ABSTRACT
Glutamine phosphoribosylpyrophosphate amidotransferase (GPATase; EC 2.4.2.14; amidophosphoribosyltransferase) catalyzes the first reaction in de novo biosynthesis of purine nucleotides: L-glutamine + 5-phospho-a-D-ribose-l-diphosphate + H20 —> L-glutamate + 5-phospho-P-D-ribosyl-l-amine + diphosphate, i.e., it transfers the glu tamine amide nitrogen to 5-phosphoribose. GPATase is also the key regulatory enzyme of this pathway because it is inhibited by adenine and guanine nucleotide end products of purine biosynthesis, which bind to an allosteric A site and a catalytic C site, respectively. The synergistic inhibition by certain nucleotide pairs has been studied in detail [59]. Numerous GPATase sequences from eukaryotes, prokaryotes, and archaea are known and show pairwise sequence identities of the order of 40%. Only the enzymes from Bacillus subtilis and E. coli are structurally characterized by X-ray crystallography (1AOO, 1GPH and 1ECB, 1ECC, 1ECF, 1ECG, 1ECJ, respec tively), and both of them are homotetramers of similar overall structure. However, they represent two classes of GPATases: enzymes of the B. subtilis class (e.g., in humans, rat, chicken, Drosophila, soybean) contain a Fe4S4 cluster that is ligated by four conserved cysteines, and are usually synthesized with an N-terminal propep tide, whereas the E. coli-type GPATases (e.g., in yeast, Haemophilus influenzae, Methanococcus jannaschii) are metal-free and lack the propeptide. Similar to the cluster in endonuclease III, the Fe4S4 cluster of B. subtilis GPATase has a redox potential of less than —600 mV and does not participate directly in catalysis. It has a structural and a regulatory function (see below).
