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(Received for publication, September 8, 1994; and in revised form, November 23,
1994) The amidotransferase or glutaminase (GLNase) domain of mammalian
carbamyl phosphate synthetase (CPSase), part of the 243-kDa CAD
polypeptide, consists of a carboxyl half that is homologous to all
trpG-type amidotransferases and an amino half unique to the carbamyl
phosphate synthetases. The two halves of the mammalian GLNase domain
have been cloned separately, expressed in Escherichia coli,
and purified. The 21-kDa carboxyl half, the catalytic subdomain, is
extraordinarily active. The k
Volume 270,
Number 5,
Issue of February 3, 1995 pp. 2190-2197
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
is 347-fold higher
and the K
is 40-fold lower than the
complete GLNase domain. Unlike the GLNase domain, the catalytic
subdomain does not form a stable hybrid complex with the E. coli CPSase synthetase subunit. Nevertheless, titration of the
synthetase subunit with the catalytic subdomain partially restores
glutamine-dependent CPSase activity. The 19-kDa amino half, the
interaction subdomain, binds tightly to the E. coli CPSase
large subunit. Thus, the GLNase domain consists of two subdomains which
can autonomously fold and function. The catalytic subdomain weakly
interacts with the synthetase domain and has all of the residues
necessary for catalysis. The interaction subdomain is required for
complex formation and also attenuates the intrinsically high activity
of the catalytic subdomain and, thus, may be a key element of the
interdomain functional linkage.
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