Building blocks of proteins
Proteins are large molecules made of carbon, hydrogen, oxygen and nitrogen (many also contain sulphur). Their building blocks are amino acids. There are about 20 different amino acids; the order in which they are joined decides which protein is formed. Because the 20 amino acids can be arranged in countless different sequences, an enormous variety of proteins can be made, each suited to a particular task in the body.
How proteins are built
Two amino acids join by a condensation reaction that removes water and forms a peptide bond. Many amino acids joined in this way make a polypeptide chain. One or more polypeptide chains fold into a precise three-dimensional shape, and this shape decides the protein's job.
Key idea
The sequence of amino acids determines the folded shape, and the shape determines the function of the protein.
Functions and denaturation
- Enzymes are proteins that speed up reactions.
- Structural proteins such as collagen and keratin support tissues.
- Transport proteins such as haemoglobin carry oxygen.
- Defence proteins such as antibodies fight pathogens; some hormones, for example insulin, are also proteins.
High temperature or extreme pH changes the shape of a protein permanently by breaking the bonds that hold it folded; the protein is then said to be denatured and can no longer work. This is why a high fever is dangerous and why enzymes stop working outside their ideal conditions. Proteins are also an important part of a balanced diet, needed for the growth and repair of body tissues.
Example
When an egg is heated, the clear albumen turns solid and white because the proteins are denatured and change shape. The Biuret test confirms protein: the blue reagent turns purple/violet.
Remember
Amino acids + peptide bonds = protein. Contains nitrogen. Biuret test turns purple for protein.