Proteins

Moving on from carbohydrates, we come to proteins. Proteins are another macromolecule, also known as polypeptides, and are formed when amino acids are linked into long chains via peptide bonds. There are many different types of proteins important to life such as enzymes and the proteins in the fluid plasma membrane of a cell.

There are seven different classifications of proteins:

Enzymes	Biological catalysts that speed up chemical reactions in the body. E.g. ATPase, an enzyme found in all cells which catalyses the breakdown and formation of ATP.
Structural proteins	Proteins that form part of a body of an organism. Keratin, a major component of hair is a structural protein.
Signal proteins	These carry messages around the body. For example, the hormone insulin, which is involved in the controlling of blood glucose levels.
Contractile proteins	These are proteins involved in movement, such as actin and myosin, which allow muscles to contract.
Storage proteins	These are proteins that store nutrients, for example the albumen in eggs.
Defensive proteins	Proteins that form part of the immune system, mainly the blood antibodies.
Transport proteins	These transport materials around organism, e.g. the haemoglobin in blood.

Proteins are made up of combinations of several different amino acids; there are 20 amino acids in total. Amino acids are composed of an Amine group (NH₂), a Carboxylic acid group (COOH) and an R group. The R group is replaced with any of the 20 R side chains, which determines the properties of the amino acid formed. The simplest amino acid is glycine, which has an R group of just a single hydrogen atom. The amino group, carboxylic group, a hydrogen atom and the R group are each bonded to a central alpha carbon atom with a covalent bond. Some R groups are basic, others alkali, some hydrophilic, others hydrophobic, and some contain the compound benzene. Amino acids are amphoteric, which means they can act as acids or alkalis.

An amino acid can also be known as a monopeptide. Through condensation polymerisation, two or more amino acids can combine to form a dipeptide, or a polypeptide. The bonds that form between each amino acid are called peptide bonds. When a polypeptide consists of enough amino acids, it is known as a protein.

Proteins are each built up of four structures, each structural layer determining the next. The primary structure is the exact sequence of the amino acids that make up its polypeptide chain. By switching the position of just two of these amino acids the whole protein structure could be changed.

A protein’s secondary structure is the way in which the polypeptide chain coils or folds into either an alpha-helix or a beta-pleated sheet. The shape of these structures is held b regularly spaced hydrogen bonds which form between the N-H group of one amino acid and the C=O group of another.

The tertiary structure is the 3D shape of the polypeptide chain; proteins can be classified as either fibrous or globular, depending on their tertiary structure. Fibrous proteins are composed of parallel polypeptide chains cross linked to form long fibres or sheets. They are usually insoluble in water and physically tough, which makes them suitable for their mainly structural functions. In globular proteins, the chain of amino acids is tightly folded to form a spherical shape. Many of them are folded so that their hydrophobic ends are inside the molecule, and the hydrophilic ends facing outside, making the molecule water soluble. Many hormones, antibodies and enzymes are globular proteins. The precise shape of the globular protein determines its every property.

If the bonds holding a protein together are broken, then denaturation occurs. The polypeptide is chemically unchanged but the tertiary structure of the chain is lost, and hence they lose their specific shape and cannot function. The process of denaturation is nearly always irreversible, and can be caused by changes in pH, temperature or salt concentration.

The quaternary structure is the way in which more than one polypeptide chains bond together, and the way in which they are arranged.