4.3 Protein

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1. Proteins are complex compounds that made up of carbon, hydrogen and nitrogen. Some proteins contain sulphur and phosphorus.
2. It is needed by the body for 
1. growth, 
2. repair of damaged tissues and 
3. synthesis of secretions (enzymes, mucus, hormones.)
3. Proteins are polymer that made up of monomers called amino acids. Each molecule of amino acid has one amino group (-NH₂) and one carboxyl group ( -COOH).
4. Two molecules of amino acids are joined by a peptide bond to form a dipeptide through condensation.
   
   Amino acid + amino acid → dipeptide + water
5. Polypeptides (protein) are formed when many amino acids are joined together by condensation.
6. Proteins can be broken down through hydrolysis into amino acids.
7. There are 20 types of amino acids in cells.
8. Amino acid can be divided into essential amino acid and nonessential amino acid.
1. Essential amino acids are amino acids that cannot be synthesised by the body. They can only be obtained from diets. An example is leucine.
2. Non-essential amino acids are amino acids that can be synthesised by the body. They are derived from other amino acids. There are 11 non-essential amino acids.
9. Proteins can be grouped into four levels of organisation according to their structures.
1. Primary structure (linear sequence of amino acids in a polypeptide.)
2. Secondary structure (polypeptide is coiled to form an alpha-helix or folded into beta-pleated sheets)
3. Tertiary structure (helix or beta-pleated sheets are folded in many ways into a three dimensional shape of a polypeptide.)
4. Examples are hormones, enzymes, antibodies and plasma proteins.
5. Quarternary structure (Two or more tertiary structure polypeptide chains combine to form a large and complex protein molecule. Example: haemoglobin.)

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