Proteins - structure and functions

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Proteins

Proteins play a number of vital roles in all organisms. It is an essential part of the diet and helps the body repair muscles, grow tissue, control metabolism as well as many more things. In fact, half of the non-water mass of the body is protein. Unlike carbohydrates or lipids, proteins all contain nitrogen as well as carbon, hydrogen and oxygen. Sulphur and phosphorus are also sometimes present.

The Structure of Proteins

Proteins are built up from amino acids. There are over 100 amino acids and they all have an NH2 (amino) and a COOH (carboxyl acid) group, as shown below. This is why they are called amino acids. Both groups have opposite charges - there is a positive charge at one end of the molecule and a negative charge at the other end. The ‘R’ group represents a side chain which in this molecule of glycine (the simplest amino acid) is a hydrogen atom:

        

On its own the body can manufacture 14 of the 22 amino acids it needs. The other 8 must be obtained from diets. Amino acids bond to form proteins. They are joined by peptide bonds. The reaction in which this happens is a condensation reaction and includes the formation of a water molecule. A peptide bond is a covalent bond between the 2 amino acids.

Hydrolysis the breaking up of the peptide bond to leave amino acids free. When 2 amino acids bond, a tripeptide is formed as the 2 amino acids become joined by a peptide link and when 3 amino acids bond, a tripeptide is formed, which has 2 peptide links. Multiple amino acid chains (like with polysaccharides) are called polypeptides. Polypeptides may be made up of up to 400 amino acids and although only 20 amino acids are used in the synthesis of proteins, the number of ways they could be formed (isomers) is almost unlimited.

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Polypeptides may fold up to form complex 3d structures. The possible structures are broken down into 4 levels: primary, secondary, tertiary and quaternary. The primary structure of a protein is the order of the amino acids of which its polypeptides are composed. At this level, only the covalent bonds between corresponding amino acids are recognised. The secondary structure refers to the way the chain of amino acids fold or turns upon itself as a result of the hydrogen bonding (the hydrogen-oxygen attraction that holds the molecule together). There are two secondary structures- the alpha-helix and the bet-pleated sheet.

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