Electron transfer between cytochrome c and the binuclear center of cytochrome oxidase

Effects of Ca2+ signal on the activities of key enzymes and expression of related genes in yeast ethanol metabolism and mitochondrial function during high sugar fermentation

BACKGROUND

During high sugar fermentation, yeast is mainly affected by high sugar stress in the early stage. It becomes jointly affected by high sugar and ethanol stress as ethanol accumulates during fermentation. Ca2+, as the second messenger of the cell, mediates various metabolic processes. In this study, the effects of the Ca2+ signal on the activities of key enzymes, expression of related genes of ethanol metabolism, and mitochondrial function were investigated.

RESULTS

The results showed a significant increase in the activities of enzymes related to ethanol metabolism in yeast cells under a high sugar environment. Ca2+ significantly promoted the activities of enzymes related to mitochondrial respiratory metabolism and regulated the carbon flow between ethanol metabolism and the tricarboxylic acid cycle. The high sugar environment affected the expression of genes related to carbon metabolism, while the addition of Ca2+ stabilized the expression of related genes.

CONCLUSION

Ca2+ signal participated in ethanol and mitochondrial metabolism and regulated the key enzymes and related gene expression to enhance the resistance of yeast to stress during high sugar fermentation. © 2024 Society of Chemical Industry.

Heptamethine Cyanine-Based Molecule Release Triggered by Mitochondrial ROS

Conditionally activated molecule release in live cells would provide spatiotemporal control for the study and intervention of biological processes, e.g., bioactive molecule monitoring and controlled drug release. Mitochondria are the main sites of reactive oxygen species (ROS) production in cells. Here, we report an ROS-triggered molecule release strategy in mitochondria. A molecule IRTO with dual targeting groups was designed by covalently linking IR-780 (a mitochondrial targeted heptamethine cyanine) and 4-aminobutyl-thiazole orange (NH2-TO, a nuclear dye). IRTO diffused into live cells and first accumulated in mitochondria. As the cyanine moiety reacted with mitochondrial ROS directly or with the help of mitochondrial cytochromes, NH2-TO was released, escaped from mitochondria, and finally located in the nucleus. This process could be visualized by fluorescent imaging, i.e., red fluorescence (from the cyanine moiety of IRTO) first located in mitochondria, and green fluorescence (from NH2-TO) appeared and gradually enhanced in the nucleus with the increase of incubation time. The addition of H2O2 or lipopolysaccharide (LPS, an ROS accelerator) could accelerate the release of NH2-TO, whereas N-acetyl-l-cysteine (NAC, an ROS inhibitor) and mitoquinone mesylate (MitoQ, a mitochondrial ROS scavenger) could obviously decrease the release of NH2-TO. These results suggest that IRTO could serve as a fluorescent probe for monitoring ROS in mitochondria and that IR-780 might be a promising endogenous ROS-triggered molecule release platform.

In Silico Modeling of the Mitochondrial Pumping Complexes with Markov State Models

The mechanism of proton pumping by the mitochondrial electron transport chain complexes is still enigmatic after decades of research. Recently, there has been interest in in silico Markov state models to model the mitochondrial pumping complexes at the microscopic level, and this chapter describes the methods of constructing and simulating such models.

The proton pumping mechanism of the bc1 complex

The bc₁ complex is a proton pump of the mitochondrial electron transport chain which transfers electrons from ubiquinol to cytochrome c. It operates via the modified Q cycle in which the two electrons from oxidation of ubiquinol at the Q_o center are bifurcated such that the first electron is passed to Cytc via an iron sulfur center and c₁ whereas the second electron is passed across the membrane by b_L and b_H to reduce ubiquinone at the Q_i center. Proton pumping occurs because oxidation of ubiquinol at the Q_o center releases protons to the P-side and reduction of ubiquinone at the Q_i center takes up protons from the N-side. However, the mechanisms which prevent the thermodynamically more favorable short circuit reactions and so ensure precise bifurcation and proton pumping are not known. Here we use statistical thermodynamics to show that reaction steps that originate from high energy states cannot support high flux even when they have large rate constants. We show how the chemistry of ubiquinol oxidation and the structure of the Q_o site can result in free energy profiles that naturally suppress flux through the short circuit pathways while allowing high rates of bifurcation. These predictions are confirmed through in-silico simulations using a Markov state model.

Bacterial Respiratory Chain Diversity Reveals a Cytochrome c Oxidase Reducing O2 at Low Overpotentials

Cytochrome c oxidases (CcOs) are the terminal enzymes in energy-converting chains of microorganisms, where they reduce oxygen into water. Their affinity for O₂ makes them attractive biocatalysts for technological devices in which O₂ concentration is limited, but the high overpotentials they display on electrodes severely limit their applicative use. Here, the CcO of the acidophilic bacterium Acidithiobacillus ferrooxidans is studied on various carbon materials by direct protein electrochemistry and mediated one with redox mediators either diffusing or co-immobilized at the electrode surface. The entrapment of the CcO in a network of hydrophobic carbon nanofibers permits a direct electrochemical communication between the enzyme and the electrode. We demonstrate that the CcO displays a μM affinity for O₂ and reduces O₂ at exceptionally high electrode potentials in the range of +700 to +540 mV vs NHE over a pH range of 4-6. The kinetics of interactions between the enzyme and its physiological partners are fully quantified. Based on these results, an electron transfer pathway allowing O₂ reduction in the acidic metabolic chain is proposed.