title: Reaction mechanism of guanidinoacetate methyltransferase, concerted or step-wise.
abstract: We describe a quantum mechanics/molecular mechanics investigation of the guanidinoacetate methyltransferase catalyzed reaction, which shows that proton transfer from guanidinoacetate (GAA) to Asp-134 and methyl transfer from S-adenosyl-L-methionine (AdoMet) to GAA are concerted. By self-consistent-charge density functional tight binding/molecular mechanics, the bond lengths in the concerted mechanism's transition state are 1.26 A for both the OD1 (Asp-134)-H(E) (GAA) and H(E) (GAA)-N(E) (GAA) bonds, and 2.47 and 2.03 A for the S8 (AdoMet)-C9 (AdoMet) and C9 (AdoMet)-N(E) (GAA) bonds, respectively. The potential-energy barrier (DeltaE++) determined by single-point B3LYP/6-31+G*//MM is 18.9 kcal/mol. The contributions of the entropy (-TDeltaS++) and zero-point energy corrections Delta(ZPE)++ by normal mode analysis are 2.3 kcal/mol and -1.7 kcal/mol, respectively. Thus, the activation enthalpy of this concerted mechanism is predicted to be DeltaH++ = DeltaE++ plus Delta(ZPE)++ = 17.2 kcal/mol. The calculated free-energy barrier for the concerted mechanism is DeltaG++ = 19.5 kcal/mol, which is in excellent agreement with the value of 19.0 kcal/mol calculated from the experimental rate constant (3.8 +/- 0.2.min(-1)).
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA.
authors.forename: Xiaodong
authors.initials: X
authors.lastname: Zhang
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Bruice
mesh_terms: D001665:Binding Sites; D018360:Crystallography, X-Ray; D018698:Glutamic Acid / Q000737:chemistry / Q000378:metabolism; D050937:Guanidinoacetate N-Methyltransferase / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D017434:Protein Structure, Tertiary
chemical_list: D018698:Glutamic Acid; D050937:Guanidinoacetate N-Methyltransferase