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Volume 271, Number 47, Issue of November 22, 1996 pp. 29865-29869
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Engineered Complementation in Escherichia coli Aspartate Transcarbamoylase
HETEROTROPIC REGULATION BY QUATERNARY STRUCTURE STABILIZATION

(Received for publication, June 13, 1996, and in revised form, September 11, 1996)

From the Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02167

Escherichia coli aspartate transcarbamoylase regulates pyrimidine biosynthesis by altering its activity homotropically in response to one of its substrates and heterotropically in response to nucleotide effectors. The mechanism of this regulation involves two structurally and functionally different forms of the enzyme, one with low activity and low affinity for substrates (T state) and the other with high activity and high affinity for substrates (R state). Heterotropic regulation may be due to the direct transmission of a regulatory "signal" between the regulatory site and the active site some 60 Å away and/or may involve altering the relative stability of the two forms of the enzyme. By combining a T state-stabilized mutant version of the enzyme, previously thought to have a defect in a heterotropic transmission pathway, with a known R state-stabilized mutant enzyme, we were able to restore some properties of the wild-type enzyme. These data imply that the relative stabilization of the T and R states of the enzyme is in part responsible for the homotropic and heterotropic properties of aspartate transcarbamoylase and that direct pathways for transmission of the heterotropic signals are unlikely.

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