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Title: Direct determination of protonation states and visualization of hydrogen bonding in a glycoside hydrolase with neutron crystallography

Journal Article · · Proceedings of the National Academy of Sciences of the United States of America
 [1]; ORCiD logo [2];  [3];  [4];  [5];  [6]; ORCiD logo [7];  [7]; ORCiD logo [7]; ORCiD logo [7]
  1. Nanjing Agricultural Univ., Nanjing (People's Republic of China)
  2. Univ. of Tennessee, Knoxville, TN (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  3. Univ. of Toledo, Toledo, OH (United States)
  4. European Spallation Source, Lund (Sweden)
  5. Technische Univ. Munchen, Garching (Germany)
  6. Forschungszentrum Julich GmbH, Garching (Germany)
  7. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

Glycoside hydrolase (GH) enzymes apply acid/base chemistry to catalyze the decomposition of complex carbohydrates. These ubiquitous enzymes accept protons from solvent and donate them to substrates at close to neutral pH by modulating the pKa values of key side chains during catalysis. Yet, it is not known how the catalytic acid residue acquires a proton and transfers it efficiently to the substrate. To better understand GH chemistry, we used macromolecular neutron crystallography to directly determine protonation and ionization states of the active site residues of a family 11 GH at multiple pD (pD = pH + 0.4) values. The general acid glutamate (Glu) cycles between two conformations, upward and downward, but is protonated only in the downward orientation. Here, we performed continuum electrostatics calculations to estimate the pKa values of the catalytic Glu residues in both the apo- and substrate-bound states of the enzyme. The calculated pKa of the Glu increases substantially when the side chain moves down. The energy barrier required to rotate the catalytic Glu residue back to the upward conformation, where it can protonate the glycosidic oxygen of the substrate, is 4.3 kcal/mol according to free energy simulations. Lastly, these findings shed light on the initial stage of the glycoside hydrolysis reaction in which molecular motion enables the general acid catalyst to obtain a proton from the bulk solvent and deliver it to the glycosidic oxygen.

Research Organization:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
Sponsoring Organization:
USDOE Laboratory Directed Research and Development (LDRD) Program
Grant/Contract Number:
AC05-00OR22725
OSTI ID:
1265915
Journal Information:
Proceedings of the National Academy of Sciences of the United States of America, Vol. 112, Issue 40; ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)Copyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 28 works
Citation information provided by
Web of Science

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Cited By (5)

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Neutron structure of the T26H mutant of T4 phage lysozyme provides insight into the catalytic activity of the mutant enzyme and how it differs from that of wild type: Neutron Structure of the T26H Mutant of T4L journal July 2017
Structural and functional characterization of a highly stable endo-β-1,4-xylanase from Fusarium oxysporum and its development as an efficient immobilized biocatalyst journal September 2016
Titration of ionizable groups in proteins using multiple neutron data sets from a single crystal: application to the small GTPase Ras journal January 2019