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JBC, Vol. 251, Issue 9, 2820-2824, May, 1976

The dissociated tryptophanase subunit is inactive

O. Raibaud and M. E. Goldberg

The dissociation into dimers of apotryptophanase has been studied from two points of view: the nature of the interactions which govern the dimer-tetramer equilibrium and the effect of dissociation on the functional properties of the enzyme. It is shown that the order in which different anions are able to shift the dimer-tetramer equilibrium is that of the Hofmeister series, thus showing that the main contribution to the interaction between two dimers is of hydrophobic nature. It is also shown that, when dimeric apotryptophanase is incubated in the presence of its cofactor and substrate, the kinetics of appearance of active molecules is of second order in enzyme and is independent of the pyridoxal-P concentration; its rate constant has been determined (5-10(4) M-1 S-1). These results indicate that the reassociation of dimers into tetramers is the rate-limiting step in the appearance of enzymatic activity, and that the tryptophanase dimer is not functional.
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