Chapter 11: Carbohydrates: Structure & Function

Carbohydrates: Structure & Function details how these sugars cyclize into pyranose and furanose rings through hemiacetal or hemiketal formation, creating alpha and beta anomers that can adopt specific conformations like chair or boat forms. The text explains how monosaccharides link via O-glycosidic and N-glycosidic bonds to form complex structures, ranging from common disaccharides like sucrose, lactose, and maltose to vast polysaccharides. Key storage homopolymers are differentiated, specifically glycogen in animals and starch (amylose and amylopectin) in plants, which utilize alpha-1,4-glycosidic linkages for accessible energy storage. In contrast, the structural polymer cellulose relies on beta-1,4-linkages to form rigid, straight chains essential for plant cell walls. The chapter further examines glycoconjugates, where carbohydrates covalently attach to proteins to form glycoproteins, proteoglycans, and mucins. It highlights the role of N-linked (asparagine) and O-linked (serine/threonine) glycosylation, processes occurring within the endoplasmic reticulum and Golgi complex involving dolichol phosphate and specific glycosyltransferases. Clinical correlations are drawn through examples like the hormone erythropoietin (EPO), the ABO blood group antigens determined by specific glycosyltransferase activity, and mucopolysaccharidoses like Hurler disease and I-cell disease caused by errors in glycosaminoglycan degradation or targeting. Finally, the chapter introduces lectins, specific carbohydrate-binding proteins that facilitate cell-cell contact and pathogen interaction, illustrating this with the mechanism of the influenza virus, which binds to sialic acid residues on host cells via hemagglutinin.