Many enzymes require cofactors to be catalytically active. A cofactor can be either a coenzyme or an inorganic ion, such as Ca2+ ions. A coenzyme is a small organic molecule, derived from vitamins, that is required by some enzymes for activity. Vitamins are organic molecules which are needed in very small amounts in the diets of animals. Coenzymes accept electrons or small groups of molecules and unlike enzymes, are altered during the reaction. Coenzymes can be restored to their original state in subsequent reactions where the electrons or small molecules that they have have accepted are removed.


Vitamins are required in very small doses and with the exception of vitamin D, which is syntesised by the body, must be consumed in the diet of animals, hence are termed essential nutrients. Two groups of vitamins exist, water soluble and fat soluble. Water soluble vitamins move freely throughout the body and are excreted via the kidneys. Water soluble vitamins are needed on a daily basis and because they are constantly removed they are unlikely to reach toxic levels as fat solbule vitamins are.

Fat soluble vitamins, on the other hand, are stored in the body and are not excreted as easily as water soluble vitamins. Fat soluble vitamins do not have to be consumed on a daily basis as do water soluble vitamins. Fat soluble vitamins run the risk of becoming toxic if consumed in excessive doses as they accumulate in the body of animals, in particular the liver.

Fat soluble vitamins can be distinguished from their chemical structure. Consider vitamin D, shown on the right. It is predominantly a hydrocarbon with one lone OH group at the end. This makes the molecule hydrophobic as it is unable to interact with the polar water molecules and allows for a high degree of interaction between lipid molecules.

Consider the molecule of vitamin C, shown on the right. In contrast to the lipophilic
(fat loving) vitamin D molecule, this has a number of polar (OH) groups protruding from the molecule. Hydroxy groups create hydrogen bonding between the vitamin C molecule and water molecules. This high degree of interaction with the water molecules allows the molecule to be highly soluble in water.

1) Consider the molecule of vitamin A, shown on the right.

a) Is this vitamin hydrophobic or hydrophilic?

b) Is it likely to be required in the diet on a daily basis?

c) Is this molecule rapidly excreted via the kidneys?

d) Is it toxic in large doses?


2) Vitamins are sometimes called coenzymes. Why is this strictly incorrect?

3) Consider the three molecules shown on the right.

a) Identify the three molecules shown on the right as hydrophobic or hydrophilic?

b) Which are likely to be required by the body on a daily basis?

c) Which woulld be expected to appear in the urine soon after ingestion?