proteins 1

What is the general structure of fibrous proteins?

Long, parallel polypeptide chains with occasional cross-linkages forming fibres; little or no tertiary structure; insoluble in water; very tough.

Why are fibrous proteins insoluble in water?

Their long, parallel chains with cross-linkages expose mostly hydrophobic amino acid side chains, preventing interaction with water molecules.

Give three examples of fibrous proteins and their biological roles.

1. Collagen – structural support in tendons, ligaments, bones, skin. 2. Keratin – structural component in hair, nails, feathers, horns. 3. Silk (spider web/silkworm) – structural threads for webs and cocoons.

What is the most common structural protein in animals?

Collagen – makes up to ~35% of body protein.

Describe the primary structure of collagen.

Three polypeptide chains, each up to 1000 amino acids long, with repeating glycine–X–Y sequence (X often proline, Y often hydroxyproline).

What secondary/tertiary structure arrangement does collagen have?

Three α-chains wound into a triple helix, held together by many hydrogen bonds.

Why is collagen so strong?

Triple helix arrangement, high number of hydrogen bonds, formation into fibrils, then fibres, giving tensile strength comparable to steel.

How does collagen combine with bone tissue?

Collagen fibres are embedded in bone matrix, acting like steel rods in reinforced concrete to provide tensile strength.

What happens in osteogenesis imperfecta?

Collagen triple helix fails to form properly due to genetic mutation → bone has reduced tensile strength → brittle and fractures easily.

What is the general structure of globular proteins?

Complex tertiary (and sometimes quaternary) structures folded into compact, spherical shapes; often soluble as colloids due to size.

Why do globular proteins form colloids instead of true solutions in water?

Molecules are too large for a true solution; their ionic groups interact with water to form a dispersed but stable suspension (colloid).

List three key roles of globular proteins.

1. Enzymes (catalysis). 2. Hormones (signalling). 3. Antibodies (immune defence). Also maintain cytoplasmic structure.

Describe the structure of haemoglobin.

Globular protein with 4 polypeptide chains (574 amino acids total), held by disulfide bonds, each with an iron-containing haem group; conjugated protein.

What type of protein is haemoglobin?

Both a globular protein and a conjugated protein (prosthetic haem group contains iron).

How does haemoglobin bind oxygen?

Iron in haem group binds O₂; quaternary structure arrangement influences oxygen affinity and release.

Define conjugated protein.

A protein molecule joined to a non-protein component (prosthetic group) which affects its structure and function.

Give two examples of conjugated proteins and their prosthetic groups.

1. Haemoglobin – haem group with Fe²⁺. 2. Chlorophyll – prosthetic group with Mg²⁺.

What are glycoproteins and their functions?

Proteins with a carbohydrate prosthetic group; carbohydrate helps bind water, makes them resistant to protease digestion; found in mucus, synovial fluid (lubrication, reducing friction, protection).

Why does stomach mucus protect the stomach wall from digestion?

Glycoprotein’s carbohydrate part holds water and resists protease action, forming a protective barrier over protein walls of the stomach.

What are lipoproteins and their role?

Proteins conjugated with lipids; transport cholesterol in the blood.

Distinguish between LDLs and HDLs.

LDLs: low-density, ~22 nm, more lipid, less protein; deliver cholesterol to tissues (can deposit in arteries). HDLs: high-density, ~8–11 nm, more protein, less lipid; transport cholesterol from tissues to liver for excretion.

Why are HDLs denser than LDLs?

HDLs have a higher protein-to-lipid ratio; proteins are more compact molecules than lipids.

Explain why glycoproteins in synovial fluid reduce friction.

Carbohydrate chains hold water → slippery and viscous consistency → cushions joint movement, minimises wear on cartilage.

What feature of collagen’s primary structure allows tight packing in the triple helix?

Every third amino acid is glycine (small side chain), allowing close packing of chains.

What type of bonds hold collagen’s triple helix together?

Numerous hydrogen bonds between polypeptide chains.