

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