Proteins are a group of biomolecules (or biopolymers) that form enzymes and muscles in the body. They are the chief nitrogenous compounds found in living organisms but are still only approximately 15% nitrogen. The other elements they are comprised of is carbon, oxygen, hydrogen and a relatively small amount of sulphur.
Proteins are made of amino acids which, as the name implies, are made from amino groups (NH2) and carboxylic acid groups (COOH). These groups are joined together via a carbon atom that is also bonded to hydrogen and another group of elements (the ‘R’ group). The ‘R’ group is what distinguishes one amino acid from the other. It could be another hydrogen (to form the amino acid glycine) or a methyl group (CH3) to form alanine or even a CH2SH group to form cysteine. In all there are 23 amino acids found in humans and by combining together these amino acids can form a myriad of different proteins. The combinations are almost limitless.
All amino acids (with the exception of glycine) will form optical isomers. However, all amino acids in nature will be found as the ‘L’ version of the amino acid – indicating that there was a common ancestor at some point in the evolutionary tree.
Amino acids will react together in a condensation reaction. The OH part of the carboxylic acid can join with a hydrogen atom of the NH2 group. The amino acids will be joined together via a peptide bond (or a peptide link) and in doing so produce a molecule of water.
Many, many, many amino acids will join together to make a polypeptide chain. The actual order of amino acids is called the primary structure.
However, the amino acid chain will either coil around itself to form what is referred to as the alpha-helix or fold to form what is termed the beta-pleated sheet. This is also termed the secondary structure.
The secondary structure can also twist and fold itself to form the tertiary structure. The secondary structure is held together by a variety of different forces such as London dispersion forces, hydrogen bonds and ionic bonds. The presence of two sulfur groups will also form a covalent bond between each other, often termed a sulphur bridge.
Different tertiary structures can interact and combine to form a quaternary structure. The quaternary structure (as well as the tertiary structure) will give the protein a 3D shape and, in the case of enzymes, this 3D shape is vital in allowing it to function.
What option are you being taught? If you are being taught biochemistry, have covered this particular topic? Have you carried out any lab work on proteins (yet)? If so, what have you carried out?