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Ch.11 Carbohydrates and Glycoproteins

Chapter 11: Carbohydrates and Glycoproteins

  • Carbohydrates are carbon-based molecules high in hydroxyl groups.

  • Empirical formula: (CH2O)n.

  • Can have additional groups or modifications.

  • Better described as polyhydroxy aldehydes and ketones (and their derivatives).

Monosaccharides

  • Definition: Simplest carbohydrates, aldehydes or ketones that contain two or more hydroxyl groups.

  • Structure: Ranges from 3 to 7 carbons in length; exist in many isomeric forms called simple sugars.

Monosaccharide Nomenclature

  • Based on chain length:

  • Trioses (3 carbons)

  • Tetroses (4 carbons)

  • Pentoses (5 carbons)

  • Hexoses (6 carbons)

  • Heptoses (7 carbons)

  • Based on oxidized group:

  • Ketose (keto group)

  • Aldose (aldehyde group)

Isomers

  • Constitutional Isomers: Same molecular formula, different atom arrangements.

  • Stereoisomers: Same order of atoms, different spatial arrangements.

  • D or L configurations.

  • Can be enantiomers (mirror images) or diastereoisomers (not mirror images).

  • The number of possible stereoisomers = 2^n (n = number of asymmetric carbon atoms).

Common Monosaccharides

  • Examples:

  • D-Ribose, D-Glucose, D-Mannose, D-Galactose, D-Fructose.

  • Epimers: Sugars that differ in configuration at a single asymmetric center.

Cyclic Forms of Monosaccharides

  • Most monosaccharides exist as cyclic forms due to reactions with alcohols:

  • Pyranose: Forms when aldehyde reacts with alcohol.

  • Furanose: Forms when ketone reacts with alcohol.

Anomers of Glucose

  • Definition: Diastereoisomers formed upon ring closure that create a new asymmetric carbon.

  • α-D-glucopyranose: Hydroxyl group on C-1 is opposite to C-6.

  • β-D-glucopyranose: Hydroxyl group on C-1 is on the same side as C-6.

Reducing Sugars

  • Sugars that can reduce oxidizing agents, including all monosaccharides that can adopt a linear structure.

  • Example: Linear glucose reacts with Cu2+.

  • Non-reducing sugars: Do not react with oxidizing agents (e.g., sucrose).

Glycation and A1C Levels in Diabetes

  • Glycation: Nonenzymatic addition of sugars to proteins.

  • D-glucose has low tendency to glycate proteins unless concentrations are high.

  • A1C levels: Represents glycated hemoglobin; used to monitor diabetes.

  • Normal individuals: <6% A1C; uncontrolled diabetes can reach 10%.

Glycosidic Linkages

  • Monosaccharides can be modified through glycosidic linkages:

  • O-glycosidic: Between anomeric carbon of a sugar and an alcohol.

  • N-glycosidic: Between anomeric carbon of a sugar and amine.

Oligosaccharides and Polysaccharides

  • Oligosaccharides: Composed of two or more monosaccharides linked by O-glycosidic bonds.

  • Polysaccharides: Large polymers from the linkage of multiple monosaccharides, e.g. glycogen, starch, cellulose.

Glycogen and Starch - Storage Forms of Glucose

  • Glycogen: Branched polymer of glucose, most common storage form in animals (α-1,4 and α-1,6 glycosidic linkages).

  • Starch: Nutritional reservoir in plants (amylose and amylopectin).

Cellulose and Dietary Fiber

  • Cellulose: Main structural polysaccharide in plants (β-1,4 glycosidic linkages); human digestive systems cannot break it down.

  • Dietary fibers: Helps in digestion; insoluble fibers increase passage rates, soluble fibers slow digestion.

Chitin in Fungi and Arthropods

  • Chitin: Main structural polysaccharide; consists of β-1,4 linked N-acetylglucosamine.

