title: Quantum delocalization of protons in the hydrogen-bond network of an enzyme active site.
abstract: Enzymes use protein architectures to create highly specialized structural motifs that can greatly enhance the rates of complex chemical transformations. Here, we use experiments, combined with ab initio simulations that exactly include nuclear quantum effects, to show that a triad of strongly hydrogen-bonded tyrosine residues within the active site of the enzyme ketosteroid isomerase (KSI) facilitates quantum proton delocalization. This delocalization dramatically stabilizes the deprotonation of an active-site tyrosine residue, resulting in a very large isotope effect on its acidity. When an intermediate analog is docked, it is incorporated into the hydrogen-bond network, giving rise to extended quantum proton delocalization in the active site. These results shed light on the role of nuclear quantum effects in the hydrogen-bond network that stabilizes the reactive intermediate of KSI, and the behavior of protons in biological systems containing strong hydrogen bonds.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305.
authors.forename: Lu
authors.initials: L
authors.lastname: Wang
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305.
authors.forename: Stephen D
authors.initials: SD
authors.lastname: Fried
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305.
authors.forename: Steven G
authors.initials: SG
authors.lastname: Boxer
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305 tmarkland@stanford.edu.
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Markland
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D020134:Catalytic Domain; D006860:Hydrogen Bonding; D016958:Pseudomonas putida / Q000201:enzymology* / Q000235:genetics; D013251:Steroid Isomerases / Q000737:chemistry* / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D013251:Steroid Isomerases
keywords: ab initio path integral molecular dynamics; enzyme; hydrogen bonding; nuclear quantum effects; proton delocalization