The main residues involved in substrate binding are shown in
Fig. 1B (6). Arg-144 (helix V) and Glu-126 (helix IV) are the
major determinants. Arg-144 forms H-bonds with the O3 and O4
atoms of the galactopyranosyl ring, whereas Glu-126 may interact
with the O4, O5, or O6 via water molecules. Although not
shown in the structure, biochemical evidence indicates that these
residues form a charge pair in the absence of the substrate
(21–25). Recent experiments (26) using carbodiimide labeling in
the absence or presence of ligand in conjunction with mass
spectroscopy indicate that Glu-269 (helix VIII) may be implicated
in sugar binding, possibly interacting with the C3OHof the
galactopyranosyl ring. The x-ray crystal structure reveals that
this residue forms a salt bridge with Arg-144 as well as an H-bond
with Trp-151 (6). Because all replacements for Glu-269, with the
exception of aspartic acid, do not bind sugar and because all
modes of translocation are abrogated, it is likely that Glu-269
may be a key residue, coupling H translocation and sugar
binding.