Glycoproteins and Their Functions

  • Glycoproteins: Proteins with covalently attached carbohydrates, important for cell recognition and protection. Include three classes: proteoglycans, mucins, and glycoproteins.

  • N-Linked and O-Linked: Differences in glycosylation based on linkages to amino acid side chains (Asn, Ser, Thr).

Lectins and Cell Interactions

  • Lectins: Carbohydrate-binding proteins that mediate cell signaling and immune responses; examples include selectins that aid in leukocyte recruitment.

Importance of Glycosylation in Blood Groups

  • Blood type determined by presence of specific carbohydrate antigens on red blood cells, influenced by specific glycosyltransferases.

I-Cell Disease

  • A lysosomal storage disorder caused by lack of proper glycosylation leading to accumulation of undegraded materials, causing severe health issues.

Conclusion

  • Carbohydrates, including their structures and modifications, play essential roles in biological systems, impacting energy storage, structural integrity, signaling, and disease mechanisms.

What to Know About Chapter 11:

Structure and Main Roles of Carbohydrates in Nature

  • Structure:

  • Carbohydrates are carbon-based molecules rich in hydroxyl groups, described by the empirical formula (CH2O)n.

  • They can exist in various forms, primarily as monosaccharides, oligosaccharides, and polysaccharides.

  • Monosaccharides: The simplest type, ranging from 3 to 7 carbons. They include aldehydes or ketones and have two or more hydroxyl groups. Common examples include D-Ribose and D-Glucose.

  • Polysaccharides: Formed from multiple monosaccharides linked by glycosidic bonds, with structures like glycogen (energy storage in animals), starch (nutritional reservoir in plants), and cellulose (structural component in plants).

  • Main Roles:

  • Energy Storage: Carbohydrates like glycogen in animals and starch in plants serve as vital energy reservoirs.

  • Structural Functions: Cellulose provides rigidity in plant cell walls, while chitin offers structural integrity in fungi and arthropods.

  • Cell Communication: Glycoproteins and glycolipids, which have carbohydrate components, play crucial roles in cell recognition and signaling.

  • Dietary Fiber: Non-digestible carbohydrates like cellulose aid in digestion and promote gut health.

Simple carbohydrates, or monosaccharides, are linked to form complex carbohydrates through a type of covalent bond known as glycosidic linkages. This process involves the reaction between the hydroxyl group of one monosaccharide and the anomeric carbon of another, resulting in the formation of an O-glycosidic bond.

  • Oligosaccharides: Formed by linking two or more monosaccharides via glycosidic bonds. These structures can include disaccharides, like sucrose (glucose + fructose) and lactose (glucose + galactose).

  • Polysaccharides: Composed of many monosaccharides linked together in long chains. They can have varied structures and functions depending on the types of monosaccharides involved and the nature of glycosidic bonds. Examples include starch, which is a polymer of glucose linked by α-1,4 and α-1,6 glycosidic bonds, and cellulose, which has β-1,4 glycosidic bonds.

Carbohydrates are linked to proteins primarily through glycosylation, resulting in glycoproteins. This process can involve two types of linkages: N-linked and O-linked, depending on whether the carbohydrate is attached to the nitrogen of an asparagine (Asn) side chain or the hydroxyl group of serine (Ser) or threonine (Thr) side chains.

  • Functions of Linked Carbohydrates:

  • Cell Recognition: Glycoproteins are essential for cell-cell interactions and recognition processes, allowing cells to identify and communicate with one another.

  • Protection: The carbohydrate component can provide a protective function by forming a mucus layer that coats and shields the protein from degradation.

  • Signaling: Glycoproteins can act as signaling molecules, playing roles in immune responses and cellular signaling pathways by binding to specific receptors.

  • Structural Stability: The carbohydrates contribute to the overall stability and folding of the protein structure, helping maintain proper protein conformation.

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Classes of Glycoproteins and Their Biochemical Roles

  1. Proteoglycans:

  • Structure: Composed of core proteins covalently attached to long chains of glycosaminoglycans (GAGs), which are sulfated polysaccharides.

