71] Tyrosine and phenylalanine biosynthesis: The T and P proteins (Aerobacter aerogenes); chorismate mutase (Pisum sativum)

Abstract

This chapter presents the tyrosine and phenylalanine biosynthesis. The terminal reactions in the biosynthesis of tyrosine and phenylalanine involve the conversion of chorismate through prephenate to 4-hydroxyphenylpyruvate or phenylpyruvate, which are then trans-aminated to give the amino acids. Of the two activities carried out by the T protein, chorismate mutase is more readily assayed in crude cell extracts. Chorismate mutase P is also present in crude cell extracts. Chorismate mutase activity is assayed by estimating the prephenate formed. Prephenate dehydrogenase activity is measured by a fluorometric determination of the diphosphopyridine nucleotide (DPNH) formed during the conversion of prephenate into 4-hydroxyphenylpyruvate. One unit of enzyme activity is defined as the amount catalyzing the formation of 0.1 micromole of prephenate in 20 minutes at 37°. On the other hand, the P protein has not been extensively purified. The P protein has two enzymatic activities—namely, chorismate mutase and prephenate dehydratase. Chorismate mutase is assayed by the similar method as T protein. Prephenate dehydratase is assayed by estimating the phenylpyruvate formed from prephenate.