Enzymatic protecting group techniques for glyco- and phosphopeptide chemistry: synthesis of a glycophosphopeptide...

The covalent modification of proteins by phosphorylation and by glycosylation with GlcNAc residues are important regulatory processes which mediate biological signal transduction. For the study of such biological phenomena in molecular detail characteristic peptides which embody both types of modification may serve as efficient tools. However, their synthesis is complicated by their pronounced acid and base lability as well as their multifunctionality. For this purpose the enzyme-labile choline ester was developed. The choline ester can be removed selectively and in high yields from various GlcNAc-glycopeptides and phosphopeptides at pH 6.5 and 37 degrees C. The conditions under which the enzymatic deprotections proceed are so mild that no undesirable side reactions are observed (i.e., no cleavage or anomerization of the glycosidic bonds and no beta-elimination of the phosphate or the carbohydrate occur). The specificity of the biocatalyst guarantees that neither the peptide bonds nor the other protecting groups present are being attacked. When this enzymatic protecting group technique was combined with the enzyme-labile 4-(phenylacetoxy)benzyloxycarbonyl (PhAcOZ) urethane protecting group a complex glycophosphopeptide could be built up. The glycopeptide is equipped with a biotin label by which it can be traced in biological systems. This peptide represents a characteristic partial structure of a glycosylated and phosphorylated sequence from the transactivation domain of serum response factor (SRF), a widely occurring human transcription factor.