The active-site arginine of S-adenosylmethionine synthetase orients the reaction intermediate.
S-Adenosylmethionine (AdoMet) synthetase catalyzes the formation of AdoMet and tripolyphosphate (PPPi) from ATP and L-methionine and the subsequent hydrolysis of the PPPi to PPi and Pi before product release. Little is known about the roles of active-site residues involved in catalysis of the two sequential reactions that occur at opposite ends of the polyphosphate chain. Crystallographic studies of Escherichia coli AdoMet synthetase showed that arginine-244 is the only arginine near the polyphosphate-binding site. Arginine-244 is embedded as the seventh residue in the conserved sequence DxGxTxxKxI which is also found at the active site of inorganic pyrophosphatases, suggesting a potential pyrophosphate-binding motif. Chemical modification of AdoMet synthetase by the arginine-specific reagents phenylglyoxal or p-hydroxyphenylglyoxal inactivates the enzyme. ATP and PPPi protect the enzyme from inactivation, consistent with the presence of an important arginine residue in the vicinity of the polyphosphate-binding site. Site-specific mutagenesis has been used to change the conserved arginine-244 to either leucine (R244L) or histidine (R244H). In the overall reaction, the R244L mutant has the kcat reduced approximately 10(3)-fold, with a 7 to 10-fold increase in substrate Km values; the R244H mutant has an approximately 10(5)-fold decrease in kcat. In contrast, the kcat values for hydrolysis of added PPPi by the R244L and R244H mutants have been reduced by less than 2 orders of magnitude. In contrast to the wild-type enzyme in which 98% of the Pi formed originates as the gamma-phosphoryl group of ATP, in the R244L mutant the orientation of the PPPi intermediate equilibrates at the active site yielding equal amounts of Pi from the alpha- and gamma-phosphoryl groups of ATP. Thus, the active-site arginine has a profound role in the cleavage of PPPi from ATP during AdoMet formation and in maintaining the orientation of PPPi in the active site, while playing a lesser role in the subsequent PPPi hydrolytic reaction.
Reczkowski
R S
RS
Institute for Cancer Research, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA.
Taylor
J C
JC
Markham
G D
GD
D019943:Amino Acid Substitution / Q000235:genetics; D001120:Arginine / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites / Q000235:genetics; D011756:Diphosphates / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology; D006639:Histidine / Q000235:genetics / Q000378:metabolism; D007930:Leucine / Q000235:genetics / Q000378:metabolism; D008716:Methionine Adenosyltransferase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding
D011756:Diphosphates; D006639:Histidine; D001120:Arginine; D008716:Methionine Adenosyltransferase; D007930:Leucine