Carbohydrate Structure and Function Lecture Notes

Chapter 13: Carbohydrate Structure and Function Part I

Chapter Outline

13.1 Carbohydrates: The Most Abundant Biomolecules in Nature
13.2 Important Biological Functions of Glycoconjugates
13.3 Biomedical Methods in Glycobiology

13.1 Carbohydrates: The Most Abundant Biomolecules in Nature

Carbohydrates, also known as glycans, are the most abundant biomolecules present in nature. They play a critical role in a variety of biological processes. Carbohydrates can be characterized as:

  • Glycoproteins: Proteins that have glycans covalently attached to them.

  • Glycolipids: Lipids that have glycans attached.

Glycans are crucial for cellular communication.

13.2 Important Biological Functions of Glycoconjugates

Common Monosaccharides Found in Glycoconjugates

The following monosaccharides are prevalent in glycoconjugates, based on the work of the Consortium for Functional Glycomics (CFG):

  • Glucose (Glc)

  • Galactose (Gal)

  • Mannose (Man)

  • N-acetylgalactosamine (GalNAc)

  • N-acetylglucosamine (GlcNAc)

  • N-acetylmuramic acid (MurNAc)

  • Xylose (Xyl)

  • Glucuronic acid (GlcA)

  • Fucose (Fuc)

  • Iduronic acid (IdoA)

  • N-acetylneuraminic acid (Neu5Ac), also known as sialic acid.

Principles of Glycobiology

The principles that describe the structure and function of glycans include:

  1. Glycan Biochemistry: Discusses the structural aspects of glycan chains — they can be branched or linear and can include modified sugar units.

  2. Glycan Biosynthesis: Describes how monosaccharides are incorporated into proteins and lipids via glycosyltransferases.

  3. Glycan Diversity: Emphasizes that cells contain a variety of both free and conjugated glycans.

  4. Glycan Recognition: Involves the binding of specific proteins, such as lectins, to glycans, which is important for numerous biological interactions.

13.3 Biomedical Methods in Glycobiology

Linking of Monosaccharides in Glycoproteins

Glycoproteins are structures comprising both protein and glycan components. The linking of monosaccharides can occur through:

  • O-linked Glycans: Mono- and oligosaccharides are added to the hydroxyl (OH) groups of amino acids like serine (Ser) or threonine (Thr).

  • N-linked Glycans: Sucrose units are linked to the amino group (NH2) of asparagine (Asn), manufactured in both Golgi and Endoplasmic Reticulum (ER).

Glycosyltransferases and Glycosidases

Glycosyltransferases are enzymes responsible for generating complex glycans by adding sugar units and are classified into two groups:

  • Enzymes adding sugars to OH groups (O links).

  • Enzymes adding sugars to NH2 groups (N links).

Glycosidases remove sugar units from complex glycans, maintaining a balance within glycan structures.

Oligosaccharides

Definition and Composition

Oligosaccharides are comprised of repeating simple sugars, ranging from 3 to 20 sugar residues, and are characterized as branched or unbranched. Examples include:

  • Lacto-N-tetraose

  • Lacto-N-fucopentaose I

These are prominently found in human breast milk, which contains approximately 100 different oligosaccharides, the most abundant following lactose and lipids.

Functions of Human Milk Oligosaccharides

Lacto-N-tetraose promotes the growth of beneficial bacteria, such as bifidobacterium, in an infant's intestinal tract. Conversely, Lacto-N-fucopentaose I helps prevent pathogenic bacteria like Salmonella and E. coli from binding to cell receptors.

Prebiotics vs. Probiotics

  • Prebiotics: These are substances that serve as nutrients for beneficial bacteria.

  • Probiotics: These refer to live beneficial bacteria often found in yogurt cultures.

Oligosaccharides Found in Plants

The raffinose series consists of oligosaccharides found in fruits and vegetables:

  • Raffinose (sucrose + 1 galactose)

  • Stachyose (sucrose + 2 galactose)

  • Verbascose (sucrose + 3 galactose)

These oligosaccharides contain (\alpha(1 \rightarrow 6)) glycosidic bonds that link sucrose and galactose. Humans lack the enzyme to cleave these bonds, leading to fermentation by the gut bacteria E. coli, which produces gas.

