COUPLED FORMATION OF AN AMIDOTRANSFERASE INTERDOMAIN AMMONIA CHANNEL AND A PHOSPHORIBOSYLTRANSFERASE ACTIVE-SITE

Activation of glutamine phosphoribosylpyrophosphate (PRPP) amidotransf erase (GPATase) by binding of a PRPP substrate analog results in the f ormation of a 20 Angstrom channel connecting the active site for gluta mine hydrolysis in one domain with the PRPP site in a second domain. T his solvent-inaccessible channel permits transfer of the NH3 intermedi ate between the two active sites. Tunneling of NH3 may be a common mec hanism for glutamine amidotransferase-catalyzed nitrogen transfer and for coordination of catalysis at two distinct active sites in complex enzymes. The 2.4 Angstrom crystal structure of the active conformer of GPATase also provides the first description of an intact active site for the phosphoribosyltransferase (PRTase) family of nucleotide synthe sis and salvage enzymes. Chemical assistance to catalysis is provided primarily by the substrate and secondarily by the enzyme in the propos ed structure-based mechanism. Different catalytic and inhibitory modes of divalent cation binding to the PRTase active site are revealed in the active conformer of the enzyme and in a feedback-inhibited GMP com plex.