B1.2 Proteins

Structure of an amino acid

A central carbon atom bonded to an amine, a carboxylic acid, a hydrogen atom, and a R group (which is how each amino acid differs)

Non-essential amino acids

Amino acids which human cells can synthesise from other amino acids. There are 11

Essential amino acids

Amino acids which human cells cannot synthesise and need to be consumed via our diets

Denaturation

The irreversible change of protein conformation, caused by high temperatures or extreme pH environments. The bonds between the R groups are relatively weak, and can be broken easily, changing the conformation of the protein

Denaturation in changes in egg white

Egg white is mainly the protein albumin. Heating causes the hydrophobic amino acids in albumin to appear at the edges, where they cause the protein to become insoluble. A harder, solid layer forms, which is the cooked egg white

Primary structure of proteins

The sequence of amino acids

Secondary structure of proteins

Forms due to pleating and coiling of the amino acid chain, which can give rise to alpha helices or beta-pleated sheets. It is held together by weak hydrogen bonds which form between the carbonile and amine groups on parallel strands.

Tertiary structure of proteins

The complex, three-dimensional shape into which the secondary structure folds due to the interactions between R groups. This gives proteins a very specific shape

Tertiary structure: amino acids

Amino acids are either polar or non-polar depending on the structure of their R groups. Proteins with polar amino acids form structures that are soluble in water, developing a globular structure with the polar amino acids on the outside and the non-polar clustered in the centre

Quaternary structure of proteins

Proteins that contain multiple polypeptide chains functioning together as a single protein. Each polypeptide chain is a subunit. It can be either conjugated or non-conjugated

Conjugated proteins

Contain non-protein components known as prosthetic groups

Non-conjugated proteins

Do not contain non-protein components

Haemoglobin

Has a quarternary structure (4 poplypeptide subunits with a haem group with a iron ion) and is a conjugated protein

Insulin

Has a quarternary structure and is non-conjugated. Consists of two polypeptide subunits joined by three disulfide bridges

Collagen

Has a quarternary structure and is non-conjugated. It is a fibrous protein with three polypeptide subunits in a helix

Globular proteins

Compact, roughly spherical in shape and soluble in water

Fibrous proteins

Long strands of polypeptide chains that have cross-linkages due to hydrogen bonds. They are insoluble in water due to hydrophobic R groups and are more suitable for structural roles