Heterocycles in Nature 159The (non-essential) amino acid proline is based on pyrrolidine. Hydroxyproline, which is synthesised (after incorporation
into a protein) from proline units in the body, is a major component of collagen, the fi brous structural protein that supports
tissues and is the main component of cartilage and the most abundant protein in the human body.
Heterocyclic vitamins – co-enzymes
Vitamins are substances essential for a healthy life; humans must ingest vitamins via their diet because there is no mecha-
nism for their biosynthesis in the body. There are 14 vitamins – absolutely essential dietary components, indeed the name
was coined when the fi rst vitamin chemically identifi ed (vitamin B 1 in 1910) turned out to be an amine – a vitalamine.
A typical vitamin is folic acid, a complex molecule in which the functionally important section is the bicyclic pyrazino-
pyrimidine ring, known as a pteridine and its aminomethyl substituent. The name ‘pteridine’ was coined when the fi rst
natural substances shown to involve this ring system were the wing pigments of butterfl ies (Lepidoptera). This vitamin
is converted in the body into a tetrahydrofolic acid (FH 4 ), which is crucial in carrying one-carbon units, at various oxi-
dation levels, for example in the biosynthesis of purines, and is mandatory for healthy development of the fetus during
pregnancy. Other essential cofactors that contain pteridine units must and can be biosynthesised in humans – without
them we cannot survive – these are oxygen-transfer enzymes based on molybdenum in which the metal is liganded by a
complex ene-dithiolate in a ubiquitous cofactor. Xanthine oxidase is an example.
Several highly signifi cant vitamins are water-soluble and heterocyclic in nature and, further, their utility in the enzyme
cofactors into which they are incorporated can only be understood on the basis of their intrinsic heterocyclic reactivity.
We deal in detail fi rst with the two important pyridine-containing vitamins – vitamin B 3 (niacin or nicotinamide) and
vitamin B 6 (pyridoxine) and then with the thiazole-containing thiamin (vitamin B 1 ).