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Adenylate Cyclase from Brevibacterium liquefaciens

REVERSIBILITY AND THERMODYNAMIC STUDIES

From the ¹ From the Department of Physiological Chemistry and Nutrition, Faculty of Medicine, The University of Tokyo, Tokyo, Japan

The stoichiometric formation of ATP from adenosine 3' : 5'-monophosphate and PP_i was established using a homogeneous adenylate cyclase preparation from Brevibacterium liquefaciens. Thermodynamic parameters were calculated based on the measurement of equilibrium constants of the reaction. The standard Gibb's free energy change (G⁰') for the conversion of ATP to adenosine 3' : 5'-monophosphate and PP_i varied as a function of pH and was + 0.8 to 2.1 Cal per mole between pH 7.7 and 6.2 at 25° in the presence of 5 mm MgSO₄. This indicates that the formation of adenosine 3' : 5'-monophosphate from ATP is an endergonic reaction. The standard enthalpy change of the formation of adenosine 3' : 5'-monophosphate and PP_i from ATP at pH 7.3 was determined to be +5.0 Cal per mole according to the measurement of equilibrium constants as a function of temperature. The standard entropy change of the conversion of ATP to adenosine 3' : 5'monophosphate and PP_i was calculated to be 12.4 e.u. at pH 7.3.

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