Structural factors controlling ligand binding to myoglobin: A kinetic hole-burning study

Abstract

Using temperature-derivative spectroscopy in the temperature range below 100 K, we have studied the dependence of the Soret band on the recombination barrier in sperm whale carbonmonoxy myoglobin (MbCO) after photodissociation at 12 K. The spectra were separated into contributions from the photodissociated species, Mb*CO, and CObound myoglobin. The line shapes of the Soret bands of both photolyzed and liganded myoglobin were analyzed with a model that takes into account the homogeneous bandwidth, coupling of the electronic transition to vibrational modes, and static conformational heterogeneity. The analysis yields correlations between the activation enthalpy for rebinding and the model parameters that characterize the homogeneous subensembles within the conformationally heterogeneous ensemble. Such couplings between spectral and functional parameters arise when they both originate from a common structural coordinate. This effect is frequently denoted as ''kinetic hole burning.'' The study of these correlations gives direct insights into the structure-function relationship in proteins. On the basis of earlier work that assigned spectral parameters to geometric properties of the heme, the connections with the heme geometry are discussed. We show that two separate structural coordinates inf luence the Soret line shape, but only one of the two is coupled to the enthalpy barrier for rebinding. We give evidence that this coordinate, contrary to widespread belief, is not the iron displacement from the mean heme plane.

Ligand binding and protein dynamics in neuroglobin

“…The extremely low geminate rebinding barriers determined in this work are consistent with an iron displacement toward the distal side because it is generally assumed that the iron position with respect to the heme plane is an important structural coordinate governing the ligand-binding barrier (25,(35)(36)(37).…”

Searching for Neuroglobin's role in the brain

“…This heterogeneity also affects the axial residues HisE7 and HisF8. In the dominant conformation (*70% of the total), the heme geometry is almost planar, with a 0.20 Å out-of-plane displacement of the iron towards the distal side, which is consistent with the high ligand affinity of Ngb (31,32). Rather surprising was the presence of a huge internal volume of *290 Å 3 that connects the distal and proximal sides and also features a channel to the bulk.…”

Influence of Distal Residue B10 on CO Dynamics in Myoglobin and Neuroglobin

“…This phenomenon has been termed 'kinetic hole burning' (KHB). It reflects structural heterogeneity and indicates that the same structural parameter governs both the position of the individual spectral line and the enthalpy barrier for recombination [70,71]. The pronounced KHB effect in the A and B substates suggests that slight structural changes in the HisE7/PheB10 side chain positions translate into substantial changes in the rebinding enthalpy of this mutant.…”