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The three-dimensional structure of the maltose- or maltodextrin-binding protein
(Mr = 40,622) with bound maltose has been obtained by crystallographic analysis
at 2.8-A resolution. The structure, which has been partially refined at 2.3 A,
is ellipsoidal with overall dimensions of 30 x 40 x 65 A and divided into two
distinct globular domains by a deep groove. Although each domain is built from
two peptide segments from the amino- and carboxyl-terminal halves, both domains
exhibit similar supersecondary structure, consisting of a central beta-pleated
sheet flanked on both sides with two or three parallel alpha-helices. The
groove, which has a depth of 18 A and a base of about 9 x 18 A, contains the
maltodextrin-binding site. We have previously observed the same general features
in the well-refined structures of six other periplasmic receptors with
specificities for L-arabinose, D-galactose/D-glucose, sulfate, phosphate,
leucine/isoleucine/valine, and leucine. The bound maltose is buried in the
groove and almost completely inaccessible to the bulk solvent. The groove is
heavily populated by polar and aromatic groups many of which are involved in
extensive hydrogen-bonding and van der Waals interactions with the maltose. All
the disaccharide hydroxyl groups, which form a peripheral polar surface
approximately in the plane of the sugar rings, are tied in a total of 11 direct
hydrogen bonds with six charged side chains, one Trp side chain, and one peptide
backbone NH, and five indirect hydrogen bonds via water molecules. The maltose
is wedged between four aromatic side chains. The resulting stacking of these
aromatic residues on the faces of the glucosyl units provides a majority of the
van der Waals contacts in the complex. The nonreducing glucosyl unit of the
maltose is involved in approximately twice as many hydrogen bonds and van der
Waals contacts as the glucosyl unit at the reducing end. The binding
protein-maltose complex shows the best example of the extensive use of polar and
aromatic residues in binding oligosaccharides. The tertiary structure of the
maltodextrin-binding protein, along with the results of genetic studies by a
number of investigators, has also enabled us for the first time to map the
different regions on the surface of the protein involved in the interactions
with the membrane-bound protein components necessary for transport of and
chemotaxis toward maltodextrins. These sites permit distinction of the "open
cleft" (without bound sugar) and closed (with bound sugar) conformations of the
binding protein by the chemotactic signal transducer with which the
maltodextrin-binding protein interacts.(ABSTRACT TRUNCATED AT 250 WORDS)
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