8 Glycoproteins Study Guide

Glycoproteins

  • Definition: Glycoproteins are proteins that have a small amount of carbohydrates attached to them.

  • Contrast with Proteoglycans: Proteoglycans are predominantly carbohydrates with only a small protein component.

  • Functionality: Glycoproteins typically have defined functions, and they are predominantly found outside the cell or in the bloodstream.

Types of Glycoproteins

O-linked Glycoproteins

  • Attachment: O-linked carbohydrates are attached to serine or threonine residues through a free hydroxyl group.

  • Variety: O-linked glycans exhibit a greater variety in structure.

N-linked Glycoproteins

  • Attachment: N-linked carbohydrates are attached to asparagine residues through the amide nitrogen in the R group of asparagine.

  • Predictability: N-linked glycans are generally more predictable in structure compared to O-linked.

  • Core Structure: Commonly starts with two N-acetylglucosamine (GlcNAc) residues followed by mannose.

    • **Classification: **N-linked glycans can further be categorized into the following:

    • High Mannose Type: Composed primarily of mannose along with N-acetylglucosamine.

    • Hybrid Type: Begins with N-acetylglucosamine, has mannose, and then features additional structures such as N-acetylglucosamine and galactose.

    • Complex Type: Contains N-acetylglucosamine, mannose, galactose, and includes a sialic acid residue at the terminal end.

ABO Blood Type System and Glycoproteins

  • Overview: The ABO blood type system includes antigens that are glycoproteins or glycolipids with specific carbohydrate structures.

  • Common Structures:

    • All blood types have a common structure consisting of:

    • Galactose residue

    • N-acetylglucosamine residue (GlcNAc)

    • N-acetylgalactosamine residue (GalNAc)

    • Fucose

  • Differentiation of Blood Types:

    • Type O: No additional attachment on the galactose.

    • Type B: Contains an additional galactose residue attached to the initial galactose.

    • Type A: Contains an N-acetylgalactosamine attached to the initial galactose.

Antibody Interaction with Antigen

  • Antibody in Type B Blood: The antibody that recognizes the type A antigen is the anti-A antibody.

  • Interactions: The interaction involves several key amino acid residues:

    • Histidine Residue: Interacts with the oxygen atom of the acetyl group of the type A antigen.

    • Tyrosine Residue (Tyrosine L32): Forms a hydrogen bond with the hydroxyl group of the fucose residue.

    • Tryptophan Residue: Forms hydrophobic interactions with the nonpolar side of N-acetylgalactosamine (GalNAc).

Carbohydrate-Protein Interactions

  • Hydrophobic and Hydrogen Bonding: Normal interactions between carbohydrates and proteins include specific hydrogen bonding and non-specific hydrophobic interactions.

  • Examples of Non-Solubility: Cellulose and amylose, which are generally insoluble in water.

  • Hexose Structures: Hexoses with the C3 hydroxyl group pointed upward (mannose, galactose, glucose) tend to exhibit hydrophobic interactions with amino acids.

Selectins and Cellular Interactions

  • Definition of Selectins: Selectins are a specific type of lectin that facilitate cell-to-cell interactions by binding carbohydrates.

  • Function in Leukocyte Movement:

    • Interaction with Endothelial Cells: Selectins on endothelial cells (E and P selectins) interact with L-selectin on leukocytes, promoting rolling and migration.

    • Inflammatory Response: Under inflammation, endothelial cells increase E-selectin expression.

    • Rolling to Binding: Leukocyte rolls through the bloodstream and binds tightly to sites of inflammation due to the presence of selectins.

Glycoproteins and Inflammation

  • Evolutionary Perspective: Inflammation was historically vital for healing; however, advancements in medicine have made it less essential, leading to potential overreactions in modern contexts.

  • Examples of Receptor Structures:

    • Oligosaccharide Composition: Consists of N-acetylglucose, fucose, galactose, and sialic acid.

    • Calcium’s Role: Calcium aids in mediating interactions between carbohydrates on receptors and selectins, enhancing leukocyte adhesion effectiveness.

Fish Antifreeze Proteins

  • Description: A unique protein that enables fish in polar regions to survive freezing conditions.

  • Mechanism: The protein contains a repeat sequence of alanine, alanine, and threonine, and features a disaccharide (galactose attached to N-acetylgalactosamine).

  • Function: The hydroxyl groups on the antifreeze protein interact with water molecules, preventing ice crystal formation, allowing fish to thrive in frigid oceans.

  • Biomass Consideration: Notably, these antifreeze proteins are crucial to the ecology of regions such as Antarctica where species relying on these adaptations constitute a significant portion of local fish biomass (approximately 90%).