THE ROLE OF CARBOHYDRATE RESIDUES IN MAMMALIAN FERTILIZATION

The fertilization process in mammals involves binding and fusion of fr ee-swimming sperm and ovulated eggs. This review focuses on the role o f carbohydrate residues in the process of sperm-egg interaction in mam mals. The zona pellucida (ZP), the acellular glycoprotein coat surroun ding the egg is highly glycosylated and posssess both Asn- (N-) linked and Ser/Thr- (O-)linked oligosaccharides, with an extreme structural heterogeneity between the different species. Different carbohydrates o n ZP3, such as Galactose in alpha-linkage, N-acetylglucosamine in beta -linkage, were suggested as the complementary sperm receptors, mediati ng the primary binding between the spermatozoon and the ZP. Several su ggested complementary ZP3 binding proteins on the sperm are sp56, beta -1,4-galactosyltransferase and p95. Some carbohydrate residues of the ZP undergo postfertilization modifications that might alter the sperm receptor, thus assisting in the establishment of the block to polysper my. The studies summarized in this review imply a main role for the ca rbohydrate residues in the process of sperm egg interaction.