The Binding of the Coenzyme Pyridoxal 5'-Phosphate and Analogues of the Substrate-Coenzyme Complex to Tyrosine Decarboxylase

Phosphopyridoxyl derivatives, which are stable analogues of a substrate-coenzyme complex, are bound at the active site with great affinity. From a comparison of the interaction of a number of such compounds with the apoenzyme the AGO values for the binding of the substrate carboxy and phenyl groups and of the coenzyme aldehydic group were determined to be 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 AGO for the binding of the coenzyme phosphate group was shown to be more negative than -20.5kJ/mol (-4.9 kcal/mol). Two features of the binding process of the coenzyme-substrate analogues to tyrosine decarboxylase have already been found in the case of tyrosine aminotransferase [Borri-Voltattorni, Orlacchio, Giartosio, Conti & Turano (1975) Eur. J. Biochem. 53, 151-160]: (1) in the binding of the substrate to the enzyme a significant fraction of the intrinsic AGO appears to be used for some associated endoergonic process; (2) the AH0 and AS0 of binding appear to be very sensitive indicators of the correct alignment of the substrate-coenzyme analogues at the active site.