The binding of the coenzyme pyridoxal 5'-phosphate and analogues of the substrate-coenzyme complex to tyrosine decarboxylase

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Abstract
Phosphopyridoxyl derivatives, which are stable analogs of a substrate-coenzyme complex, are bound at the active site [of Streptococcus saccalis tyrosine decarboxylase] with great affinity. A comparison of the interaction of a number of such compounds with the apoenzyme showed that the .DELTA.Go [standard free energy change] values for the binding of the substrate carboxy and phenyl groups and of the coenzyme aldehydic group were equal to (or more negative than) -3.8, -8.4 and -12.5 kJ/mol (-0.9, -1.9 and -3 kcal/mol) respectively; the .DELTA.Go for the binding of the coenzyme phosphate group was more negative than -20.5 kJ/mol (-4.9 kcal/mol). Two features of the binding process of the coenzyme-substrate analogs to tyrosine decarboxylase were already found in the case of tyrosine aminotransferase: in the binding of the substrate to the enzyme a significant fraction of the intrinsic .DELTA.Go appears to be used for some associated endoergonic process and the .DELTA.Ho [standard enthalpy change] and .DELTA.So [standary entropy changes] of binding appear to be very sensitive indicators of the correct alignment of the substrate-coenzyme analogs at the active site.