Delineating Heme-Mediated versus Direct Protein Oxidation in Peroxidase-Activated Cytochrome c by Top-Down Mass Spectrometry

Alkaline State of the Domain-Swapped Dimer of Human Cytochrome c: A Conformational Switch for Apoptotic Peroxidase Activity

A 2.08 Å structure of an alkaline conformer of the domain-swapped dimer of K72A human cytochrome c (Cytc) crystallized at pH 9.9 is presented. In the structure, Lys79 is ligated to the heme. All other domain-swapped dimer structures of Cytc have water bound to this coordination site. Part of Ω-loop D (residues 70-85) forms a flexible linker between the subunits in other Cytc domain-swapped dimer structures but instead converts to a helix in the alkaline conformer of the dimer combining with the C-terminal helix to form two 26-residue helices that bracket both sides of the dimer. The alkaline transition of the K72A human dimer monitored at both 625 nm (high spin heme) and 695 nm (Met80 ligation) yields midpoint pH values of 6.6 and 7.6, respectively, showing that the Met80 → Lys79 and high spin to low spin transitions are distinct. The dimer peroxidase activity increases rapidly below pH 7, suggesting that population of the high spin form of the heme is what promotes peroxidase activity. Comparison of the structures of the alkaline dimer and the neutral pH dimer shows that the neutral pH conformer has a better electrostatic surface for binding to a cardiolipin-containing membrane and provides better access for small molecules to the heme iron. Given that the pH of mitochondrial cristae ranges from 6.9 to 7.2, the alkaline transition of the Cytc dimer could provide a conformational switch to tune the peroxidase activity of Cytc that oxygenates cardiolipin in the early stages of apoptosis.

Rapid Online Oxidation of Proteins and Peptides via Electrospray-Accelerated Ozonation

Mass spectrometry-based analyses of protein conformation continue to grow in utilization due their speed, low sample requirements, and applicability to most protein systems. These techniques typically rely on chemical derivatization of proteins and as with all label-based analyses must ensure the integrity of the protein conformation throughout the duration of the labeling reaction. Hydroxyl radical footprinting of proteins and the recently developed fast fluoroalkylation of proteins attempt to bypass this consideration via rapid reactions that occur on time scales faster than protein folding, but they often require microfluidic setups or electromagnetic radiation sources. In this work, we demonstrate that ozonation of proteins and peptides, which normally occurs in the second to minute time scales, can be accelerated to the submillisecond to millisecond time scale with an electrospray ionization source. This rapid ozonation results in selective labeling of tryptophan and methionine residues. When applied to cytochrome C and carbonic anhydrase, this labeling technique is sensitive to solution conditions and correlates with solution-phase analyses of conformation. While significant work is still needed to characterize this fast chemical labeling strategy, it requires no complicated sample handling, electromagnetic radiation sources, or microfluidic systems outside of the electrospray source and may represent a facile alternative to other rapid labeling technologies that are utilized today.

How to Turn an Electron Transfer Protein into a Redox Enzyme for Biosensing

Cytochrome c is a small globular protein whose main physiological role is to shuttle electrons within the mitochondrial electron transport chain. This protein has been widely investigated, especially as a paradigmatic system for understanding the fundamental aspects of biological electron transfer and protein folding. Nevertheless, cytochrome c can also be endowed with a non-native catalytic activity and be immobilized on an electrode surface for the development of third generation biosensors. Here, an overview is offered of the most significant examples of such a functional transformation, carried out by either point mutation(s) or controlled unfolding. The latter can be induced chemically or upon protein immobilization on hydrophobic self-assembled monolayers. We critically discuss the potential held by these systems as core constituents of amperometric biosensors, along with the issues that need to be addressed to optimize their applicability and response.