Cytochrome has served as a model system for studying redox reactions, protein folding, and more recently peroxidase activity induced by partial unfolding on membranes. This review illuminates some important aspects of the research on this biomolecule. The first part summarizes the results of structural analyses of its active site. Owing to heme-protein interactions the heme group is subject to both in-plane and out-of-plane deformations. The unfolding of the protein as discussed in detail in the second part of this review can be induced by changes of pH and temperature and most prominently by the addition of denaturing agents. Both the kinetic and thermodynamic folding and unfolding involve intermediate states with regard to all unfolding conditions. If allowed to sit at alkaline pH (11.5) for a week, the protein does not return to its folding state when the solvent is switched back to neutral pH. It rather adopts a misfolded state that is prone to aggregation via domain swapping. On the surface of cardiolipin containing liposomes, the protein can adopt a variety of partially unfolded states. Apparently, ferricytochrome c can perform biological functions even if it is only partially folded. 1. Introduction Among the multitude of biomolecules, the family of heme proteins has played a peculiar and prominent role in biophysical research over many decades [1]. All members of this family contain at least one, sometimes multiple, heme group as active sites. Heme groups are iron-protoporphyrin derivatives. Figure 1 exhibits heme c, the prosthetic group of the electron carrier cytochrome c. The simplest heme proteins are monomers with a variety of secondary and tertiary structures, the former being dominated by helical segments in most cases (Figure 2). However, for some of these proteins the monomers assemble to form a quaternary structure. The most prominent example is hemoglobin, which contains four globular heme proteins as subunits. This quaternary structure is pivotal for the protein’s biological function in that it enables a cooperative communication between the four binding sites, which allows for an effective uptake and release of oxygen in the lungs and the tissue, respectively. Figure 1: Structure of heme c, the prosthetic group in cytochrome c. Figure 2: Visualization of the secondary and tertiary structures of ferro-horse heart cytochrome c obtained from pdb 2FRC. The figure has been produced with VMD software [ 22]. All heme proteins contain a heme group which functions as their active site for performing a diverse set of tasks, such as ligand
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