Examples of dietary aids include Beano®, which contains (\alpha(1 \rightarrow 6)) glycosidase to help digest these carbohydrates.

Cellulose: A Structural Carbohydrate

Cellulose is characterized as a homopolymer of cellobiose (which is a disaccharide) with repeating units connected through β-1,4 glycosidic bonds between glucose units. The cellulose strands form layers similar to β-sheets in proteins, providing rigidity to the plant cell wall due to hydrogen bonding. Cellulase is an enzyme that can hydrolyze these β-1,4 glycosidic bonds, yet most animals and humans lack this enzyme. Ruminating herbivores, however, can utilize cellulose as an energy source with the assistance of microorganisms.

Chitin: A Structural Carbohydrate

Chitin, recognized as the second most abundant polysaccharide after cellulose, is found in the exoskeletons of insects and crustaceans, as well as in the cell walls of fungi. Approximately 1 billion tons of chitin are produced annually in the biosphere. Chitin is a linear polysaccharide formed of N-acetylglucosamine (GlcNAc) units and contains β-1,4 glycosidic bonds. It is degraded by chitinase, but humans cannot digest chitin. It has been used medicinally due to its wound healing and antimicrobial properties.

Glycosaminoglycans (Proteoglycans)

Glycosaminoglycans are linear hexosamine polysaccharides that covalently attach to proteins to create proteoglycans.

Structure and Function

  • Proteoglycans consist of 20–50 repeating disaccharides within a single polypeptide chain.

  • Examples include:

    • Chondroitin sulfate: Found around joints, aiding lubrication.

    • Keratan sulfate: Present in the cornea, facilitating tissue structural organization.

    • Heparan sulfate: Involved in the extracellular matrix, adhering leucocytes to endothelium during inflammation.

Important Functions

  1. Joint lubrication

  2. Providing structural components

  3. Forming part of the extracellular matrix

  4. Aiding in solute diffusion within the matrix

Macular Corneal Dystrophy and Keratan Sulfate

Keratan sulfate is composed of repeating units of N-acetyllactosamine. For structural support, N-acetyllactosamine must be sulfated. The enzyme responsible for sulfation is CHST6. Macular corneal dystrophy occurs when there’s a defect in this sulfation process leading to corneal opacity. This condition is distinct from cataracts (cloudiness of the lens) and age-related macular degeneration.

Storage Form of Carbohydrates: Starch and Glycogen

Starch

Starch is stored in the stroma as a reserve of energy. It consists of glucose homopolymers linked through (\alpha(1 \rightarrow 4)) glycosidic bonds and includes:

  • Amylose: A linear polymer that consists of around 100 glucose units.

  • Amylopectin: A branched polymer featuring branch points articulated via (\alpha(1 \rightarrow 6)) bonds.

Glycogen

Glycogen is also a glucose homopolymer but is stored in the liver and muscle, and is characterized by:

  • Linear chains of 8-10 glucose units linked through (\alpha(1 \rightarrow 4)) bonds;

  • More frequent branching compared to starch, occurring at (\alpha(1 \rightarrow 6)) bonds.

Properties of Starch and Glycogen

Both amylopectin and glycogen share structural similarities; however, glycogen is significantly more branched than starch.

Amylose and Amylopectin: Iodine Test

The iodine test reveals the presence of starch through a blue color formation, which disappears upon heating. Amylose exhibits a helical structure with a specific arrangement of glucose molecules (6 glucose units per turn) stabilized by intra-strand hydrogen bonds.

Glycogen Structure and Degradation

Amylopectin displays a single reducing end and incorporates branching every 15–30 glucose residues. Conversely, glycogen has a core protein called glycogenin (homodimeric) with two glucose molecules at its center, having a higher frequency of nonreducing ends, which facilitates quicker degradation through glycogenolysis.

Glycoconjugates and Their Biological Functions

Glycoconjugates in Cell Signaling and Immunity

Glycoconjugates are pivotal in cellular communication, particularly within immune responses. They can facilitate both:

  1. Intrinsic binding: Pertaining to cell recognition, attachment, and migration.

  2. Extrinsic binding: Engaging interactions between immune cells and pathogens, helping protect host cells from infections.

