Study Notes for Chapter 10: Carbohydrates and Glycoproteins

Chapter 10: Carbohydrates and Glycoproteins

1. Introduction to Carbohydrates

  • Carbohydrates are named based on the formula ( Cn(H2O)_n ).

  • They are produced from carbon dioxide (CO₂) and water (H₂O) through the process of photosynthesis in plants.

  • They can range from small molecules like glyceraldehyde (molecular weight = 90 g/mol) to large molecules such as amylopectin (molecular weight = 200,000,000 g/mol).

  • Carbohydrates fulfill various functions:

    • Energy source: Provide energy for cells.

    • Energy storage: Store energy in forms like starch and glycogen.

    • Structural components: Serve as structural elements in cell walls and exoskeletons.

    • Informational molecules: Involved in cell signaling and communication.

  • Carbohydrates can be covalently linked to proteins to form glycoproteins and proteoglycans.

2. Monosaccharides

2.1 Definition and Properties

  • Monosaccharides are the simplest carbohydrates, characterized as aldehydes or ketones containing two or more hydroxyl (–OH) groups.

  • The smallest monosaccharides consist of three carbon atoms.

2.2 Nomenclature of Monosaccharides

  • Naming conventions based on the number of carbon atoms in the backbone:

    • 3 carbons: Triose

    • 4 carbons: Tetrose

    • 5 carbons: Pentose

    • 6 carbons: Hexose

    • 7 carbons: Heptose

2.3 Types of Monosaccharides

  • Aldoses: Monosaccharides with an aldehyde functional group.

  • Ketoses: Monosaccharides with a ketone functional group.

2.4 Isomerism in Monosaccharides

  • Monosaccharides exist in various isomeric forms:

    • Isomers: Molecules with the same molecular formula but different structures.

    • Constitutional isomers: Differ in the order of atom attachment.

    • Stereoisomers: Atoms connected in the same order but differ in spatial arrangement.

2.4.1 Types of Stereoisomers

  • Epimers: Differ at one of several asymmetric carbon atoms.

  • Enantiomers: Non-superimposable mirror images of each other.

  • Diastereoisomers: Isomers that are not mirror images of each other.

  • Anomers: Isomers that differ at a new asymmetric carbon formed during ring closure.

2.5 Drawings and Representation

  • Chiral compounds are drawn using Fischer projections.

    • In Fischer projections:

    • Horizontal bonds point towards the viewer.

    • Vertical bonds point away from the viewer.

2.6 Common Monosaccharides

  • Key monosaccharides include:

    1. D-Ribose

    2. D-Deoxyribose

    3. D-Glucose

    4. D-Fructose

    5. D-Galactose

    6. D-Mannose

3. Cyclic Structures of Monosaccharides

3.1 Formation of Cyclic Forms

  • An aldehyde reacts with an alcohol to form a hemiacetal; many sugars are cyclic due to this reaction.

  • For glucose:

    • The resulting intramolecular hemiacetal forms a six-carbon ring known as pyranose.

    • The C-5 hydroxyl performs a nucleophilic attack on the C-1 aldehyde, creating a new asymmetric carbon at the C-1 position (the anomeric carbon).

    • The position of the hydroxyl group defines whether the anomer is ( \alpha ) (below the plane of the ring) or ( \beta ) (above the plane of the ring).

3.2 Ketose Sugars

  • Fructose can form a five-carbon ring known as furanose.

  • The furanose form has both ( \alpha ) and ( \beta ) configurations based on the orientation of the hydroxyl group at C-2.

3.3 Naming Cyclic Carbohydrates

  1. Determine the number of carbons.

  2. Identify if the molecule contains a ketone or aldehyde functional group.

  3. Establish the ( \alpha ) or ( \beta ) configuration based on the position of the hydroxyl group on the anomeric carbon.

4. Reducing Sugars

  • A reducing sugar has a free aldehyde group that can be oxidized, thereby reducing another compound.

  • Example: Glucose can reduce cupric ion (Cu²⁺) to cuprous ion (Cu⁺) while being oxidized to gluconic acid.

5. Glycosidic Bonds

5.1 Types of Glycosidic Bonds

  • An O-glycosidic bond forms between the anomeric carbon and a hydroxyl group of another molecule, resulting in a glycoside.

  • An N-glycosidic bond forms between the anomeric carbon and an amine.

  • Carbohydrates can also form ester linkages to phosphates.

5.2 Modifications on Monosaccharides

  • Monosaccharides can undergo various modifications:

    • O-acetylation

    • N-acetylation

    • Phosphorylation

    • Specific examples include:

    • ( B-D-Acetylgalactosamine ) (GalNAc)

    • ( B-D-Acetylglucosamine ) (GlcNAc)

    • Glucose 6-phosphate (G6P)

    • Dihydroxyacetone phosphate (DHAP)

6. Complex Carbohydrates

6.1 Oligosaccharides and Polysaccharides

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

  • Polysaccharides are large polymeric oligosaccharides, and if all monosaccharides are the same, they form a homopolymer.

6.2 Glycogen Structure

  • Glycogen is the main storage form of glucose in animal cells characterized by:

    • Most glucose units linked by ( \alpha-1,4 )-glycosidic bonds.

    • Branches formed by ( \alpha-1,6 )-glycosidic bonds occurring every 10 glucose units.

  • Glycogen consists of approximately 30,000 glucose units surrounding a core protein of glycogenin.

6.3 Starch Structure

  • Starch, the nutritional reservoir in plants, has two forms:

    • Amylose: Unbranched form.

    • Amylopectin: Branched form.

6.4 Chitin

  • Chitin is a homopolymer of ( eta-1,4 )-linked N-acetylglucosamine, often found in cell walls of fungi and exoskeletons of arthropods. It provides rigidity and strength due to cross-linking with minerals and proteins.

7. Glycoproteins

7.1 Classes of Glycoproteins

  • Glycoproteins are proteins with carbohydrate attachments and are classified into:

    1. Glycoproteins: Protein is the largest component by weight, playing various roles, including membrane proteins.

    2. Proteoglycans: Majority carbohydrate by weight, attached to glycosaminoglycans, playing structural roles.

    3. Mucins or mucoproteins: Predominantly carbohydrate, often serving as lubricants, attached by N-acetylgalactosamine.

7.2 Linkage Types

  • Carbohydrates are attached to proteins via two types of linkages:

    • N-linkage: Attachment to the nitrogen atom of asparagine.

    • O-linkage: Attachment to the oxygen atom of serine or threonine.

  • All N-linked polysaccharides consist of a common pentasaccharide core comprising three mannoses, a six-carbon sugar, and two N-acetylglucosamine units, with potential for additional monosaccharide attachments.

7.3 Example of Glycoprotein Application

  • Erythropoietin (EPO) is a glycoprotein secreted by the kidney that stimulates red blood cell production. Glycosylation increases EPO stability in blood.

7.4 Proteoglycans

  • Comprised mainly of glycosaminoglycans (95% by weight), proteoglycans consist of repeating disaccharides featuring an amino sugar and a negatively charged sugar component. They are crucial components of the extracellular matrix and function as lubricants.

7.5 Mucins

  • Mucins are glycoproteins characterized by extensive glycosylation with a variable number of tandem repeats (VNTR), consisting of serine and threonine residues that are O-glycosylated.

8. Lectins

8.1 Definition and Function

  • Lectins are glycan-binding proteins that attach to specific oligosaccharides on cell surfaces, facilitating cell-cell interactions and may have roles in immune response and embryo attachment.

  • Some viral infections, such as influenza, exploit lectins for cell entry, binding to carbohydrate residues on surfaces that allow viral attachment and infection.