Open AccessInverse heavy enzyme isotope effects in methylthioadenosine nucleosidases
Author(s) -
Morais Brown,
Ioanna Zoi,
Dimitri Antoniou,
Hilda A. Namanja-Magliano,
Steven D. Schwartz,
Vern L. Schramm
Publication year - 2021
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.2109118118
Subject(s) - kinetic isotope effect , chemistry , enzyme , isotope , reaction coordinate , biochemistry , computational chemistry , deuterium , atomic physics , physics , quantum mechanics
Heavy enzyme isotope effects occur in proteins substituted with 2 H-, 13 C-, and 15 N-enriched amino acids. Mass alterations perturb femtosecond protein motions and have been used to study the linkage between fast motions and transition-state barrier crossing. Heavy enzymes typically show slower rates for their chemical steps. Heavy bacterial methylthioadenosine nucleosidases (MTANs from Helicobactor pylori and Escherichia coli ) gave normal isotope effects in steady-state kinetics, with slower rates for the heavy enzymes. However, both enzymes revealed rare inverse isotope effects on their chemical steps, with faster chemical steps in the heavy enzymes. Computational transition-path sampling studies of H. pylori and E. coli MTANs indicated closer enzyme-reactant interactions in the heavy MTANs at times near the transition state, resulting in an improved reaction coordinate geometry. Specific catalytic interactions more favorable for heavy MTANs include improved contacts to the catalytic water nucleophile and to the adenine leaving group. Heavy bacterial MTANs depart from other heavy enzymes as slowed vibrational modes from the heavy isotope substitution caused improved barrier-crossing efficiency. Improved sampling frequency and reactant coordinate distances are highlighted as key factors in MTAN transition-state stabilization.