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Helguera G, Beauge L: Phosphoryl Group Exchange between ATP and ADP Catalyzed by H+-ATPase from Oat Roots. Plant Physiol. 1997 Aug;114(4):1397-1403.
ATP-ADP exchange was estimated in the presence of plasma membrane H+-ATPase of oat (Avena sativa) roots partially purified with Triton X-100 by measuring [14C] ATP formation from [14C] ADP. Most studies were done at 0 [deg] C. At pH 6.0 the exchange showed: (a) Mg2+ requirement with a biphasic response giving maximal activity at 152 [mu] M and (b) insensitivity to ionic strength, [Na+], and [K+]. ATP and ADP dependence were analyzed with a model in which nucleotide-enzyme interactions are at rapid-random equilibrium, whereas E1ATP [left right arrow] E1P-ADP transitions occur in steady state. The results indicated competition between ADP and ATP for the catalytic site, whereas ATP interaction with the ADP site was extremely weak. At 0 [deg] C the exchange showed a 3-fold pH increase, from pH 5.5 to 9.0. At an alkaline pH the reaction was not affected by sodium azide and carbonyl cyanide p-trifluometoxyphenyl-hydrazone, had a biphasic response to Mg2+ (maximal at 513 [mu] m), and was insensitive to ionic strength. At 20 [deg] C ATP-ADP exchange was pH insensitive. At both temperatures ATP hydrolysis displayed a bell-shaped response, with a maximum around pH 6.0 to 6.5. Because no adenylate kinase activity was detected under any condition, these results demonstrate the existence of an ATP-ADP exchange reaction catalyzed by the plant H+-ATPase. |
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