PROTEINS

• Proteins are the most abundant organic compounds in cells and constitute 50% of total dry weight
• Proteins are compounds which are made up of carbon, hydrogen, nitrogen, oxygen and sometimes sulphur and phosphorus
• The structural units of proteins are amino acids
• The nature of a protein is determined by the types of amino acids it is made of
• There are about 20 common amino acids that make up proteins

Essential and Non-Essential Amino Acids

• Essential amino acids are those which cannot be synthesised in the body of an organism and must therefore be provided in the diet
• There are ten amino acids which are essential for humans
• These are valine, leucine, phenylalanine, lysine, tryptophan, isoleucine, methionine, threonine, histidine and arginine
• Non-essential amino acids are those which the body can synthesise and therefore need not be available in the diet
• There are ten of them
• These are glycine, alanine, glutamic acid, aspartic acid, serine, tyrosine, proline, glutamine, arginine and cysteine
• Proteins are essential in the diet because they are not stored in the body
• Excess amino acids are deaminated

Formation of Proteins

• Proteins are made up of many amino acid units joined together through peptide bonds
• When two amino acids are joined together a dipeptide is formed
• The chemical process involved is called condensation and a molecule of water is eliminated
• When many amino acids are joined together a polypeptide chain is formed
• The nature of a particular protein depends on the types, number and sequence of amino acids from which it is made

Functions of Proteins As structural materials proteins

• Are the basic building structures of protoplasms
• Proteins in conjunction with lipid form the cell membrane

Examples of structural proteins include:

• Keratin (in hair, nails, hoofs, feathers and wool)
• Silk in spider’s web
• Elastin forms ligaments that join bones to each other

Protective proteins

• Antibodies that protect the body against foreign antigens
• Fribrogen and thrombin are involved in clot formation, preventing entry of micro-organisms when blood vessel is cut

As functional chemical compounds

• Examples are hormones and enzymes that act as regulators in the body
• Respiratory pigments
• Examples are haemoglobin that transports oxygen in the blood and myoglobin that stores up oxygen in muscles
• Contractile proteins – make up muscles, i.e myosin and actin
• Proteins combine with other chemical groups to form important substances e.g. mucin in saliva

Source of energy

• Proteins are a source of energy in extreme conditions when carbohydrates and fats are not available e.g. in starvation