Carbohydrates: Structure, Function, and Biochemical Relevance

10.1 Monosaccharides Are the Simplest Carbohydrates

Monosaccharides are the fundamental building blocks of carbohydrates, characterized by their simple structure and functional groups. They serve as the basis for more complex carbohydrates and play essential roles in metabolism.

• Aldoses: Carbohydrates containing an aldehyde group (e.g., glyceraldehyde, glucose).

• Ketoses: Carbohydrates containing a keto group (e.g., dihydroxyacetone, fructose).

• D and L Series: The D or L configuration is determined by the absolute configuration of the asymmetric carbon atom farthest from the carbonyl group, compared to D-glyceraldehyde. Most natural sugars are in the D series.

• Cyclic Forms:

  • Glucose (an aldose) forms a six-membered pyranose ring via reaction between its C-1 aldehyde and C-5 hydroxyl group.

  • Fructose (a ketose) forms a five-membered furanose ring via reaction between its C-2 keto group and C-5 hydroxyl group.

  • Pentoses (e.g., ribose, deoxyribose) also form furanose rings.

• Anomeric Carbon: Cyclization creates a new asymmetric center at the anomeric carbon (C-1 in aldoses, C-2 in ketoses), leading to anomers (α and β forms).

• Glycosidic Bonds: Sugars can be joined to alcohols and amines at the anomeric carbon via glycosidic bonds.

Example: D-glucose cyclizes to form α- and β-pyranose forms, which differ in the configuration at the anomeric carbon.

10.2 Monosaccharides Are Linked to Form Complex Carbohydrates

Monosaccharides can be joined to form disaccharides and polysaccharides through O-glycosidic bonds, resulting in a wide variety of carbohydrate structures with diverse biological functions.

• Disaccharides:

  • Sucrose: α-glucose + β-fructose (α-1,2 linkage)

  • Lactose: β-galactose + β-glucose (β-1,4 linkage)

  • Maltose: α-glucose + α-glucose (α-1,4 linkage)

• Polysaccharides:

  • Starch: Polymeric form of glucose in plants, mainly α-1,4 linkages with some α-1,6 branches (amylopectin).

  • Glycogen: Animal storage polysaccharide, similar to starch but with more α-1,6 branch points.

  • Cellulose: Structural polymer in plant cell walls, composed of β-1,4 linked glucose units, forming straight, rigid fibrils.

• Structural Differences:

  • α-linkages (starch, glycogen): Form open helices, suitable for energy storage.

  • β-linkages (cellulose): Form straight chains, providing structural strength.

Example: Glycogen's highly branched structure allows rapid release of glucose during metabolic demand.

10.3 Carbohydrates Are Attached to Proteins to Form Glycoproteins

Carbohydrates can be covalently attached to proteins, forming glycoproteins, proteoglycans, and mucoproteins, each with distinct structural and functional roles.

• Glycoproteins: Proteins with oligosaccharide chains attached via N- or O-glycosidic bonds.

  • N-linked: Attachment to the amide nitrogen of asparagine.

  • O-linked: Attachment to the hydroxyl oxygen of serine or threonine.

  • Most secreted proteins are glycoproteins (e.g., erythropoietin).

• Proteoglycans: Proteins covalently linked to glycosaminoglycans (long, repeating disaccharide units), often found in connective tissue.

• Mucoproteins (Mucins): Heavily O-glycosylated proteins, with N-acetylgalactosamine, serving as lubricants.

Example: Glycoproteins on the cell surface are involved in cell recognition and signaling.

10.4 Lectins Are Specific Carbohydrate-Binding Proteins

Lectins are proteins that specifically recognize and bind to carbohydrate moieties on cell surfaces, mediating a variety of biological processes including cell-cell recognition and pathogen entry.

• Lectins: Mediate cell-cell contact by binding specific carbohydrate structures.

• Viral Recognition: Influenza virus hemagglutinin binds sialic acid residues on host cells, facilitating viral entry.

Example: Selectins are lectins involved in the immune response by mediating leukocyte-endothelial cell interactions.

Appendix: Biochemistry in Focus – Glucosidase Inhibitors and Glucose Homeostasis

Maintaining blood glucose levels (3.9–5.5 mM) is essential for health. Disruption leads to conditions such as diabetes mellitus, characterized by hyperglycemia due to absent or ineffective insulin.

• Glucose Homeostasis: Regulated by insulin and other hormones; imbalance leads to diabetes.

• Enzymatic Targets for Diabetes Treatment:

  • α-Amylase: Initiates starch and glycogen breakdown.

  • α-Glucosidase: Further digests oligosaccharides to glucose for absorption.

• Drug Inhibitors:

  • Acarbose and Miglitol: Competitive inhibitors of α-glucosidase, reduce postprandial glucose absorption, used in type 2 diabetes management.

Example: Acarbose is taken at the start of a meal to slow carbohydrate digestion and glucose absorption.

Appendix: Problem-Solving Strategies – Determining Branch Points in Polysaccharides

Methylation analysis is a classic method for determining the branching structure of polysaccharides such as glycogen and amylopectin.

• Methylation: All free hydroxyl groups of a polysaccharide are methylated; glycosidic-bonded hydroxyls are not.

• Hydrolysis: Acid hydrolysis breaks the polysaccharide into monosaccharides, which retain their methylation pattern.

• Analysis:

  • Branch points (α-1,6 linkages) are identified by the presence of 2,3-di-O-methylglucose.

  • Reducing ends yield 1,2,3,6-tetra-O-methylglucose.

• Quantification: The amount of 2,3-di-O-methylglucose corresponds to the number of branch points; the amount of tetra-O-methylglucose corresponds to the number of reducing ends.

Example: By analyzing the methylation pattern after hydrolysis, researchers can determine the degree of branching in glycogen.

Key Terms

• Monosaccharide: Simple sugar molecule, basic unit of carbohydrates.

• Epimer: Stereoisomers differing at one chiral center.

• Anomer: Isomers differing at the anomeric carbon.

• Reducing Sugar: Sugar capable of acting as a reducing agent due to a free anomeric carbon.

• Glycosidic Bond: Covalent bond joining carbohydrate to another group.

• Oligosaccharide: Short chain of monosaccharide units.

• Disaccharide: Carbohydrate composed of two monosaccharides.

• Polysaccharide: Long chain of monosaccharide units.

• Glycogen: Animal storage polysaccharide.

• Starch: Plant storage polysaccharide.

• Cellulose: Plant structural polysaccharide.

• Glycoprotein: Protein with covalently attached carbohydrate.

• Proteoglycan: Protein with covalently attached glycosaminoglycan.

• Glycosaminoglycan: Long, unbranched polysaccharide with repeating disaccharide units.

• Mucin (Mucoprotein): Heavily glycosylated protein serving as a lubricant.

• Lectin: Carbohydrate-binding protein.

• Selectin: Family of lectins involved in cell adhesion.

Table: Comparison of Major Polysaccharides

Key Equations

• General formula for monosaccharides:

• Glycosidic bond formation (example):