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Biosynthesis of the purines. 33. Catalytic properties of the glutamine site of formylglycinamide ribonucleotide amidotransferase from chicken liver.

Abstract From studies with a large number of l-γ-glutamyl derivatives, it has been found that formylglycinamide ribonucleotide (FGAR) amidotransferase can catalyze the hydrolysis of esters, thioesters, and substituted amides in addition to the amide bond of glutamine. The hydrolytic reactions are dependent on the presence of FGAR, ATP, and Mg2+; however, in contrast to the reaction of glutamine they are not accompanied by the stoichiometric formation of ADP in the case of amides and there is no ADP formation when esters and thioesters are substrates. Km and Vm values are reported for l-γ-glutamylmethoxyamide, l-γ-glutamylhydroxamate, l-γ-glutamylhydrazide, γ-methyl l-glutamate, and γ-ethyl l-glutamate. Km values of FGAR and MgATP2- for these l-γ-glutamyl derivatives are also reported. In the case of the hydrolysis of l-γ-glutamyl esters and thioesters, only a catalytic amount of FGAR is needed. In the reaction catalyzed by FGAR amidotransferase ethanol is released from γ-ethyl l-glutamate in amounts equal to the glutamate formed. Thus, when l-γ-glutamyl esters or thioesters were used as substrates, the reactions of the glutamine site can be separated from those of the whole FGAR-amidotransferase complex. Both glutamine and NH4Cl inhibit the enzymatic reaction with respect to γ-ethyl l-glutamate as substrate. Hydrazine and methylamine are competitive and mixed type inhibitors of the reaction, respectively, with glutamine as substrate. On the other hand, l-γ-glutamylmethylamide, l-γ-glutamylhydroxamate, and γ-methyl l-glutamate all acted as mixed type inhibitors of the reaction relative to NH4Cl. The effect of a number of monovalent cations on the enzyme activity is examined. Potassium ion is required for maximum activity whereas sodium ion inhibits the enzyme probably at the FGAR-MgATP site.