| Abstract
| - The covalent modification of proteins by phosphorylation andaddition of GlcNAc residues are importantregulatory processes which mediate biological signal transduction.For instance, the cytosolic form of RNA polymeraseII is heavily glycosylated but during its transition from an initiatingto an elongating complex the carbohydrates areremoved and the protein is phosphorylated. For the study of suchbiological phenomena, characteristic peptideswhich embody both types of modifications may serve as efficient tools.However, their synthesis is complicated bytheir pronounced acid and base lability as well as theirmultifunctionality. These properties make the applicationofprotecting groups necessary which can be removed under the mildestconditions. For the construction of such peptideconjugates the enzyme labile PhAcOZ urethane blocking group wasdeveloped. This protecting group embodies (a)a functional group (a phenylacetate) that is recognized by thebiocatalyst (penicillin G acylase) and that is bound byan enzyme labile linkage (an ester) to (b) a functional group (ap-hydroxybenzyl urethane) that undergoes aspontaneousfragmentation upon cleavage of the enzyme-sensitive bond resulting in(c) the liberation of a carbamic acid derivativewhich decarboxylates to give the desired peptide or peptide conjugate.When this enzymatic protecting group techniquewas combined with classical chemical methods, a complexphosphoglycohexapeptide was built up, which embodiestwo glycosylated, one phosphorylated, and one underivatizedhydroxyamino acid. This peptide represents acharacteristic partial structure of the repeat sequence of the largesubunit of RNA polymerase II which becomesglycosylated or phosphorylated while the enzyme carries out itsbiological functions. The conditions under whichthe enzymatic deprotections proceed are so mild that no undesired sidereaction is observed (i.e., no rupture oranomerization of the glycosidic bonds and no β-elimination of thephosphate or a carbohydrate occur). In addition,the specificity of the biocatalyst guarantees that the peptide bondsand the other protecting groups present are notattacked either.
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