2.2 - globular and fibrous proteins

Features of a globular protein

reactive, soluble in water, weak(er), often form spherical shape,

Features of a fibrous protein

insoluble in water, unreactive, strong, form structural components, repeating sequences of amino acids in one chain

Examples of globular proteins

Haemoglobin, insulin, pepsin

Examples of fibrous proteins

Elastin, collagen, keratin

Function of haemoglobin

To transport oxygen from the lungs to the tissues

Structure of haemoglobin

Four polypeptide chains: two alpha, two beta; each one has as haem prosthetic group that contains the iron ion that oxygen will bind to

Function of insulin

Targets muscle and fat cells to tell them to increase their uptake of glucose from blood - regulates blood glucose concentrations

Structure of insulin

Two polypeptide chains: one with a section of alpha-helix, and one with a section of Beta-pleat.

Function of pepsin

Enzyme that digests protein in the stomach

Structure of pepsin

One long polypeptide chain - folds into a 3D tertiary structure, has lots of a acidic R groups so it is stable in the acidic conditions of the stomach

Function of keratin

To be hard and strong, also is an impermeable and waterproof barrier to prevent pathogens entering

Structure of keratin

Has lots of cysteine amino acids so lots of disulphide bridges are formed, this plus hydrogen bonding makes the protein very strong

Function of collagen

To provide mechanical strength

Examples of collagen in the body

Prevents artery from bursting, tendons made of collagen, bones made of collagen to make them hard, cartilage and connective tissue made from collagen

Function of elastin

Allows body parts to stretch and adapt their shape

Structure of elastin

Cross-linking and coiling makes the structure strong and extensible