Hydrophobic Effect Drives Oxygen Uptake in Myoglobin via Histidine E7

Leonardo Boechi,Mehrnoosh Arrar,Marcelo A Martí,John S Olson,Adrián E Roitberg,Darío A Estrin

doi:10.1074/jbc.m112.426056

Abstract

Since the elucidation of the myoglobin (Mb) structure, a histidine residue on the E helix (His-E7) has been proposed to act as a gate with an open or closed conformation controlling access to the active site. Although it is believed that at low pH, the His-E7 gate is in its open conformation, the full relationship between the His-E7 protonation state, its conformation, and ligand migration in Mb is hotly debated. We used molecular dynamics simulations to first address the effect of His-E7 protonation on its conformation. We observed the expected shift from the closed to the open conformation upon protonation, but more importantly, noted a significant difference between the conformations of the two neutral histidine tautomers. We further computed free energy profiles for oxygen migration in each of the possible His-E7 states as well as in two instructive Mb mutants: Ala-E7 and Trp-E7. Our results show that even in the closed conformation, the His-E7 gate does not create a large barrier to oxygen migration and permits oxygen entry with only a small rotation of the imidazole side chain and movement of the E helix. We identify, instead, a hydrophobic site in the E7 channel that can accommodate an apolar diatomic ligand and enhances ligand uptake particularly in the open His-E7 conformation. This rate enhancement is diminished in the closed conformation. Taken together, our results provide a new conceptual framework for the histidine gate hypothesis.

Highlights

The distal histidine in myoglobin is thought to act as a gate regulating oxygen uptake
Relationship between His-E7 Tautomeric and Conformational States—We performed extensive molecular dynamics (MD) simulations to determine whether the His-E7 protonation state controls its conformation
This interaction serves as a “latch” for the His-E7 gate, maintaining it in the open conformation

Summary

Introduction

The distal histidine in myoglobin is thought to act as a gate regulating oxygen uptake. Results: Neither the open nor closed conformation hinders oxygen uptake; a hydrophobic site is more apparent in the open conformation. Since the elucidation of the myoglobin (Mb) structure, a histidine residue on the E helix (His-E7) has been proposed to act as a gate with an open or closed conformation controlling access to the active site. It is believed that at low pH, the His-E7 gate is in its open conformation, the full relationship between the His-E7 protonation state, its conformation, and ligand migration in Mb is hotly debated. We observed the expected shift from the closed to the open conformation upon protonation, but more importantly, noted a significant difference between the conformations of the two neutral histidine tautomers. Instead, a hydrophobic site in the E7 chan-

Objectives

Methods

Results

Discussion

Conclusion