ISSN:
0091-7419
Keywords:
Life Sciences
;
Molecular Cell Biology
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Biology
,
Chemistry and Pharmacology
,
Medicine
Notes:
A new approach to the direct estimation of the value of the off constant for dissociation of ATP from myosin subfragment 1 (S1) has been developed. From measurements of the extremely slow rate of release of [32P]-ATP formed from 32Pi by S1 catalysis and the amount of rapidly formed [32P]-ATP tightly bound to S1, the value of the off constant is approximately 2.8 × 10-4 sec-1 at pH 7.4.The concentration dependencies for Pi ⇌ H18 OH exchange and for 32Pi incorporation into myosin-bound ATP give direct measurements of the dissociation constant of Pi from S1. Both approaches show that the enzyme has a very low affinity for Pi, with an apparent Kd of 〉 400 mM.Measurement of the average number of water oxygens incorporated into Pi released from ATP by S1-catalyzed hydrolysis in the presence of Mg2+ suggests that the hydrolytic step reverses an average of at least 5.5 times for each ATP cleaved. With the Ca2+-activated hydrolysis, less than one oxygen from water appears in each Pi released. This finding is indicative of a possible isotope effect in the attack of water on the terminal phosphoryl group of ATP.
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/jss.400030208
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