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Figure 1.
Figure 1. The structure of sucrose (Glc-α(1 → 2)β-Fru)
in complex with maltoporin, and comparison with the
corresponding maltose complex. (a) Stereo view of the cyclic
averaged electron density of sucrose with the final model
superimposed. To avoid any model bias only the protein model was
used for the calculation of the initial (2 F[obs]−F[calc])
map. The methyl-hydroxyl groups of the fructosyl residue are
labeled. For steric reasons the two sugar rings are arranged
approximately at right angles to each other, while in maltose
(see (d)), the angle between the sugar residues is considerably
smaller. (b) Depiction of sucrose in relation to the greasy
slide and the channel constriction. The extracellular vestibule
of the channel is on top, and the periplasmic outlet at the
bottom of the Figure. The barrel axis of maltoporin is oriented
vertically and tilted by about 30° towards the viewer. The
sugar model, the greasy slide residues (Trp#74, donated from an
adjacent subunit; Tyr41; Tyr6; Trp420; Trp358; Phe227) and
Tyr118 (right) are shown as stick models with the cyclic
averaged electron density superimposed. The (clipped)
C^α-tracing is colored in green. The glucosyl moiety of sucrose
is in van der Waals contact with the greasy slide, while the
fructosyl residue is found above the channel constriction. (c)
Stereographic representation of the complex viewed from the
vestibule at the extracellular side onto the constriction site,
and along the pore axis. Three of the four hydroxyl groups of
the glucosyl moiety are H-bonded to residues of the ionic
tracks, hydroxyl O2-H is facing the channel entrance. The O6-H
hydroxyl group of the fructosyl residue is bonded to Asp121, a
residue that is not part of the constriction zone. During
translocation, steric interactions of the fructosyl moiety with
Tyr118 and Arg109 appear to hinder the movement of the
saccharide across the pore constriction. (d) Superposition of
the sucrose-maltoporin complex (atom colors) with the two
maltose molecules (steel-blue) as observed in complex with
maltoporin [Dutzler et al 1996]. The protein structures of both
complexes are virtually identical. The view is the same as in
(b). The individual glucosyl moieties of the maltose molecules
are bound to subsites S1 to S4 (labeled in green). The glucosyl
of sucrose adopts a position intermediate between S2 and S3.
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