Proteins

Proteins

Different proteins can appear very different and perform diverse functions (e.g. the water-soluble antibodies involved in the immune system and the water-insoluble keratin of hair, hooves and feathers). Despite this, each one is made up of amino acid subunits.

There about 20 different amino acids that all have a similar chemical structure but behave in very different ways because they have different side groups. Hence, stringing them together in different combinations produces very different proteins.

Proteins

Each amino acid has an amino group (NH2) and a carboxylic acid group (COOH). The R group is a different molecule in different amino acids which can make them neutral, acidic, alkaline, aromatic (has a ring structure) or sulphur-containing.

When 2 amino acids are joined together (condensation) the amino group from one and the acid group from another form a bond, producing one molecule of water. The bond formed is called a peptide bond.

Hydrolysis is the opposite of condensation and is the breaking of a peptide bond using a molecule of water.

Due to the bonding and the shape and chemical nature of different amino acids, the shape of a whole chain of amino acids (a polypeptide or protein) is specific.

This will affect the properties of the protein, just as the type of a necklace depends on the type of beads and how they are strung together. Therefore, the primary structure depends on the order and number of amino acids in a particular protein.

For example:Haemoglobin is made up of 4 polypeptide chains, 2α chains and 2β chains, each with a haem group attached. There are 146 amino acids in each chain. If just one of these is wrong, serious problems can arise (e.g. sickle cell anaemia). The red blood cells become distorted, the amount of oxygen they can carry is reduced and blood capillaries can be blocked, leading to acute pains called crises.

This is the basic shape that the chain of amino acids takes on. The 2 most common structures are the α-helix and the β-pleated sheet.

Secondary structure of proteins

This has a regular coiled structure like a spring, with the R groups pointing towards the outside of the helix. Hydrogen (H) bonds are relatively weak but because there are so many, the total binding effect is strong and stable. The helix is flexible and elastic.

Pleated sheet

This is composed of 'side by side' chains connected by H bonds. All the peptide linkages are involved in inter-chain H bonding so the structure is very stable.

This is the overall 3-D structure of the protein.

The shape of the protein is held together by H bonds between some of the R groups (side chains) and ionic bonds between positively and negatively charged side chains. These are weak interactions, but together they help give the protein a stable shape. The protein may be reinforced by strong covalent bonds called disulphide bridges which form between two amino acids with sulphur groups on their side chains.

These interactions may be electrostatic attractions between charged groups e.g. NH3+ and O- or van der Waal's forces.

Fibrous proteins are made of long molecules arranged to form fibres (e.g. in keratin). Several helices may be wound around each other to form very strong fibres. Collagen is another fibrous protein, which has a greater tensile strength than steel because it consists of three polypeptide chains coiled round each other in a triple helix. We are largely held together by collagen as it is found in bones, cartilage, tendons and ligaments.

Globular proteins are made of chains folded into a compact structure. One of the most important classes are the enzymes. Although these folds are less regular than in a helix, they are highly specific and a particular protein will always be folded in the same way. If the structure is disrupted, the protein ceases to function properly and is said to be denatured. An example is insulin, a hormone produced by the pancreas and involved in blood sugar regulation.

A globular protein based mostly on an α-helix is haemoglobin.

A globular protein based mostly on a β-pleated sheet is the immunoglobulin antibody molecule.

If a protein is made up of several polypeptide chains, the way they are arranged is called the quaternary structure. Again, each protein formed has a precise and specific shape (e.g. haemoglobin)

The majority of proteins are assisted in their functions by a prosthetic group. This may a simple metal ion such as zinc in the enzyme carboxypeptidase, or it may be a complex organic molecule, such as the haem group in haemoglobin.

  1. Virtually all enzymes are proteins.
  2. Structural: e.g. collagen and elastin in connective tissue, keratin in skin, hair and nails.
  3. Contractile proteins: actin and myosin in muscles allow contraction and therefore movement.
  4. Hormones: many hormones have a protein structure (e.g. insulin, glucagon, growth hormone).
  5. Transport: for example, haemoglobin facilitates the transport of oxygen around the body, a type of albumin in the blood transports fatty acids.
  6. Transport into and out of cells: carrier and channel proteins in the cell membrane regulate movement across it.
  7. Defence: immunoglobulins (antibodies) protect the body against foreign invaders; fibrinogen in the blood is vital for the clotting process.

Biochemical test:

The reagent used to test for proteins is called biuret reagent. It can be used as two separate solutions of copper sulphate and potassium or sodium hydroxide or as a ready-made biuret solution. In either case, a purple colour indicates a positive result.

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