Journal Article

Quantum delocalization of protons in the hydrogen-bond network of an enzyme active site

Markland, Thomas E., Boxer, Steven G., Fried, Stephen D., Wang, Lu

2014

Overview

Significance Because of the low mass of the proton, nuclear quantum effects can dramatically alter the properties of hydrogen-bond networks, especially when short and strong hydrogen bonds occur. Here, we combine experiments and state-of-the-art simulations that include the quantum nature of both the electrons and nuclei to show that the enzyme ketosteroid isomerase contains a hydrogen-bond network in its active site that facilitates extensive quantum proton delocalization. This leads to a 10,000-fold increase in the acidity of an active-site residue compared with the limit where the nuclei are classical particles. This work opens up new avenues for understanding the interplay between quantum effects and hydrogen bonding in biological systems containing strong hydrogen bonds. 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.

• Publisher: National Academy of Sciences,National Acad Sciences
• Subject: Acidity / Active sites / Atoms / Bacterial Proteins - chemistry / Bacterial Proteins - genetics / Biochemistry / Catalytic Domain / Depersonalization / electrons / Enzymes / Hydrogen / Hydrogen Bonding / Hydrogen bonds / Isotope effects / Ketosteroids / Molecular dynamics / Physical Sciences / Proteins / Protons / Pseudomonas putida - enzymology / Pseudomonas putida - genetics / Quantum mechanics / Quantum physics / Simulation / Steroid Isomerases - chemistry / Steroid Isomerases - genetics