Lecture 6 - Globular Proteins, Fibrous Proteins, and Post-Translational Modifications

What are the structural components of antibodies?

Two heavy chains and two light chains. The light chains consist of one constant and one variable domain (where antigens bind), and the heavy chains consist of three constant and one variable domains. The heavy and light chains are joined by disulfide bonds.

What aspect of variable domains make variable domains of antibodies highly specific?

The hypervariable regions in the variable domains create high specificity in antibodies to recognize the epitopes of antigens.

What is the function of the hinge region in an antibody?

To allow molecular flexibility when the molecule binds an antigen.

What are the types of non-covalent interactions that occur between an antibody and antigens?

Electrostatic forces, hydrogen bonds, van der waals forces, hydrophobic forces, and cation-pi interations

What are the functions of antibodies and what does each entail?

Neutralization: the antibody binds to bacterial toxins and prevents their ability to infect cells - Opsonization: the antibody Fc region binds to Fc receptor on accessory cells to ingest and kill the pathogen when bound - complement activation: antibody triggers complement by activating C1, the first step in the complement pathway.

What are the two types of immunodeficiency disease and what causes them?

Primary ID is caused by mutations in any immune system gene, which is less common among the two. Secondary ID is more common and takes place because of diseases like HIV/AIDS or other environmental factors (starvation, adverse impact of medical treatment)

What is collagen and where is it mostly found?

Collagen is a protein that provides structural suport to extracellular spaces of connective tissues. It is found in skin, tendons, bones, and ligaments.

What types of collagen are the fibril forming collagens and where are they each found?

Type I is in skin, bone, blood vessels, and cornea. Type II is in cartilage, intervertebral disks, and the vitreous body. Type III is in the blood vessels, skin, and muscles.

What types of collagen are network forming and where are they each found?

Type IV is in the basement membrane. Type VIII is found in the corneal and vascular endothelium.

What types of collagen are fibril associated and where are they each found?

Type IX is found in the cartilage. Type XII is found in tendon, ligaments, and other tissues.

Detail the synthesis of collagen.

Polypeptide precursors of collagen are synthesized in fibroblasts. Proline and lysine residues are hydroxylated. Hydroxylysine residues are glycosylated with glucose and galactose. Three of these precursor helices assemble into an alpha helix, forming procollagen. The procollagen exits the cell via the Golgi vacuoles. The N-terminal and C-terminal propeptides are cleaved by procollagen peptidcase, forming tropocollagen. Tropocollagen forms into cross-linking fibrils, creating collagen.

What characteristics do alpha helices and beta sheets adopt as fibrous proteins?

Beta sheets adopt soft and flexible properties, while alpha helices adopt tough and insoluble properties that vary in hardness/flexibility

What is the amino acid composition of collagen type I?

33% glycine, 21% proline and hydroxyproline.

Why is glycine important to collagen structure and what is the amino acid formula of collagen?

glycine is the only amino acid that can fit into the triple helix of collagen because its lack of a side chain. it does not interfere with the other two strands when they are in contact. the formula is Gly-X-Y

What are the respective functions of hydroxyproline and hydroxylysine?

Hydroxyproline is involved in hydrogen bond formation that stabilizes the triple helix (through its extra -OH group). Hydroxylysine molecules are the sites of attachment of disaccharide moieties (galactose--glucose on extra -OH)

What important molecule is needed for the hydroxylation of lysine and proline?

Ascorbate (vitamin C)

What causes scurvy and what changes result from the disease?

The absence of vitamin C causes scurvy. Hydroxyproline and hydroxylysine are in defecit and the melting temperature of collagen sharply decreases. The hydrogen bond formation between strands of collagen is most and leads to bleeding gums, swelling joints, and poor wound healing.

How are crosslinks formed in collagen?

Lysine side chains are oxidized by lysyl oxidase to form an aldehyde. A second lysine residue reacts with the aldehyde in allysine. Aldol condensation between the two moieties forms a covalent bond.

What causes Ehlers-Danlos syndrome?

Caused by a deficieny of collagen processing enzymes lysyl hydroxylase or N-procollagen peptidase. Can also be caused by mutations in type I, III, or V collagen.

What are the general characteristics of EDS?

Skin extensibility and fragility; joint hypermobility

What are the characteristics of type III collagen EDS?

This is the vascular form of EDS - associated with lethal arterial rupture

What is Osteogenesis Imperfecta and what are its causes?

OI is known is brittle bone syndrome, leading to bones that are easily fractured. This is caused by dominant mutations in type I collagen, replacing glycine with amino acids with bulky side chains

What is the treatment for osteogenesis imperfecta and how does it work?

OI is treated with biphosphates. They inactivate osteoclasts (cells that break down bone tissue), increase apoptosis of osteoclasts, and decrease apoptosis of osteoblasts (cells that lay down new bone matrix)

Mutations in which type of collagen leads to lethal OI in infants?

Type II collagen

What is each type of post-translation modification?

Acetylation, ADP-ribosylation, carboxylation, fatty acylation, glycosylation, hydroxylation, methylation, and phosphorylation