Heme proteins carry out a diverse array of functions in vivo while maintaining a well-conserved 3-over-3 α-helical structure. Human hemoglobin (Hb) is well-known for its oxygen transport function. Type 1 non-symbiotic hemoglobins (nsHb1) in plants and bacterial flavohemoglobins (fHb) from a variety of bacterial species have been predicted to carry out a nitric oxide dioxygenase function. In nsHb1 and fHb this function has been linked to protection from nitrosative stress. Herein, I combine photoacoustic calorimetry (PAC), transient absorption spectroscopy (TA), and classical molecular dynamics (cMD) simulations to characterize molecular mechanism of diatomic ligand interactions with a hexa-coordinate globin from plant (rice hemoglobin), bacterial flavohemoglobins and human hemoglobin.
In rice type 1 non-symbiotic hemoglobin (rHb1), the dynamics and energetics of structural changes associated with ligand photodissociation is strongly impacted by solvent and temperature, namely CO escape from the protein matrix is slower at pH = 6.0 compare to neutral pH (ns) due to the CD loop reorganization which forms a pathway for ligand escape. In human hemoglobin, exogenous allosteric effectors modulate energetics of conformational changes associated with the CO and O2 escape although the effectors impact on rate constants for ligand association is small. The conformational dynamics associated with ligand photorelease from fHbs from Cupriavidus necator (FHP) and Staphylococcus aureus (HMPSa) are strongly modulated by the presence of azole drugs indicating that drug association modulates structural properties of the heme binding pocket.
In addition, we carried out a study of the formation of the DNA intercalated motif (i-motif). The formation of the structure is strongly favored at acidic pH; therefore, PAC was combined with a 2-nitrobenzaldehyde pH-jump to probe formation of the i-motif on fast timescales. i-Motif folding is two-step process with the initial protonation of cytosine residues being endothermic with ΔHfast=8.5 ± 7.0 kcal mol-1 and ΔVfast=10.4 ± 1.6 mL mol-1 and subsequent nucleation/i-motif folding (τ = 140 ns) with ΔHslow=-51.5 ± 4.8 kcal mol-1 and ΔVslow=-6.6 ± 0.9 mL mol-1. The above results indicate that PAC can be employed to study diverse biochemical reactions such as DNA folding, drug binding and ligand photorelease from proteins.
