4Q49

Room temperature neutron crystal structure of apo human carbonic anhydrase at pH 7.5

Summary for 4Q49

• Entry DOI: 10.2210/pdb4q49/pdb
• Descriptor: Carbonic anhydrase 2, ZINC ION (3 entities in total)
• Functional Keywords: mixed alpha beta, proton transfer, cytosolic, lyase
• Biological source: Homo sapiens (human)
• Total number of polymer chains: 1
• Total formula weight: 29354.47

Authors

Fisher, S.Z.,McKenna, R. (deposition date: 2014-04-14, release date: 2015-02-25, Last modification date: 2023-09-20)

Primary citation

Michalczyk, R.,Unkefer, C.J.,Bacik, J.P.,Schrader, T.E.,Ostermann, A.,Kovalevsky, A.Y.,McKenna, R.,Fisher, S.Z.
Joint neutron crystallographic and NMR solution studies of Tyr residue ionization and hydrogen bonding: Implications for enzyme-mediated proton transfer.
Proc.Natl.Acad.Sci.USA, 112:5673-5678, 2015

PubMed Abstract: Human carbonic anhydrase II (HCA II) uses a Zn-bound OH(-)/H2O mechanism to catalyze the reversible hydration of CO2. This catalysis also involves a separate proton transfer step, mediated by an ordered solvent network coordinated by hydrophilic residues. One of these residues, Tyr7, was previously shown to be deprotonated in the neutron crystal structure at pH 10. This observation indicated that Tyr7 has a perturbed pKa compared with free tyrosine. To further probe the pKa of this residue, NMR spectroscopic measurements of [(13)C]Tyr-labeled holo HCA II (with active-site Zn present) were preformed to titrate all Tyr residues between pH 5.4-11.0. In addition, neutron studies of apo HCA II (with Zn removed from the active site) at pH 7.5 and holo HCA II at pH 6 were conducted. This detailed interrogation of tyrosines in HCA II by NMR and neutron crystallography revealed a significantly lowered pKa of Tyr7 and how pH and Tyr proximity to Zn affect hydrogen-bonding interactions.

PubMed: 25902526
DOI: 10.1073/pnas.1502255112

Experimental method

NEUTRON DIFFRACTION (1.8 Å)