STRUCTURE OF THE ALLOSTERIC REGULATORY ENZYME OF PURINE BIOSYNTHESIS

Multiwavelength anomalous diffraction (MAD) has been used to determine the structure of the regulatory enzyme of de novo synthesis of purine nucleotides, glutamine 5-phosphoribosyl-1-pyrophosphate (PRPP) amidot ransferase, from Bacillus subtilis. This allosteric enzyme, a 200-kilo dalton tetramer, is subject to end product regulation by purine nucleo tides. The metalloenzyme from B. subtilis is a paradigm for the higher eukaryotic enzymes, which have been refractory to isolation in stable form. The two folding domains of the polypeptide are correlated with functional domains for glutamine binding and for transfer of ammonia t o the substrate PRPP. Eight molecules of the feedback inhibitor adenos ine monophosphate (AMP) are bound to the tetrameric enzyme in two type s of binding sites: the PRPP catalytic site of each subunit and an unu sual regulatory site that is immediately adjacent to each active site but is between subunits. An oxygen-sensitive [4Fe-4S] cluster in each subunit is proposed to regulate protein turnover in vivo and is distan t from the catalytic site. Oxygen sensitivity of the cluster is dimini shed by AMP, which blocks a channel through the protein to the cluster . The structure is representative of both glutamine amidotransferases and phosphoribosyltransferases.