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Title
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Crystal structure of Escherichia coli cystathionine gamma-synthase at 1.5 A resolution.
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Authors
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T.Clausen,
R.Huber,
L.Prade,
M.C.Wahl,
A.Messerschmidt.
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Ref.
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Embo J, 1998,
17,
6827-6838.
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PubMed id
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Abstract
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The transsulfuration enzyme cystathionine gamma-synthase (CGS) catalyses the
pyridoxal 5'-phosphate (PLP)-dependent gamma-replacement of
O-succinyl-L-homoserine and L-cysteine, yielding L-cystathionine. The crystal
structure of the Escherichia coli enzyme has been solved by molecular
replacement with the known structure of cystathionine beta-lyase (CBL), and
refined at 1.5 A resolution to a crystallographic R-factor of 20.0%. The enzyme
crystallizes as an alpha4 tetramer with the subunits related by
non-crystallographic 222 symmetry. The spatial fold of the subunits, with three
functionally distinct domains and their quaternary arrangement, is similar to
that of CBL. Previously proposed reaction mechanisms for CGS can be checked
against the structural model, allowing interpretation of the catalytic and
substrate-binding functions of individual active site residues. Enzyme-substrate
models pinpoint specific residues responsible for the substrate specificity, in
agreement with structural comparisons with CBL. Both steric and electrostatic
designs of the active site seem to achieve proper substrate selection and
productive orientation. Amino acid sequence and structural alignments of CGS and
CBL suggest that differences in the substrate-binding characteristics are
responsible for the different reaction chemistries. Because CGS catalyses the
only known PLP-dependent replacement reaction at Cgamma of certain amino acids,
the results will help in our understanding of the chemical versatility of PLP.
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