Allosteric and catalytic functions of the PPi-binding motif in the ATP sulfurylase-GTPase system

ATP sulfurylase, from Escherichia coli K-12, catalyzes and couples the Gibbs potentials of two reactions, GTP hydrolysis and activated sulfate (APS, adenosine 5'-phosphosulfate) synthesis. Coupling these potentials requires that the catalytic cycle include reaction stage-dependent conformational changes that gate the activities of the two active sites. These interactions were probed in a mutagenesis study of a highly conserved pyrophosphate-binding motif (SXGXDS), which is located at the APS-forming active site. The motif appears to be unique to the N-type PPi synthetase family, and mutations in it are linked, in other systems, to citrullinemia, an often fatal orphan disease. The conserved sites in the motif were evaluated individually for their ability to activate GTP hydrolysis (which reports interactions among the activator (AMP or Mg2+.PPi), the enzyme, and GTP), to affect the energetic coupling of the two reactions, and to alter the kinetic constants of the adenylyl transfer reaction in the absence of guanine nucleotide. What emerges from this first mutagenic exploration of the PPi motif in any adenylyltransferase is that the residues of the motif participate differently, and in sometimes profoundly important ways, in the different functions of the enzyme.