TIMB3 Consortium Seminar with Ekaterina Pletneva

The next TIMB3 Seminar will take place on Thursday, November 18, with invited speaker Ekaterina Pletneva.
 

Title: Ligands, Protons, Neighboring Redox Centers, and Protein Fold in Redox Reactions of Heme Proteins

Affiliation: Department of Chemistry, Dartmouth College, Hanover, New Hampshire, USA

Abstract:  Electron-transfer reactions are essential to the 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.