Chemistry Reference
In-Depth Information
throughout by threonine. This simple substitution lead to a fivefold increase in the
catalytic rate constant up to k
cat
/k
uncat
¼
1.3 min
1
) for substrate
90,000 (k
cat
¼
2
.
15.4 GLYCOPEPTIDE DENDRIMERS
Due to their size, peptide dendrimer analogs of proteins should be optimally suited to
perturb protein-protein and carbohydrate-protein interactions. An initial study
toward this goal was carried out focusing on lectin-glycoprotein interactions [32].
Lectins are ubiquitous proteins that bind to the terminal carbohydrates of glyco-
proteins or glycolipids by weak but specific binding sites. Lectins usually possess
multiple binding sites and form oligomers, allowing avidity effects to enhance
binding to carbohydrate epitopes displayed in polyvalent format on cell surfaces.
Multivalent carbohydrate ligands generally bind strongly to lectins by virtue of such
multivalency effects [33,34].
In the case of peptide dendrimers, one can expect that attaching carbohydrates at
the end of the peptide dendrimer sequence should produce multivalent lectin-specific
ligands inwhich binding by the carbohydratemoiety can be fine-tuned by the nature of
the amino acid residues in the peptide dendrimer branches. These residues should at
least determine the physicochemical properties of the dendrimers such as aqueous
solubility, hydrophobicity, and charge, and at best engage in secondary binding
interactions with the lectin. These hypotheses were borne out in practice in the study
of fucosylated peptide dendrimer ligands to the bacterial lectin LecB as discussed
below.
15.4.1 Synthesis of Glycopeptide Dendrimers
While the preparation of natural glycopeptides requires sophisticated chemical or
enzymatic glycosylation chemistry, the decoration of synthetic macromolecules with
carbohydrates is often more conveniently realized by reacting preassembled glyco-
sidic building blocks by simpler coupling chemistries. In an initial synthetic study
aimed at the dendrimer-mediated delivery of colchicine to cancer cells, three different
strategies were investigated to obtain glycopeptide dendrimers [35]. The first
approach used oxime bond formation between a multivalent aldehyde-terminated
peptide dendrimer and an oxyamine-functionalized glycoside, as reported previously
for the glycosylation of peptides [36] and peptide dendrimers [37]. The second
approach used reductive alkylation of the free N-terminus with lactose [38], a method
previously known for modifying whole proteins [39] and reported for the synthesis of
glycodendrimers [40]. The third approach involved amide bond formation between an
acetyl-protected glycosidic carboxylic acid building block and the N-termini of the
peptide dendrimer directly on the solid support. Although this third approach required
multigram quantities of carbohydrate building blocks, the synthesis was the most
practical because it required only a single chromatographic purification and yielded
products of excellent purity. For example, the N-acetyl-glucosamine terminated
peptide dendrimer colchicine conjugate
, which shows selective cytotoxicity to
cancer cells, was obtained by this approach (Figure 15.7).
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