Because glycoproteins are involved in a wide range of fundamental biological processes, it is important to understand the reasons for the striking structural diversity in their carbohydrate chains. For N-linked glycans this diversity results from differential processing, which is caused by a combination of factors, principally variations of cellular glycosylation apparatus and the nature of steric interactions between glycopeptide substrate and the processing glycosidases and glycosyltransferases. Peptide conformation in the region of the glycosylation site is potentially an important factor in the enzyme/substrate interaction. In this proposal, the influence of peptide structure on processing will be explored by the chemical synthesis of glycopeptides with different peptide sequences adjacent to the glycosylation site. The structures of the synthetic glycopeptides will be examined by circular dichroism, Fourier-transformed infrared, and nuclear magnetic resonance spectroscopy. Processing the glycan residues of these glycopeptides will be examined by determining the rate and extent of [3H] fucose or [3H]G1cNAc incorporation into identified products, or by observing the activity of Golgi alpha-mannosidase II, in incubations with enzymes form Golgi membranes or purified processing enzymes. However,the possibility that, depending on the source of Golgi membranes, the glycopeptide substrates may be processed by reactions different form those already characterized in other systems will also be taken into account. The results of the proposed experiments will contribute to an understanding of how information directing the synthesis of specific carbohydrate structures can be coded in the polypeptide sequence of a glycoprotein. Furthermore, detailed conformational studies of glycopeptides with defined carbohydrate structures, using """"""""state of the art"""""""" spectroscopic procedures combined with molecular modeling, will yield information that will contribute to an understanding of the protein folding problem, and help to define the structural function of the oligosaccharide chains in glycoproteins.