Glycoproteins in Disease Recognition

Glycoproteins play a significant role in:

  • Cell signaling: Immune cells communicate and interact through glycoproteins on cell surfaces.

  • Pathogen recognition: Pathogens bind to host cells via glycoconjugate interactions, exemplified by a virus attaching to a host or E. coli lectin protein FimH binding to human glycoprotein uroplakin.

Types of Glycoconjugates: N- and O-Linked Oligosaccharides

N-linked Oligosaccharides

  • Found on polypeptide chains associated with asparagine (Asn) residues.

O-linked Oligosaccharides

  • Attached to serine (Ser) or threonine (Thr) residues.

Glycan Groups:

  • N-linked: Characterized by GlcNAc₂Man₃ as a common structure.

  • O-linked GalNAc glycan groups are signature attributes of O-linked glycoproteins.

ABO Blood Groups and Glycosyltransferases

ABO Glycoconjugates

ABO blood types are determined by glycoconjugates attached to proteins (75%) and membrane lipids (25%) on red blood cells, with the involvement of two glycosyltransferases (GTA and GTB). These enzymes differ at only four amino acids, influencing the selectivity of sugar attachment leading to distinct blood group expressions:

  • Type A: Expression of GTA only, no GTB.

  • Type B: Expression of GTB, no GTA.

  • Type AB: Both enzymes expressed.

  • Type O: No expression of GTA or GTB.

Blood Group Compatibility

The Rhesus (Rh) factor is also significant in blood type compatibility, with explained percentages of Rh positive (99%) and Rh negative (1%). Groups are classified as:

  • Group AB: Universal acceptor

  • Group O: Universal donor

The hemagglutination test is typically applied to assess blood compatibility.

Proteoglycan Structures & Functions

Proteoglycans consist mainly of carbohydrates, serving critical functions in:

  1. Protein binding activities.

  2. Regulation of signal transduction.

  3. Facilitation of cell migration.

  4. Enhancing cell adhesion.

These proteoglycans are categorized into two major classes:

  1. Cell surface proteoglycans

  2. Extracellular matrix proteoglycans

Peptidoglycans in Bacterial Cell Walls

Peptidoglycans are structural components of bacterial cell walls made of hexosamine polysaccharides, specifically consisting of repeating units of a β(1→4)-linked disaccharide (MurNAc & GlcNAc). These polysaccharide chains are covalently cross-linked through oligopeptides.

Lysozyme is an enzyme important in cleaving β(1→4) glycosidic bonds and is present in various bodily fluids, playing a role in innate immunity.

Gram Staining Technique

Gram staining differentiates bacteria into:

  • Gram-positive: Characterized by a thick peptidoglycan layer that traps dye (appears purple).

  • Gram-negative: Has a thinner peptidoglycan layer that does not retain dye (appears red/pink).

Penicillin in Bacterial Defense

Penicillin inhibits bacterial cell wall synthesis by specifically blocking the transpeptidase enzyme necessary for forming oligopeptide linkages in the peptidoglycan layer. This occurs because penicillin mimics the natural substrate of the enzyme, consequently irreversibly inhibiting its activity.

Discovery of Penicillin

  • Discovered in 1928 by Alexander Fleming from mold Penicillium notatum, leading to significant medical advancements and a Nobel Prize in 1945.

Antibiotic Resistant Bacteria

The mechanisms for antibiotic resistance include:

  • β-lactamase: An enzyme that cleaves the β-lactam ring, rendering antibiotics like penicillin ineffective.

  • Mutated variants of peptidases that do not bind to antibiotics, including variants involved in MRSA through lateral gene transfer.

Summary of Chapter 13

Carbohydrates constitute a diverse group, essential for functionality in structure, energy storage, and communication within biological systems. They can be classified into simple sugars (monosaccharides, disaccharides, oligosaccharides, polysaccharides) and complex carbohydrates which are often linked to proteins and lipids, forming glycoconjugates. Additionally, the structure and composition of carbohydrates play foundational roles in cellular interactions, such as blood group determination and bacterial cell wall construction, highlighting their overarching significance in biological contexts.