Enzymes of Aromatic Biosynthesis

Abstract

The higher plants form many kinds of aromatic compounds, such as aromatic amino acids, lignins, flavonoids, phenolic glycosides, coumarins, and alkaloids. Some of these, like the aromatic amino acids, are universally distributed in nature and others are found in a rather restricted number of plants. The biosynthetic pathways of these compounds in plants have been actively investigated recently by tracer experiments using C¹⁴-labelled compounds. It has been assumed that the universally occurring aromatic amino acids are made from carbohydrates following essentially the same pathway — “Davis’ scheme of aromatic biosynthesis” — as in microorganisms, and that the aromatic compounds restricted to certain species might be derived secondarily from the fundamental aromatic amino acids or intermediates involved in their biosynthesis (Neish, 1960 a). The investigations of aromatic biosynthesis in plants have now reached the stage where hypotheses about biochemical pathways obtained by the tracer experiments have to be confirmed by the series of biochemical reactions based on the enzymes or enzyme systems, as has been done in aromatic biosynthesis of microorganisms. However, few investigations on the enzymes of aromatic biosynthesis have hitherto been carried out.