Reaction of (sodium(1+)-potassium(1+) ion)-activated ATPase with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole: evidence for an essential tyrosine at the active site

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Abstract
The reaction of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole [NBD-Cl] with purified eel electroplax (Na-K)ATPase was monitored by changes in the (Na-K)ATPase activity, the K+ stimulated p-nitrophenyl phosphatase [PNPase] activity, and the protein UV absorption spectrum. The NBD-Cl reacts with 2 tyrosine residues/mol of enzyme (.apprx. 6-7 nmol/mg of protein), as judged by changes in protein absorption spectra and incorporation of [14C]NBD-Cl. The modified tyrosine groups are located on the 95,000 MW polypeptide chain and react at different rates. Only 1 tyrosine modification is necessary for complete inhibition of (Na-K)ATPase activity, although both must be modified for complete inhibition of PNPase activity. Reversal of these modifications by 2-mercaptoethanol restores 65% of both activities. Na+ increases the rate of tyrosine modification, K+ decreases the rate, and ATP affords the more reactive tyrosine group complete protection. NBD-Cl modification of .apprx. 6-7 nmol of tyrosine groups/mg of protein results in a large decrease in ATP affinity as judged by equilibrium binding. These results are compared with similar results obtained from NBD-Cl modification of the coupling factors of oxidative phosphorylation and photophosphorylation. A model is presented suggesting an asymmetric arrangement of 2 95,000 MW polypeptide chains with a single tyrosine residue at the ATP site.

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