Effects of site-directed mutagenesis on structure and function of recombinant rat liver S-adenosylhomocysteine hydrolase. Crystal structure of D244E mutant enzyme.
A site-directed mutagenesis, D244E, of S-adenosylhomocysteine hydrolase (AdoHcyase) changes drastically the nature of the protein, especially the NAD(+) binding affinity. The mutant enzyme contained NADH rather than NAD(+) (Gomi, T., Takata, Y., Date, T., Fujioka, M., Aksamit, R. R., Backlund, P. S., and Cantoni, G. L. (1990) J. Biol. Chem. 265, 16102-16107). In contrast to the site-directed mutagenesis study, the crystal structures of human and rat AdoHcyase recently determined have shown that the carboxyl group of Asp-244 points in a direction opposite to the bound NAD molecule and does not participate in any hydrogen bonds with the NAD molecule. To explain the discrepancy between the mutagenesis study and the x-ray studies, we have determined the crystal structure of the recombinant rat-liver D244E mutant enzyme to 2.8-A resolution. The D244E mutation changes the enzyme structure from the open to the closed conformation by means of a approximately 17 degrees rotation of the individual catalytic domains around the molecular hinge sections. The D244E mutation shifts the catalytic reaction from a reversible to an irreversible fashion. The large affinity difference between NAD(+) and NADH is mainly due to the enzyme conformation, but not to the binding-site geometry; an NAD(+) in the open conformation is readily released from the enzyme, whereas an NADH in the closed conformation is trapped and cannot leave the enzyme. A catalytic mechanism of AdoHcyase has been proposed on the basis of the crystal structures of the wild-type and D244E enzymes.
Komoto
J
J
Department of Molecular Biosciences, University of Kansas, Lawrence, Kansas 66045-210, USA.
Huang
Y
Y
Gomi
T
T
Ogawa
H
H
Takata
Y
Y
Fujioka
M
M
Takusagawa
F
F
D043383:Adenosylhomocysteinase; D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites; D002384:Catalysis; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D006867:Hydrolases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008099:Liver / Q000201:enzymology*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009154:Mutation / Q000235:genetics; D009243:NAD / Q000378:metabolism; D017433:Protein Structure, Secondary; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013329:Structure-Activity Relationship
D011994:Recombinant Proteins; D009243:NAD; D006867:Hydrolases; D043383:Adenosylhomocysteinase