Computational study of O2 affinity in adult human hemoglobin: a tertiary structure viewpoint

MAURO BRINGAS; ARIEL PETRUK; DARIO A. ESTRIN; MARCELO A. MARTI; LUCIANA CAPECE

Hamburgo

Conferencia; XIXth International conference on "Oxygen binding and sensing proteins"; 2016

Adulthuman hemoglobin (HbA) is a tetrameric (α2β2)globin which posseses the function of small ligand transport, andthis function presents a cooperative behaviour. In the MWC model, Hbpresents two states, R and T, with high and low affinity for oxygen,respectively. In this work we intend to describe the transitionsthat occur at the tertiary level when the quaternary transition takesplace, in order to shed light on the mechanisms of oxygen affinityregulation through the allosteric change. Twomicroscopic events related to oxygen binding and release process arestudied: the ligand migration to the distal pocket and the Fe-O bondclevage. Ata tertiary level, two regulatory mechanisms of oxygen affinity havebeen described in monomeric globins:the distal effect, that refers to the chance of hydrogenbond-mediated stabilization of the ligand in the binding site and theproximal effect, which is related to the relative position of theproximal histidine respect to the heme plane and its interactionswith the protein matrix. We analyzed these two effects through hybridQM/MM optimizations in wt-HbA and in two relevant mutants: HE7G, thedistal mutant, and HF8G+Imidazole, the proximal mutant. We calculated∆Eoxy in both allosteric states, in αand βsubunits. R state subunits showed higher binding energy than Tsubunits, and differential proximal and distal effects were observedin α and β,in the R and T states.Ligandmigration through the protein matrix was studied with classicalmolecular dynamics simulations. Two ligand migration-relatedprocesses were analyzed: the open-close conformational transition ofthe HE7 and the ligand entrance itself. Free energy changes wereestimated using advanced sampling methodologies coupled to classicalMD simulations. Histidine protonation state seems to be a key factorin these processes, so simulations were carried out in the threepossible protonation states of the HE7. Our results show smallerfree energy barriers towards the HE7 opening in β subunits than in αones, consistent with the experimentally observed more flexiblebehaviour of β monomers.p { margin-bottom: 0.1in; direction: ltr; color: rgb(0, 0, 0); line-height: 120%; }p.western { font-family: "Liberation Serif", "Times New Roman", serif; font-size: 12pt; }p.cjk { font-family: "Droid Sans Fallback"; font-size: 12pt; }p.ctl { font-family: "FreeSans", "Times New Roman"; font-size: 12pt; }