J. Biol. Chem., Vol. 255, Issue 11, 5154-5158, Jun, 1980
The role of tryptophan in aspartate transcarbamylase
J Foote, DM Ikeda and ER Kantrowitz
Replacement of 7-azatryptophan for tryptophan in two positions on the
catalytic chain of aspartate transcarbamylase results in changes in the
enzyme's homotropic and heterotropic interactions although there is no
change in the enzyme's specific activity. The extent of azatryptophan
incorporation was quantitated by amino acid analysis which showed that 85%
of the tryptophan residues had been replaced. The substituted enzyme is
activated by ATP and inhibited by CTP to a greater extent than is the
native enzyme. The aspartate saturation curve in the presence of ATP is
identical for the two enzymes, but the curve in the presence of CTP and
without effectors is shifted toward higher aspartate concentrations for the
azatryptophan-substituted enzyme. At low aspartate concentrations, the
native enzyme is activated to a greater extent by the substrate analog
succinate. These data suggest that the substitution renders the low
substrate affinity conformational state of the enzyme less catalytically
efficient. This interpretation is in agreement with possible side chain
interactions observed in the three-dimensional structure of the enzyme.
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