Chemistry Reference
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The fi rst TR-NOE-based NMR study in the carbohydrate fi eld investigated the
binding of methyl
- lactoside to the B -chain of the potent plant toxin ricin
(please see Chapters 15 and 18 for further information on ricin). Using one-
dimensional TR -NOE experiments and a selectively deuterated substrate,
Prestegard and coworkers showed that only minor changes in the conformation
of free methyl
β
-lactoside took place upon binding [V.L. Bevilacqua
et al.
Con-
formation of methyl
β
- lactoside bound to the ricin B - chain: interpretation of
transferred nuclear Overhauser effects facilitated by spin simulation and selec-
tive deuteration.
Biochemistry
1990;
29
, 5529-5537]. Later, Asensio
et al.
showed
that the protein caused a slight conformational variation in the glycosidic
torsion angles of methyl
β
-lactoside upon binding [J.L. Asensio
et al.
Studies
of the bound conformations of methyl
α
- allolactoside
to ricin B chain using transferred NOE experiments in the laboratory and rotat-
ing frames, assisted by molecular mechanics and dynamics calculations.
Eur J
Biochem
1995;
233
, 618-630], although the recognized conformer was still
within the lowest energy region. In the same study, it was found that different
conformations around the
α
- lactoside and methyl
β
- allolactoside
were recognized by the lectin. In fact, for this complex, only the TR- NOE spec-
troscopy cross-peaks corresponding to the protons of the galactose residue were
negative, as expected for a molecule in the slow motion regime. In contrast,
the corresponding cross-peaks for the glucose residue were nearly zero, as
expected for a molecule with motional properties practically independent from
the overall motion of the protein.
φ
,
ψ
and
ω
glycosidic bonds of methyl
β
major variations in the conformational behavior of the sugar are observed upon
protein binding. Galectin-1, as further example, recognizes the
syn
- conformer
(
- galactosyl xyloses [11] , which represents
around 90% of the free sugar population (for angle designation of glycosidic
linkage, please see Figure 2.2; for information on the conformational behavior of
lactose, please see Figure 2.3; for biological activity of galectin-1, please see Fig.
25.1). Indeed, only this conformation allows the establishment of favorable con-
tacts of the Glc/Xyl unit with the protein (Figure 13.7). Similarly, the
syn
-
φ
50 - 60 ° ,
ψ
0 °) of lactose and different
β
con-
former of
C
- lactose, a non - hydrolyzable lactose analog, is bound by galectin - 1
(Figure 13.6). In this case, this is not the predominant conformer in solution, as
C
-lactose exhibits much higher fl exibility than the
O
- analog and three different
conformational regions (
syn
,
anti
and
gauche
-
gauche
) are signifi cantly populated.
However, only the
syn
-
Φ
conformer makes the formation of three hydrogen
bonds between the Glc residue and the protein possible. Thus, this is an instruc-
tive example of selection of a minor conformer from an equilibrium mixture in
solution. In contrast,
C
-lactose is bound to ricin B-chain in the
anti
-
Φ
,
syn
-
Ψ
ψ
conforma-
tion, whereas the enzyme
Escherichia coli
β
- galactosidase recognizes the
anti
-
Φ
conformer ([10] and references therein), hereby showing that different carbo-
