Polyethylene glycol promotes autoxidation of cytochrome c

Hemoglobin-PEG Interactions Probed by Small-Angle X-ray Scattering: Insights for Crystallization and Diagnostics Applications

Protein-protein interactions, controlling protein aggregation in the solution phase, are crucial for the formulation of protein therapeutics and the use of proteins in diagnostic applications. Additives in the solution phase are factors that may enhance the protein's conformational stability or induce crystallization. Protein-PEG interactions do not always stabilize the native protein structure. Structural information is needed to validate excipients for protein stabilization in the development of protein therapeutics or use proteins in diagnostic assays. The present study investigates the impact of polyethylene glycol (PEG) molecular weight and concentration on the spatial structure of human hemoglobin (Hb) at neutral pH. Small-angle X-ray scattering (SAXS) in combination with size-exclusion chromatography is employed to characterize the Hb structure in solution without and with the addition of PEG. Our results evidence that human hemoglobin maintains a tetrameric conformation at neutral pH. The dummy atom model, reconstructed from the SAXS data, aligns closely with the known crystallographic structure of methemoglobin (metHb) from the Protein Data Bank. We established that the addition of short-chain PEG600, at concentrations of up to 10% (w/v), acts as a stabilizer for hemoglobin, preserving its spatial structure without significant alterations. By contrast, 5% (w/v) PEG with higher molecular weights of 2000 and 4000 leads to a slight reduction in the maximum particle dimension (Dmax), while the radius of gyration (Rg) remains essentially unchanged. This implies a reduced hydration shell around the protein due to the dehydrating effect of longer PEG chains. At a concentration of 10% (w/v), PEG2000 interacts with Hb to form a complex that does not distort the protein's spatial configuration. The obtained results provide a deeper understanding of PEG's influence on the Hb structure in solution and broader knowledge regarding protein-PEG interactions.

Regulation of electron transfer in the terminal step of the respiratory chain

In mitochondria, electrons are transferred along a series of enzymes and electron carriers that are referred to as the respiratory chain, leading to the synthesis of cellular ATP. The series of the interprotein electron transfer (ET) reactions is terminated by the reduction in molecular oxygen at Complex IV, cytochrome c oxidase (CcO) that is coupled with the proton pumping from the matrix to the inner membrane space. Unlike the ET reactions from Complex I to Complex III, the ET reaction to CcO, mediated by cytochrome c (Cyt c), is quite specific in that it is irreversible with suppressed electron leakage, which characterizes the ET reactions in the respiratory chain and is thought to play a key role in the regulation of mitochondrial respiration. In this review, we summarize the recent findings regarding the molecular mechanism of the ET reaction from Cyt c to CcO in terms of specific interaction between two proteins, a molecular breakwater, and the effects of the conformational fluctuation on the ET reaction, conformational gating. Both of these are essential factors, not only in the ET reaction from Cyt c to CcO, but also in the interprotein ET reactions in general. We also discuss the significance of a supercomplex in the terminal ET reaction, which provides information on the regulatory factors of the ET reactions that are specific to the mitochondrial respiratory chain.

Interaction of polyethylene glycol with cytochrome c investigated via in vitro and in silico approaches

One of the significant proteins that have attracted research groups due to virtue of being a potent selective anticancer drug target and property of triggering apoptosis upon release in cytoplasm is cytochrome c (cyt c). The mechanical transformations due to the macromolecular crowding in membrane in the mammalian cell are proposed to be useful inductors of changes in volume. It is very interesting to know that mitochondrial function were observed to be improved by polyethylene glycol (PEG) interaction, which in turn inhibits the cyt c (a pro-apoptotic cell death factor). In this work, the effect of polyethylene glycol of molecular weight 4 kilo Dalton (PEG 4 kDa) was investigated to highlight the structural transformations (tertiary and secondary structure) in cyt c using a choice of spectroscopic techniques (including UV-Vis absorption, near-UV, far-UV and Soret circular dichroism and fluorescence spectroscopy), which shows noteworthy shifts in the secondary and tertiary structures at higher concentrations of PEG 4 kDa with small changes in the heme-globular interactions. The size distribution changes of native protein treated with various concentrations of the crowder were observed and analyzed by dynamic light scattering (DLS). The interaction studies of the crowder with the protein was observed and analyzed by FTIR, isothermal titration calorimetry, time resolved fluorescence and molecular docking. The investigations suggested that the structural changes in the protein occurred due to soft interactions of PEG 4 kDa, which usually destabilizes proteins. The experimental evidence in this study proposed that crowding could be another approach to mechanical super-competition and free of certain markers that could aid in the identification and control of various diseases. This study suggests that crowders at specific concentrations, which softly interact with proteins, can be exploited as remedy for various diseases.