Mitochondrial F1-ATPase will bind and cleave ATP but only slowly release ADP after N,N'-dicyclohexylcarbodiimide or 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole derivatization

Kandpal, R.P.; Melese, T.; Stroop, S.D.; Boyer, P.D.

Journal of Biological Chemistry 260(9): 5542-5547

1985


ISSN/ISBN: 0021-9258
PMID: 2859288
Accession: 005905581

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Abstract
The ATPase from the inner mitochondrial membrane is inhibited by modification of 1 of the 3 catalytic subunits with N,N'-dicyclohexylcarbodiimide (DCCD) or 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole. Most of the residual ATPase activity observed after the usual DCCD or 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole modification is due to the presence of unmodified enzyme, although the large fraction of modified enzyme remains a weak catalytic activity. This weak catalytic activity can be stimulated by methanol or dimethyl sulfoxide. When the modified enzymes are exposed to Mg2+ and [3H]ATP, about equal amounts of [3H]ATP and [3H]ADP appear at catalytic sites. The turnover rate for these enzymes is < 1/1000 that of the native enzyme when it is calculated from the rate at which the enzyzme becomes labeled at the catalytic sites with [3H]ATP and [3H]ADP during steady state hydrolysis. A higher ATP concentration is required for steady state turnover and, after ATP binding, the principal rate-limiting step is the capacity of the derivatized enzyme to undergo the binding changes necessary for the release of ADP and Pi. When the modified enzymes were not hydrolyzing ATP, they convert to form(s) that show a distinct lag in the replacement of bound nucleotides at catalytic sites. The replacement of bound nucleotides is still promoted by MgATP, even though the enzymes have been converted to sluggish forms. Contrary to a recent suggestion based on the study of the DCCD-modified enzyme, the data provide evidence for the existence of catalytic cooperativity between at least 2 alternating sites in the modified enzyme and are consistent with continued sequential participation of all 3 sites.