title: Kinetic isotope effect studies on aspartate aminotransferase: evidence for a concerted 1,3 prototropic shift mechanism for the cytoplasmic isozyme and L-aspartate and dichotomy in mechanism.
abstract: The C alpha primary hydrogen kinetic isotope effects (C alpha-KIEs) for the reaction of the cytoplasmic isozyme of aspartate aminotransferase (cAATase) with [alpha-2H]-L-aspartate are small and only slightly affected by deuterium oxide solvent (DV = 1.43 +/- 0.03 and DV/KAsp = 1.36 +/- 0.04 in H2O; DV = 1.44 +/- 0.01 and DV/KAsp = 1.61 +/- 0.06 in D2O). The D2O solvent KIEs (SKIEs) are somewhat larger and are essentially independent of deuterium at C alpha (D2OV = 2.21 +/- 0.07 and D2OV/KAsp = 1.70 +/- 0.03 with [alpha-1H]-L-aspartate; D2OV = 2.34 +/- 0.12 and D2OV/KAsp = 1.82 +/- 0.06 with [alpha-2H]-L- aspartate). The C alpha-KIEs on V and on V/KAsp are independent of pH from pH 5.0 to pH 10.0. These results support a rate-determining concerted 1,3 prototropic shift mechanism by the multiple KIE criteria [Hermes, J. D., Roeske, C. A., O'Leary, M. H., & Cleland, W. W. (1982) Biochemistry 21, 5106]. The large C alpha-KIEs for the reaction of mitochondrial AATase (mAATase) with L-glutamate (DV = 1.88 +/- 0.13 and DV/KGlu = 3.80 +/- 0.43 in H2O; DV = 1.57 +/- 0.05 and DV/KGlu = 4.21 +/- 0.19 in D2O) coupled with the relatively small SKIEs (D2OV = 1.58 +/- 0.04 and D2OV/KGlu = 1.25 +/- 0.05 with [alpha-1H]-L-glutamate; D2OV = 1.46 +/- 0.06 and D2OV/KGlu = 1.16 +/- 0.05 with [alpha-2H]-L-glutamate) are most consistent with a two-step mechanism for the 1,3 prototropic shift for this isozyme-substrate pair.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry, University of California, Berkeley 94720.
authors.forename: D A
authors.initials: DA
authors.lastname: Julin
authors.forename: J F
authors.initials: JF
authors.lastname: Kirsch
mesh_terms: D000818:Animals; D001219:Aspartate Aminotransferases / Q000378:metabolism*; D001224:Aspartic Acid / Q000378:metabolism; D003600:Cytosol / Q000201:enzymology; D003903:Deuterium / Q000378:metabolism*; D017666:Deuterium Oxide; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D008962:Models, Theoretical; D009206:Myocardium / Q000201:enzymology*; D012997:Solvents; D013552:Swine; D014867:Water / Q000378:metabolism*
chemical_list: D007527:Isoenzymes; D012997:Solvents; D014867:Water; D001224:Aspartic Acid; D003903:Deuterium; D001219:Aspartate Aminotransferases; D017666:Deuterium Oxide