Molecular Dynamics of Human Methemoglobin: The Transmission of Conformational Information between Subunits in an αβ Dimer

Abstract

Spectroscopic studies indicate an interaction of the distal histidine with the heme iron as well as the transmission of distal heme perturbations across the α 1 β 1 interface. Molecular dynamics simulations have been used to explain the molecular basis for these processes. Using a human methemoglobin αβ dimer, it has been shown that at 235 K after 61 ps, a rearrangement occurs in the α-chain corresponding to the formation of a bond with the distal histidine. This transition does not take place in the β-chain during a 100-ps simulation and is reversed at 300 K. The absence of the distal histidine transition in the isolated chains and with the interface frozen indicate the involvement of the αβ interface. A detailed analysis of the simulation has been performed in terms of RMS fluctuations, domain cross-correlation maps, the disruption of helix hydrogen bonds, as well changes in electrostatic interactions and dihedral angles. This analysis shows that the rearrangements in the α-chain necessary to bring the histidine closer to the iron involve alterations primarily in the CD loop and at the interface. Communication to the β-chain distal pocket is propagated by increased interactions of the α-chain B helix with the β-chain G-GH-H segment and the flexibility in the EF loop. The G helices shown to be involved in propagation of perturbation across the α 1 β 1 interface extend into the α 1 β 2 interfaces, providing a mechansim whereby distal interactions can modulate the T⇋R transition in hemoglobin.