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PROTEINS
Proteins are compounds containing carbon, hydrogen, oxygen and nitrogen and occasionally sulphur and phosphorus.
Basic unit of protein: amino acids.
Structure of an amino acid
Figure 15 Structure of an amino acid.
There are a total of 20 different amino acids.
Egs include phenylalanine, methionine, glycine, histidine, proline, cysteine. (For whole list, refer to: http://www.chemie.fu-berlin.de/chemistry/bio/amino-acids_en.html)
Polypeptide chains
Figure 16 Three amino acids are joined by condensation reactions with the removal of two molecules of water. Different amino acids are held by peptide bonds.
Amino acids are arranged in a linear sequence by peptide bonds to form a polypeptide chain.
Different sequence of amino acids gives rise to different polypeptide chains.
Example:
Alanine, glycine, histidine and proline are examples of amino acids found in cells.
These four amino acids are arranged in a different order to form two completely different polypeptide chains.
Polypeptide Chain 1
alanine - glycine - histidine - proline
Polypeptide Chain 2
glycine - alanine - histidine - proline
Reverse of the first two amino acids leads to completely different polypeptide chains.
How do amino acids make up a protein?
Figure 17 Relationship between amino acids, a polypeptide and a protein.
Note: The 3D shape of each protein is unique. Changes in temperature and pH will cause the unfolding of the protein à denaturation
Main functions of proteins
Stabilise pH of blood (pH = 7.4) due to presence of amino and acidic groups.
As enzymes which alter the rate of chemical reactions in living systems.
Hormones, eg insulin and glucagons which regulate blood sugar level.
Antibodies to protect the body from invasion by foreign particles.
Haemoglobin (red pigment in red blood cells) for transport of oxygen.
Mechanical structures (eg. Keratin – important component of hair and collagen – component of skin and cartilage)
Source of energy. 1 g of protein provides 17 kJ of energy.
Biuret Test for identifying proteins
Step 1: To 2 cm3 of food sample, add 2 cm3 of sodium hydroxide solution.
Step 2: Shake the mixture well.
Step 3: Add 1% copper (II) sulfate solution to the mixture, drop by drop.
(+): If proteins are present, the contents turns from blue to violet.
(-): If proteins are absent, the contents remains blue.
Figure 18 Positive (left) and negative (right) result of Biuret test.
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