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Loose and tight binding of adenine nucleotides by membrane-associated chloroplast ATPase
Authors:Susanne Bickel-Sandkötter
Institution:Botanisches Institut der Tierärztlichen Hochschule Hannover, Bünteweg 17d, D-3000 Hannover F.R.G.
Abstract:Steady-state binding of adenine nucleotides by thylakoid membranes is measured by employing a centrifugation technique. By this method tightly bound nonexchangeable nucleotides can be discriminated from loosely bound, exchangeable nucleotides. Nucleotide binding requires membrane energization and is highly specific for medium ADP. In illuminated chloroplasts almost no exogenous AMP and only some ATP are incorporated, most being recovered as tightly bound nucleotides. In light-triggered chloroplasts, however, which are capable of hydrolyzing ATP, a high level of exchangeable nucleotides is found on the membranes. The sum of tightly bound and loosely bound nucleotides originating from medium ADP is about one per CF1. The ratio between them decreases with increasing proton-motive force. Exchangeable nucleotides most probably represent the ligands involved in the catalytic process, as suggested from substrate specificity and the effect of a competitive inhibitor of photophosphorylation, naphthoyl ADP. This compound in a low concentration range supresses loose binding but not tight binding of medium ADP. Under phosphorylating conditions (presence of ADP, Pi and light), some of the tightly bound nucleotides exist as ATP even in the presence of a hexokinase system. The results are discussed in the context of the regulation of chloroplast ATPase by tight nucleotide binding.
Keywords:Photophosphorylation  ATPase  Adenine nucleotide binding  Thylakoid membrane  Proton-motive force  (Spinach chloroplast)  chloroplast coupling factor 1  Chl  chlorophyll  FCCP  naphthoyl ADP  naphthoyl ATP  Tricine
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