Rat liver glycine methyltransferase. Cooperative binding of S-adenosylmethionine and loss of cooperativity by removal of a short NH2-terminal segment.
Rat liver glycine methyltransferase, a homotetramer, exhibits sigmoidal rate behavior with respect to S-adenosylmethionine (Ogawa, H., and Fujioka, M. (1982) J. Biol. Chem. 257, 3447-3452). The binding experiment shows that the sigmoidicity observed in initial velocity kinetics is explained by the cooperative binding of S-adenosylmethionine to the catalytic sites residing on each subunit. Limited proteolysis of glycine methyltransferase with trypsin in the presence of S-adenosylmethionine yields an enzyme lacking the NH2-terminal 8 residues. The proteolytically modified enzyme retains a tetrameric structure. The truncated enzyme shows no cooperativity with respect to S-adenosylmethionine binding and kinetics. It has values of Vmax and Km for glycine identical to those of the native enzyme, but a 3-fold lower [S]0.5 value for S-adenosylmethionine. The proteolytic modification is without effect on the circular dichroism and fluorescence spectra. Furthermore, the protein fluorescence of the modified enzyme is quenched upon addition of S-adenosylmethionine to the same extent as observed with the native enzyme. These results suggest that a short NH2-terminal segment, which lies outside the active site, is important for communication between subunits.
Konishi
K
K
Department of Biochemistry, Toyama Medical and Pharmaceutical University Faculty of Medicine, Japan.
Fujioka
M
M
D000818:Animals; D004591:Electrophoresis, Polyacrylamide Gel; D050938:Glycine N-Methyltransferase; D008099:Liver / Q000201:enzymology*; D008780:Methyltransferases / Q000378:metabolism*; D051381:Rats; D012436:S-Adenosylmethionine / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014357:Trypsin / Q000378:metabolism
D012436:S-Adenosylmethionine; D008780:Methyltransferases; D050938:Glycine N-Methyltransferase; D014357:Trypsin