THE SOURCE OF THE NITROGEN ATOM FOR THE BIOSYNTHESIS OF ANTHRANILIC ACID

An anthranilate synthase of the extreme aminase type in a species of blue-green bacteria (algae)

Anthranilate synthase of Agmenellum quadruplicatum, a unicellular species of blue-green bacteria, consists of two nonidentical subunits. A 72,000 dalton protein has aminase activity but is incapable of reaction with glutamine (amidotransferase) unless a second protein (18,000 molecular weight) is present. The small subunit was first detected through its ability to complement a partially purified aminase subunit from Bacillus subtilis to produce a hybrid complex capable of amidotransferase function. Conditions for the function of the heterologous complex were less stringent than for the homologous A. quadruplicatum complex. A reducing agent such as dithiothreitol stabilizes the A. quadruplicatum aminase subunit and is obligatory for amidotransferase function. L-Tryptophan feedback inhibits both the aminase and amidotransferase reactions of anthranilate synthase; Ki values of 6 X 10(-8) M for the amidotransferase activity and 2 X 10(-6) M for the aminase activity were obtained. The Km value calculated for ammonia (2.2 mM) was more favorable than the Km value glutamine (13 mM). Likewise, the Vmax of anthranilate synthase was greater with ammonia than with glutamine. Starvation of a tryptophan auxotroph results in a threefold derepression of the aminase subunit, but no corresponding increase in the small 18,000 M subunit occurs. While microbial anthranilate synthase complexes are remarkably similar overall, the relatively good aminase activity of the A. quadruplicatum enzyme may be of physiological significance in nature.

Study of a corn (Zea mays L.) mutant (blue fluorescent-1) which accumulates anthranilic acid and its beta-glucoside

A corn (Zea mays L.) mutant, blue fluorescent-1 (bf), is described that shows ultraviolet light induced blue fluorescence in young seedling leaves if homozygous for the mutant gene, and in anthers if either homozygous or heterozygous. The blue fluorescent compounds were extracted with acetone and separated by paper chromatography. Anthranilic acid was present and the beta-glucoside was also identified by paper chromatography and beta-glucosdase and acid treatment. A third major fluorescent compound was not identified, but it was convertible to anthranilic acid by acid treatment. Anthranilate synthetase from mutant plants was 3-40 times more active and was also more resistant to feedback inhibition by tryptophan than was the enzyme from normal plants. The high activity and feedback resistance would both lead to anthranilate accumulation. Anthranilate-phosphoribosylpyrophosphate phosphoribosyltransferase (PR transferase), the enzyme which usually utilizes anthranilate in the tryptophan pathway, was inhibited by the beta-glucoside of anthranilic acid in a noncompetitive manner and showed very little activity in the mutant plant extract. This inhibition of the enzyme which utilizes anthranilate would also lead to accumulation. Apparently the oversynthesis of anthranilate leads to the formation of the beta-glucoside, which inhibits anthranilate utilization. The fluorescent compounds are absent in seed, but form on germination. The levels decrease with age after 35 days postgermination, but are still present in leaves during grain filling.

Anthranilate synthase-anthranilate 5-phosphoribosyl 1-pyrophosphate phosphoribosyltransferase from Aerobacter aerogenes

1. Anthranilate synthase and phosphoribosyltransferase from Aerobacter aerogenes purify simultaneously and sediment together on sucrose gradients, showing that they occur as an enzyme aggregate. Both activities of the intact aggregate are subject to inhibition by tryptophan. 2. By using appropriate auxotrophic mutants it was shown that an intact active enzyme aggregate is formed when the components come from separate mutant strains. An intact active aggregate can also be formed when one component is from Escherichia coli and the other from A. aerogenes. 3. Phosphoribosyltransferase of A. aerogenes is active when not in an aggregate with anthranilate synthase, but is not subject to tryptophan inhibition, indicating that the inhibitor site is on the anthranilate synthase component. 4. Anthranilate synthase can be active and sensitive to tryptophan inhibition when complexed with an inactive phosphoribosyltransferase. 5. Kinetic studies on the anthranilate synthase activity show that tryptophan is a competitive inhibitor with respect to chorismate and a non-competitive inhibitor with respect to either glutamine or NH(4) (+) ions. This is consistent with a sequential mechanism of the ordered type in which chorismate is the first reactant.

Regulation of glutamine synthetase. II. Patterns of feedback inhibition in microorganisms

The feedback inhibition of glutamine synthetase was investigated by use of partially purified enzyme preparations from Salmonella typhimurium, Micrococcus sodonensis, Pseudomonas fluorescens, Bacillus cereus, Bacillus licheniformis, Clostridium pasteurianum, Rhodospirillum rubrum, Neurospora crassa, Candida utilis, and Chlorella pyrenoidosa. Inhibition analyses indicated that the enzyme of each organism can be effectively regulated with mixtures of end products from the diverse pathways of glutamine metabolism. When tested individually, tryptophan, histidine, alanine, glycine, glutamine, 5'-adenylate (AMP), cytidine-5'-triphosphate, carbamyl phosphate, and glucosamine-6-phosphate gave limited inhibition. In most cases, the inhibitors were independent in their action, and cumulative degrees of inhibition were obtained with mixtures of these end products. In contrast, with the glutamine synthetases of the two Bacillus species, the simultaneous presence of AMP and histidine (or AMP and glutamine) gave inhibition greater than the sum of the amounts of inhibition caused by either inhibitor alone. Also, alanine and carbamyl phosphate acted synergistically to inhibit the enzyme from N. crassa. The remarkable similarity in the overall patterns of end-product inhibition observed with the enzymes from different sources indicates that these diverse organisms have evolved comparable mechanisms for the regulation of glutamine metabolism. Nevertheless, the enzymes from different sources do differ significantly in their physical and catalytic properties, as was demonstrated by dissimilarities in their purification behaviors, specificity for nucleotide substrate, ability to catalyze the glutamyl transfer reaction, and ability to utilize Mn(++) and Mg(++) as activators for the biosynthetic reaction.