Thermodynamics and stoicheiometry of the binding of substrate analogues to uricase

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Abstract
The subunit composition, metal content, substrate-analogue binding and thermal stability of Aspergillus flavus uricase were determined. A. flavus uricase is a tetramer and contains no copper, iron or any other common prosthetic group. Analytical-gel-filtration and equilibrium-dialysis experiments showed one binding site per subunit for urate analogues. The free energy of xanthine binding was −30.5 kJ (-7.3 kcal)/mol of subunit by equilibrium dialysis and −30.1 kJ (-7.2 kcal)/mol of subunit by microcalorimetry. The enthalpy change for xanthine binding was −15.9 kJ (-3.8 kcal)/mol of subunit when determined from the temperature-dependence of the equilibrium constant and −18.0 kJ (-4.3 kcal)/mol of subunit when measured microcalorimetrically. The thermal inactivation rate of A. flavus uricase increases as protein concentration is decreased. This concentration-dependent instability is not due to subunit dissociation.