Chemoenzymatic synthesis of a characteristic phosphorylated and glycosylated peptide fragment of the large subunit of mammalian RNA polymerase II

The covalent modification of proteins by phosphorylation and addition of GlcNAc residues are important regulatory processes which mediate biological signal transduction. For instance, the cytosolic form of RNA polymerase II is heavily glycosylated but during its transition from an initiating to an elongating complex the carbohydrates are removed and the protein is phosphorylated. For the study of such biological phenomena, characteristic peptides which embody both types of modifications may serve as efficient tools. However, their synthesis is complicated by their pronounced acid and base lability as well as their multifunctionality. These properties make the application of protecting groups necessary which can be removed under the mildest conditions. For the construction of such peptide conjugates the enzyme labile PhAcOZ urethane blocking group was developed. This protecting group embodies (a) a functional group (a phenylacetate) that is recognized by the biocatalyst (penicillin G acylase) and that is bound by an enzyme labile linkage (an ester) to (b) a functional group (a p-hydroxybenzyl urethane) that undergoes a spontaneous fragmentation upon cleavage of the enzyme-sensitive bond resulting in (c) the liberation of a carbamic acid derivative which decarboxylates to give the desired peptide or peptide conjugate. When this enzymatic protecting group technique was combined with classical chemical methods, a complex phosphoglycohexapeptide was built up, which embodies two glycosylated, one phosphorylated, and one underivatized hydroxyamino acid. This peptide represents a characteristic partial structure of the repeat sequence of the large subunit of RNA polymerase II which becomes glycosylated or phosphorylated while the enzyme carries out its biological functions. The conditions under which the enzymatic deprotections proceed are so mild that no undesired side reaction is observed (i.e., no rupture or anomerization of the glycosidic bonds and no beta-elimination of the phosphate or a carbohydrate occur). In addition, the specificity of the biocatalyst guarantees that the peptide bonds and the other protecting groups present are not attacked either