Synthesis of biotinylated glycosulfopeptides by chemoselective ligation

Recent progress in the field of neoglycoconjugate chemistry

Glycosylation is probably the most complex secondary gene event that affects the vast majority of proteins in nature resulting in the occurrence of a heterogeneous mixture of glycoforms for a single protein. Many functions are exerted by single monosaccharides, well-defined oligosaccharides, or larger glycans present in these glycoproteins. To unravel these functions it is of the utmost importance to prepare well-defined single glycans conjugated to the underlying aglycon. In this review, the most recent developments are described to address the preparation of carbohydrate-amino acid (glyco-conjugates). Naturally occurring N- and O-linked glycosylation are described and the preparation of non-natural sugar-amino acid linkages are also included.

Glycation methods for bombesin analogs containing the (NalphaHis)Ac chelator for 99mTc(CO)3 radiolabeling

The overexpression of peptide receptors in a variety of human carcinomas has generated considerable interest in peptide-based radiopharmaceuticals for peptide receptor imaging and peptide receptor radiotherapy. The gastrin-releasing peptide receptor is overexpressed in human prostate-, breast-, colon- and small cell lung carcinoma cells. We have developed metabolically stable (99m)Tc-radiolabeled bombesin ([Cha(13), Nle(14)]BBS(7-14)) analogs, which bind with high affinity to the gastrin-releasing peptide receptors. However, because of their lipophilicity, they showed unfavorable biodistribution with high hepatic accumulation and hepatobiliary excretion. We now report a study of different glycation methods for [Cha(13), Nle(14)]BBS(7-14) analogs to improve their biodistribution profile. Whereas the glycation using the Maillard reaction was problematic, resulting in low yields, selective introduction of the glycomimetic shikimic acid to the side chain of a Lys residue was possible. A chemoselective ligation of alpha-D-glucose to an amino-oxyacetylated [Cha(13), Nle(14)]BBS(7-14) analog could be achieved, but was complicated by the co-elution of starting peptide and glycopeptide. The best procedure consisted of the [1,3]-cycloaddition of N(3)-beta-D-glucose to a propargylglycine-containing [Cha(13), Nle(14)]BBS(7-14) analog, using a catalytic amount of Cu(I)I. All glycated [Cha(13), Nle(14)]BBS(7-14) analogs showed high affinity for the gastrin-releasing peptide receptor and rapid accumulation into PC-3 tumor cells.

Site-Specific Incorporation of Glycosylated Serine and Tyrosine Derivatives into Proteins

Glycosylation of proteins can have a dramatic effect on their physical, chemical, and biological properties. Analogues of dihydrofolate reductase and firefly luciferase containing glycosylated amino acids at single, predetermined sites have been elaborated. Misacylated suppressor tRNAs activated with glycosylated serine and tyrosine derivatives were used for suppression of the nonsense codons in a cell-free protein biosynthesizing system, thereby permitting the preparation of the desired glycosylated proteins. In this fashion, it was possible to obtain proteins containing both mono- and diglycosylated amino acids, including glycosylated serine and tyrosine moieties. For the modified firefly luciferases, the effect of these substitutions on the wavelength of the light emitted by firefly luciferase was investigated. The maximum wavelength for mutants containing peracetylated glycosylated serine derivatives at position 284 showed a red shift in the emission spectra. For mutants containing glycosylated tyrosines, the red shift was observed only when the carbohydrate moiety was fully deacetylated.

A versatile set of aminooxy amino acids for the synthesis of neoglycopeptides

Four N-alkylaminooxy amino acids have been synthesized in 22-56% overall yield from readily available amino acid precursors. Each amino acid can be efficiently incorporated into peptides using Boc-chemistry-based solid-phase peptide synthesis, and in three of the four cases the resulting peptides can be chemoselectively glycosylated at the aminooxy side chains to generate neoglycopeptides. The range of N-alkylaminooxy amino acids prepared allows attachment of sugars at two-, three-, or four-atom distances from the peptide backbone, and each ensures that attached sugars adopt cyclic conformations. These derivatives provide convenient access to arrays of biologically relevant neoglycopeptides that may be used to probe the influence of attached sugars on the structure and function of peptides and proteins.