Complex sentence: The anabolism of oligosaccharides was not clear and hence the role of nucleotide sugars until 1950s when Leloir and his coworkers found that the key enzyme in this process is glycosyltranserases which transfer glycosyl group from a sugar nucleotide to an acceptor.
Split into simpler sentences: The anabolism of oligosaccharides was not clear-and, hence, the role of nucleotide sugars-until 1950s when Leloir and his coworkers found that the key enzymes in this process are the glycosyltranserases. These enzymes transfer a glycosyl group from a sugar nucleotide to an acceptor.