Professor Ekaterina Pletneva
Professor Ekaterina Pletneva
Department of Chemistry
Host: Professor Ambika Bhagi-Damodoran
Ligands, Protons, Neighboring Redox Centers, and Protein Fold in Redox Reactions of Heme Proteins
Electron-transfer reactions are essential to function of heme proteins as enzymes and electron carriers. In many of these systems the movement of electrons is coupled to other processes such as changes in protonation and protein conformation. Further, hemes are often incorporated into strings of multiple redox centers and their redox properties are strongly affected by their redox neighbors. Our understanding of these important redox-linked processes is incomplete, in part because they cannot be always readily observed. We employ a number of approaches to probe these elusive phenomena in c-type cytochromes and their relevance to biological redox mechanisms.
A small protein cytochrome c (cyt c) with its flexible coordination loop offers opportunities for engineering differently-ligated heme proteins within the cyt c scaffold. We have engineered a variety of switchable proteins in which the interchanging heme iron ligands are Met, Lys, Cys, and His. Analysis of protein stability demonstrates that the protein scaffold and the polypeptide interactions with the solvent play an important role in stabilizing particular heme coordination. Ligand exchange and accompanying protein rearrangements control the rates of redox reactions in these systems. Variations in the identity and location of the dissociating or incoming ligand alter reaction rates by orders of magnitude. Protonation of the heme iron ligands and neighboring groups modify redox reactivity of our model proteins. We show that enthalpies of protonation from isothermal titration calorimetry can be used to identify the number of involved protons and sites of protonation (deprotonation) in protein redox reactions. Finally, our in vitro and in vivo studies of bacterial electron carriers cyt c4, proteins with two heme groups, illustrate the role of the diheme architecture in tuning the electron injection efficiency of these proteins in their ET reaction with cbb3 oxidases.
Heme proteins are the main subjects of Professor Pletneva's research. In particular, researchers in her group have been focusing on ligand substitution reactions at the heme as a common platform for switching the protein structure and redox reactivity in signaling processes. They are investigating conformational properties of cytochrome c in apoptosis and correlate them to the protein peroxidase activity, which is critical for execution of this cellular pathway. We are also studying redox reactivity and folding of native sensors and engineered "switchable" proteins, in which changes in the oxidation state of the heme are linked to heme ligand substitution resulting in protein conformational rearrangements.