The binding of thiamin diphosphate (ThDP) to yeast dimeric apotransketolase (apoTK) is accompanied by the appearance of a band in the absorption spectrum with maximum at 320 nm. The saturation function has been analyzed using a scheme that involves binding of ThDP to each subunit followed by the conformational transition of this subunit. It is assumed that the binding of ThDP to one subunit may affect the conformational transition of the other subunit. Rigorous mathematical expressions describing the dependence of the optical absorption on the total concentration of ThDP are first developed. Equilibrium constants and corresponding rate constants for the binding of ThDP to apoTK have been estimated. The negative cooperativity in the ThDP binding has been characterized by the function reflecting the dependence of the conformational change on the saturation of apoTK by ThDP.
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