5.3.1.24 ) is an enzyme that catalyzes the third step of tryptophan biosynthesis . [ 1 ] N-(5-phospho-beta-D-ribosyl)anthranilate 1-(2-carboxyphenylamino)-1-deoxy-D-ribulose 5-phosphate Hence, this enzyme has one substrate , N-(5-phospho-beta-D-ribosyl)anthranilate , and one product , 1-(2-carboxyphenylamino)-1-deoxy-D-ribulose 5-phosphate . This enzyme belongs to the family of isomerases , specifically those intramolecular oxidoreductases interconverting aldoses