The Common-Precursor Pathway of Aromatic Biosynthesis

Davis (184) selected a large number of mutants of Escherichia coli requiring two or more aromatic amino acids for growth, and then tested a large number of substances to see if any could relieve the growth inhibition. Success was attained with shikimic acid (215, 268), at that time a relatively obscure natural product. This indicated either that shikimic acid (structure diagram 1) was a true aromatic precursor or that it could readily be transformed into a true precursor. The likelihood that shikimic acid was a true precursor was increased when other mutants were found to accumulate shikimic acid in the medium, from which it could be isolated (184, 185). 

Not all the mutants responding to shikimic acid responded to a mixture of 

Davis concluded that shikimic acid was a common precursor of phenylalanine, tyrosine, tryptophan, p-aminobenzoic acid, p-hydroxybenzoic acid, and an unknown sixth factor, and he next set out to determine other substances lying on the biosynthetic pathway. The various mutants were therefore tested for syntrophism, i.e., for the ability of one mutant to produce a substance necessary for the growth of another mutant. There was thus found a thermolabile substance, X, which was a true precursor of shikimic acid (184). X was isolated from culture filtrates and identified as 5-dehydroshikimic acid (744). Similar experiments revealed a substance, W, which was a true precursor of substance X (187, 193). This also was isolated and shown to be 5-dehydroquinic acid (906). The enzyme, named 5-dehydroquinase, converting dehydroquinic acid to dehydroshikimic acid has been partially purified (606). It is fairly stable, has a high specificity, appears to have no cofactors, and is of wide occurrence in bacteria, algae, yeasts, and plants but, as expected, could not be found in mammalian liver. 

The enzyme, 5-dehydroshikimic reductase, converting dehydroshikimic to shikimic acid, has also been studied (964a). It requires triphosphopyri-dinenucleotide as cofactor. 

These results are summarized in diagram 1. Substance CP represents a common precursor which has not yet been identified but the existence of which is shown by the existence of mutants which require all six aromatic factors but excrete shikimic acid into the medium. 

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