Type 1 nonsymbiotic hemoglobins are
found in a wide variety of
land plants and exhibit very high affinities for exogenous gaseous
ligands. These proteins are presumed to have a role in protecting
plant cells from oxidative stress under etiolated/hypoxic conditions
through NO dioxygenase activity. In this study we have employed photoacoustic
calorimetry, time-resolved absorption spectroscopy, and classical
molecular dynamics simulations in order to elucidate thermodynamics,
kinetics, and ligand migration pathways upon CO photodissociation
from WT and a H73L mutant of type 1 nonsymbiotic hemoglobin from <i>Oryza sativa</i> (rice). We observe a temperature dependence
of the resolved thermodynamic parameters for CO photodissociation
from CO-rHb1 which we attribute to temperature dependent formation
of a network of electrostatic interactions in the vicinity of the
heme propionate groups. We also observe slower ligand escape from
the protein matrix under mildly acidic conditions in both the WT and
H73L mutant (τ = 134 ± 19 and 90 ± 15 ns). Visualization
of transient hydrophobic channels within our classical molecular dynamics
trajectories allows us to attribute this phenomenon to a change in
the ligand migration pathway which occurs upon protonation of the
distal His73, His117, and His152. Protonation of these residues may
be relevant to the functioning of the protein in vivo given that etiolation/hypoxia
can cause a decrease in intracellular pH in plant cells