  • Biochemical Roles:

    • Provide structural support in the extracellular matrix.

    • Play key roles in cell signaling and regulating interactions between cells and their environment.

    • Involved in maintaining the hydration and swelling pressure of tissues.

  1. Mucins:

  • Structure: High molecular weight glycoproteins characterized by a net-like structure due to extensive O-glycosylation.

  • Biochemical Roles:

    • Form protective mucus barriers on epithelial surfaces, such as in the respiratory and gastrointestinal tracts.

    • Involved in lubrication and protection against pathogens and environmental irritants.

    • Play a role in cell signaling and immune responses.

  1. Conventional Glycoproteins:

  • Structure: Generally contain shorter, branched carbohydrate chains linked to proteins via N-linked or O-linked glycosidic bonds.

  • Biochemical Roles:

    • Involved in a wide range of functions including cell recognition, communication, and adhesion.

    • Act as receptors for hormones and other signaling molecules, showing importance in processes such as immune response.

    • Have roles in the transport of molecules across cellular membranes and in maintaining protein stability and solubility.

Lectins are carbohydrate-binding proteins that play a significant role in mediating cell signaling and immune responses. They bind specifically to carbohydrates, allowing them to recognize and interact with glycoproteins and glycolipids on cell surfaces.

Biochemical Functions of Lectins:

  • Cell Signaling: Lectins are involved in cell signaling pathways, facilitating communication between cells.

  • Immune Responses: By binding to carbohydrates on pathogens, lectins can help the immune system recognize and respond to infections.

  • Cell Adhesion: Lectins promote adhesion of cells, which is important in processes such as inflammation and tissue repair.

  • Modulation of Cell Behavior: Lectins can influence cell growth, differentiation, and apoptosis by interacting with specific carbohydrate structures on cell surfaces.

Genes can be altered in living organisms through various techniques in genetic engineering, which involve modifying an organism's DNA to achieve desired traits. Here are some of the primary methods used:

Techniques for Altering Genes:

  1. CRISPR-Cas9:

  • A precise gene-editing tool that uses a guide RNA to direct the Cas9 enzyme to a specific location in the genome, allowing for targeted modifications such as gene knockouts, insertions, or replacements.

  1. Transgenesis:

  • The process of introducing a gene from one organism into the genome of another organism, resulting in a transgenic organism. This is commonly used in agriculture to create genetically modified crops with desirable traits, like pest resistance or enhanced nutritional content.

  1. Gene Therapy:

  • A technique to treat or prevent diseases by directly modifying the genes within a patient's cells. This can involve replacing defective genes with functional ones, inactivating genes that are not functioning properly, or introducing new genes to help treat a disease.

  1. Homologous Recombination:

  • A process where DNA sequences are exchanged between two similar or identical strands of DNA, allowing for targeted gene modification or repair within an organism's genome.

Applications of Gene Alteration:

  1. Medical Therapies:

  • Gene editing techniques like CRISPR are being explored for the treatment of genetic disorders, cancers, and infectious diseases by targeting and correcting faulty genes.

  1. Agriculture:

  • Genetically modified crops can be engineered for improved yield, pest resistance, herbicide tolerance, and enhanced nutritional profiles, helping to secure food sources and reduce reliance on chemical pesticides.

  1. Biotechnology:

  • Engineering microorganisms to produce pharmaceuticals, enzymes, and biofuels efficiently incapsulates the essence of synthetic biology, reducing costs and improving sustainability.

  1. Research Models:

  • Altered genes are used to create animal models for studying human diseases, which helps in understanding disease mechanisms and testing potential treatments.

  1. Conservation:

  • Gene editing can be used to enhance the genetic diversity and adaptability of endangered species, potentially aiding in their recovery efforts.

These techniques have the potential to revolutionize medicine, agriculture, and biotechnology, but they also raise ethical and ecological considerations that continue to be debated.