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Title
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Three-dimensional structure of the tryptophan synthase alpha 2 beta 2 multienzyme complex from Salmonella typhimurium.
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Authors
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C.C.Hyde,
S.A.Ahmed,
E.A.Padlan,
E.W.Miles,
D.R.Davies.
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Ref.
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J Biol Chem, 1988,
263,
17857-17871.
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PubMed id
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Abstract
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The three-dimensional structure of the alpha 2 beta 2 complex of tryptophan
synthase from Salmonella typhimurium has been determined by x-ray
crystallography at 2.5 A resolution. The four polypeptide chains are arranged
nearly linearly in an alpha beta beta alpha order forming a complex 150 A long.
The overall polypeptide fold of the smaller alpha subunit, which cleaves indole
glycerol phosphate, is that of an 8-fold alpha/beta barrel. The alpha subunit
active site has been located by difference Fourier analysis of the binding of
indole propanol phosphate, a competitive inhibitor of the alpha subunit and a
close structural analog of the natural substrate. The larger pyridoxal
phosphate-dependent beta subunit contains two domains of nearly equal size,
folded into similar helix/sheet/helix structures. The binding site for the
coenzyme pyridoxal phosphate lies deep within the interface between the two beta
subunit domains. The active sites of neighboring alpha and beta subunits are
separated by a distance of about 25 A. A tunnel with a diameter matching that of
the intermediate substrate indole connects these active sites. The tunnel is
believed to facilitate the diffusion of indole from its point of production in
the alpha subunit active site to the site of tryptophan synthesis in the beta
active site and thereby prevent its escape to the solvent during catalysis.
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