Activation of gluatmine phosphoribosylpyrophosphate (RPPP) amidotransferase
(GPATase) by binding of a PRPP substrate analog results in the formation of a 20
A channel connecting the active site for glutamine hydrolysis in one domain with
the PRPP site in a second domain. This solvent-inaccessible channel permits
transfer of the NH3 intermediate between the two active sites. Tunneling of NH3
may be a common mechanism for glutamine amidotransferase-catalyzed nitrogen
transfer and for coordination of catalysis at two distinct active sites in
complex enzymes. The 2.4 A 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 synthesis and salvage
enzymes. Chemical assistance to catalysis is provided primarily by the substrate
and secondarily by the enzyme in the proposed 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 complex.
