112 Globular and Fibrous Proteins 113 What You Know So Far:

Key Idea: Globular proteins have mainly catalytic and regulatory functions, while fibrous proteins have a mainly structural function.

Globular proteins

Globular proteins have a roughly spherical shape and are soluble in water. Many globular proteins are enzymes and have a catalytic role in regulating metabolic pathways.

Their properties include: • Easily water soluble • Tertiary structure is critical to function • Polypeptide chains folded into a roughly spherical shape

Their functions include: • Regulatory, e.g. hormones (insulin) • Catalytic, e.g. enzymes • Transport, e.g. haemoglobin • Defensive, e.g. antibodies Their functions include: • Structural role in cells and organisms, e.g. collagen in connective tissue, cartilage, bones, tendons, and blood vessel walls. • Contractile in the cytoskeleton and muscle

Bonds between different polypeptide chains help hold globular proteins in their required shape.

Human insulin is a relatively small globular protein involved in regulating blood glucose. It has two polypeptide subunits.

RuBisCo is a large multiunit enzyme. It catalyses the first step of carbon fixation in photosynthesis. It consists of 8 large and 8 small subunits and is the most abundant protein on Earth.

1. Contrast the properties of globular and structural proteins:

2. How does the shape of a fibrous protein relate to its functional role?

3. How does the shape of a catalytic protein (enzyme) relate to its functional role?

Fibrous proteins

Fibrous proteins are fibre-like and not water soluble. They are often made up of repeating units and give stiffness and rigidity to the fluid components of cells and tissues. Their roles are structural and contractile. Their properties include: • Water insoluble • Physically very tough. They are supple or stretchy • Parallel polypeptide chains in long fibres or sheets

Covalent cross links between the collagen molecules. Hydrogen bond

Glycine

Collagen (above) consists of three helical polypeptides wound around each other to form a ‘rope’ and held together by hydrogen bonds. Many collagen molecules form fibrils and the fibrils group together to form larger fibres.