Time-resolved thermodynamics studies of heme signaling proteins and model systems

Heme-based gas sensor proteins have the ability to sense diatomic molecules such as O2 (FixL, EcDos or HemAT), CO (CooA, a CO-sensing protein of Rhodospirillum rubrum) and NO (guanylate cyclase) molecules and subsequently regulate numerous important biological processes in prokaryotic and eukaryotic organisms. The sensing function of these proteins is initiated by the binding of an effector (i.e., O2, CO, etc5) to the heme iron which then leads to a cascade of conformational events which gives rise to changes in kinase activity, DNA-binding activity, etc... In order to better understand the mechanism heme-based signaling, time resolved photothermal methods as well as transient optical techniques were utilized to obtain thermodynamic profiles for ligand binding/release in heme based signaling proteins including HemAT from Bacillus subtilis (aerotactic transducer), FixL from Sinorhizobium meliloti (regulation of the nitrogen fixation) and CooA from Rhodospirillum rubrum (transcriptional activator). In addition, a number of model systems were examined to understand the underlying thermodynamic processes involved in heme ligation. The variation of volume and enthalpy changes associated with spin state change of the iron from high-spin to low-spin where examined using the spin crossover Fe(III)(salten)(mepepy) complex. In addition, the experimental determination of the volume change due to electrostriction events were using Ru(II)(L)3 and the Debye-Hückel equation. Finally, different model heme proteins were studied to understand how a signal is conformationaly transmitted within a heme protein matrix. Sandbar shark hemoglobin was examined as an example of a non-signaling an allosteric protein. Two different peroxidases (horseradish and soybean) which have a direct channel between the heme pocket and the solvent involving no barrier energetic for the photodissociated ligand leaving the heme pocket were examined as example of non-signaling, non-allosteric proteins. The results show that each protein has a unique thermodynamic profile to conformationaly transmit signals subsequent to photodissociation of CO, even within the same class of protein (i.e. PAS domains, globins, etc...)