7 peptides polysachccharides glycosaminoglycans

Peptides and Polysaccharides

  • Discussion begins with peptide bonds and their characterization through phi (φ) and psi (ψ) angles, which are significant in the context of protein structures.

    • These angles are relating to bonds within the peptide backbone.

    • A comparison is made to polysaccharides, highlighting the difficulty of applying these definitions to them.

    • Reference to limitations in literature regarding specific definitions for phi and psi in polysaccharides.

  • Proposed alternative approach for considering angles in polysaccharides involves examining the angles between planes created by individual residues rather than conceptualizing specific angles.

    • Example provided: Cellulose structure

    • Defined planes can be drawn around the molecule, emphasizing a 180-degree bond rotation around its structure, leading to an extended plane shape.

    • Example provided: Amylose structure

    • Displays approximately a 60-degree bond rotation compared to cellulose, indicating significant differences in structure and flexibility at the residue level.

    • Emphasis on the rigidity of these molecules due to their ring structures, which limit flexibility.

Dextrins and Additional Angles

  • Dextrins introduce an additional angle known as the omega (ω) angle, which describes the rotation about the bond between carbon-6 and carbon-5 in the residue.

    • This rotation contributes to the overall structural flexibility present in dextrins compared to more rigid polysaccharides.

  • Each residue serves as an individual plane, with overarching angles summarizing the rotation between residues.

Glycosaminoglycans (GAGs)

  • Glycosaminoglycans are identified as heteropolysaccharides, typically composed of two different monomers repeating in sequence.

    • Example of GAG: Hyaluronic acid (or hyaluronate)

    • Structure involves alternating molecules of glucuronate and N-acetylglucosamine.

    • Notable features:

    • Amino sugar content: N-acetylglucosamine is the amino component, often found acetylated.

    • Presence of a negatively charged sugar due to the carboxylic acid residue in its deprotonated form.

    • Example: In Chondroitin sulfate, the sulfate ester adds another negative charge.

    • Example: Keratin sulfate exhibits similar characteristics with the presence of sulfated groups.

  • Glycosidic bonds appear to alternate, commonly containing beta linkages:

    • Hyaluronic acid shows β1-3 and β1-4 linkages following an alternating sequence between glucuronic acids.

    • Exception: Heparin, which contains α linkages, diverging from the usual patterns observed in other GAGs.

Proteoglycans and Peptidoglycans

  • Peptidoglycans and proteoglycans derive their names from the peptide and sugar components, respectively.

    • The molecular weight of both is primarily due to the carbohydrate portion rather than the peptide.

    • Major roles:

    • Present in cell membranes and the glycocalyx surrounding cells.

  • Composition of proteoglycans often involves glycosaminoglycans linked to protein core, typically at serine residues adjacent to glycine residues.

    • Example: Syndecan

    • Core protein associated with heparin sulfate and chondroitin sulfate residues.

    • Initial three monomers include xylose and two galactose residues.

  • Linkage Characteristics

    • Carbohydrate attachments are always extracellular—never found on the intracellular side of proteins.

    • Glycosylation occurs as proteins are directed for secretion from the cell.

  • Role in cellular processes (example: Syndecan):

    • Inserts core protein into the membrane with amine terminus facing the extracellular space and carboxyl terminus interacting with the cytoskeleton.

    • Proteoglycans form a matrix with connectivity involving fibronectin, which itself has positively charged lysine and arginine residues that interact with negatively charged glycosaminoglycans.

Bacterial Cell Walls: Peptidoglycan Structure

  • Two types of bacterial cell walls:

    • Gram-positive bacteria: Thick peptidoglycan layers for protection.

    • Gram-negative bacteria: Thinner peptidoglycan layers situated between two membranes.

  • Gram-negative peptidoglycan layer:

    • Composed of D-alanine, L-lysine, isoglutamate, and L-alanine residues interconnected by peptide bonds.

    • Oligosaccharides associated are typically made of N-acetylglucosamine, contributing to structure.

    • Unique Structural Features: Characteristic ether linkages enhance stability and resistance to hydrolysis.

  • Gram-positive peptidoglycan layer:

    • Utilizes a pentaglycine bridge for structural separation, reflecting differences in strength and protection mechanisms.

Lipopolysaccharides in Gram-Negative Bacteria

  • Composed of lipid and carbohydrate components:

    • Core carbohydrate and O-specific antigen variant.

    • Structural elements aligned with immune recognition processes.

  • Highlight on unique amino acids present in lipopolysaccharide, such as heptose and 3-deoxy-L-fucose, underlining their role in immune evasion and recognition by antibodies.

    • The presence of acetyl groups vs hydroxyls is critical for distinguishing between bacterial strains and their immune responses.