Reactive sulfur species (RSS) entail a diverse family of sulfur derivatives that have emerged as important effector molecules in H2S-mediated biological events. RSS (including H2Show more
Reactive sulfur species (RSS) entail a diverse family of sulfur derivatives that have emerged as important effector molecules in H2S-mediated biological events. RSS (including H2S) can exert their biological roles via widespread interactions with metalloproteins. Metalloproteins are essential components along the metabolic route of oxygen in the body, from the transport and storage of O2, through cellular respiration, to the maintenance of redox homeostasis by elimination of reactive oxygen species (ROS). Moreover, heme peroxidases contribute to immune defense by killing pathogens using oxygen-derived H2O2 as a precursor for stronger oxidants. Coordination and redox reactions with metal centers are primary means of RSS to alter fundamental cellular functions. In addition to RSS-mediated metalloprotein functions, the reduction of high-valent metal centers by RSS results in radical formation and opens the way for subsequent per- and polysulfide formation, which may have implications in cellular protection against oxidative stress and in redox signaling. Furthermore, recent findings pointed out the potential role of RSS as substrates for mitochondrial energy production and their cytoprotective capacity, with the involvement of metalloproteins. The current review summarizes the interactions of RSS with protein metal centers and their biological implications with special emphasis on mechanistic aspects, sulfide-mediated signaling, and pathophysiological consequences. A deeper understanding of the biological actions of reactive sulfur species on a molecular level is primordial in H2S-related drug development and the advancement of redox medicine. Show less
Ligand substitution at the metal center is common in catalysis and signal transduction of metalloproteins. Understanding the effects of particular ligands, as well as the polypeptide surrounding, is c Show more
Ligand substitution at the metal center is common in catalysis and signal transduction of metalloproteins. Understanding the effects of particular ligands, as well as the polypeptide surrounding, is critical for uncovering mechanisms of these biological processes and exploiting them in the design of bioinspired catalysts and molecular devices. A series of switchable K79G/M80X/F82C (X = Met, His, or Lys) variants of cytochrome (cyt) c was employed to directly compare the stability of differently ligated proteins and activation barriers for Met, His, and Lys replacement at the ferric heme iron. Studies of these variants and their nonswitchable counterparts K79G/M80X have revealed stability trends Met < Lys < His and Lys < His < Met for the protein FeIII-X and FeII-X species, respectively. The differences in the hydrogen-bonding interactions in folded proteins and in solvation of unbound X in the unfolded proteins explain these trends. Calculations of free energy of ligand dissociation in small heme model complexes reveal that the ease of the FeIII-X bond breaking increases in the series amine < imidazole < thioether, mirroring trends in hardness of these ligands. Experimental rate constants for X dissociation in differently ligated cyt c variants are consistent with this sequence, but the differences between Met and His dissociation rates are attenuated because the former process is limited by the heme crevice opening. Analyses of activation parameters and comparisons to those for the Lys-to-Met ligand switch in the alkaline transition suggest that ligand dissociation is entropically driven in all the variants and accompanied by Lys protonation at neutral pH. The described thiolate redox-linked switches have offered a wealth of new information about interactions of different protein-derived ligands with the heme iron in cyt c model proteins, and we anticipate that the strategy of employing these switches could benefit studies of other redox metalloproteins and model complexes. Show less