Tertiary Structure

• Tertiary structure of proteins= spatial arrangement of the secondary structures (folding of domains)

• Stabilization: between side chains of AAs

• 1) hydrogen bonds

• 2) ionic (electrostatic) interactions

• 3) hydrophobic interactions

• 4) disulfide bonds

Classification of proteins according to tertiary structure

• 1) globular proteins(sphero proteins)

• spheroidal shape

• both secondary structures are abundant

• 2) fibrous proteins(sclero proteins)

• rod-like shape

• one secondary structure predominates

• e.g. α-keratin, collagen

Quaternary Structure

• oligomeric structure of a protein (2 or more

• subunits ~ monomers)

• i.e. the structure is found only in proteins

• composed from 2 or more chains (subunits)

• Stabilization: non covalent interactions

Denaturation

Denaturation is the change in structure of protein molecules. The process results in the unfolding of molecules. Factors which contribute to denaturation are heat, salts, pH and mechanical action.

Denaturation is a partially reversible change. For example, when an egg white is whisked it incorporates air to form a foam.

If the foam is left to stand, it will

collapse back to form liquid

egg white.

Coagulation

Coagulation follows denaturation. For example, when egg white is cooked it changes colour and becomes firmer or sets.

The heat causes egg proteins to unfold from their coiled state and form a solid stable network.

This change is irreversible.

Coagulation

Another form of coagulation occurs in the production of cheese. Rennin (an enzyme from a calf’s stomach) is added to milk causing the protein casein to clot, producing curds (solid) and whey (liquid).

Other applications of coagulation are:

• yogurt production;

• thickening of sauces with beaten egg;

• binding ingredients together, e.g. fish, cakes,

reformed meats;

• providing a coating for products, e.g. scotch eggs.

Gluten development

A cross section is shown below of under developed

dough and weak or soft flour 8% respectively.

Products that require short or non-elastic textures,

such as biscuits and cakes, use flours with lower

protein contents.

Gelation

Gelatine is a protein which is extracted from collagen, present in connective tissue in meat.

When it is mixed with warm water the gelatine protein molecules start to unwind.

Although on cooling a stable network is formed, trapping the liquid.

Gelation is reversible.