Protein structure is the key to protein function

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Protein structure is the key to protein function

All amino acids contain an amine group -NH2, a carboxylic group - COOH, and a hydrogen - H, attached to the central atom. Each type of amino acid has a different side chain, called a residual or R group. Glycine is the simplest amino acid with -H forming its residual group. 

Primary structure
Two amino acids join in a condensation reaction to form a dipeptide, with a peptide bond forming between the two sub-units. This process can be repeated to form polypeptide chains which may contain thousands of amino acids. A protein is made up from one or more of these polypeptide chains. The sequence of amino acids in the polypeptide chains is known as the primary structure of the protein.

Secondary structure
The chain of amino acids may twist to form an a-helix. Within the helix, hydrogen bonds form between the C=O of the carboxylic acid and the -NH of the amine group of different amino acids, stabilising the shape. Several chains may link together, with hydrogen bonds holding the parallel chains in an arrangement known as a B-pleated sheet.

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Protein structure is the key to protein function

Tertiary and quaternary structure
A polypeptide chain often bends and fold to produce a precise 3D shapeChemical bonds and hydrophobic interactions between R groups maintain this final tertiary structure. An R group is polar when the sharing of the electrons within it is not quite even. Polar R groups attract polar molecules, like water, and are therefore hydrophilic. The non-polar groups are hydrophobic. Non-polar, hydrophobic R groups are arranged so they face the inside of the protein, excluding water from the centre of the molecule. Only proteins with several polypeptide chains, like haemoglobin. have a quaternary structure. Single chain proteins top at the tertiary structure. 

Globular and fibrous proteins
In globular proteins the polypeptide chain is folded into a compact spherical shape. These proteins are soluble due to the hydrophilic side chains that project from the outside of the molecules and are therefore important in metabolic reactions. Enzymes are globular proteins. Their 3D shape is crucial to their ability to form enzyme-substrate complexes and catalyse reactions within cells.

Conjugated proteins
Conjugated proteins have another chemical group associated with their polypeptide chains

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Protein structure is the key to protein function

Globular and fibrous proteins
The 3D shapes of globular proteins are critical to their roles in binding to other substances. Examples include transport proteins within membranes and the oxygen-transport pigments haemoglobin (in red blood cells) and myoglobin (in muscle cells). Antibodies are globular and rely on their shapes to bind to the microorganisms that enter our bodies.

Fibrous proteins do not fold up into a ball shape but remain as long chains. Several polypeptide chains can be cross-linked for additional strength. These insoluble proteins are important structural molecules. Keratin in hair and skin, and collagen in the skin tendons, bones, cartilage and blood vessel walls, are also examples of fibrous proteins.  

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