The yeast Saccharomyces cerevisiae uses fermentation as the preferred pathway to obtain ATP and requires the respiratory chain to re-oxidize the NADH needed for activity of Glyceraldehyde-3-phosphate. This process is favored by uncoupling of oxidative phosphorylation (OxPhos), which is at least partially controlled by the mitochondrial unspecific pore (ScMUC). When mitochondrial ATP synthesis is needed as in the diauxic phase or during mating, a large rise in Ca2+ concentration ([Ca2+]) closes ScMUC, coupling OxPhos. In addition, ScMUC opening/closing is mediated by the ATP/ADP ratio, which indicates cellular energy needs. Here, opening and closing of ScMUC was evaluated in isolated mitochondria from S. cerevisiae at different incubation times and in the presence of different ATP/ADP ratios or varying [Ca2+]. Measurements of the rate of O2 consumption, mitochondrial swelling, transmembrane potential and ROS generation were conducted. It was observed that ScMUC opening was reversible, a high ATP/ADP ratio promoted opening and [Ca2+] closed ScMUC even after several minutes of incubation in the open state. In the absence of ATP synthesis, closure of ScMUC resulted in an increase in ROS.

From 2.5 to 2.0 billion years ago, atmospheric oxygen concentration [O2] rose thousands of times, leading to the first mass extinction. Reactive Oxygen Species (ROS) produced by the non-catalyzed partial reduction of O2 were highly toxic eliminating many species. Survivors developed different strategies to cope with ROS toxicity. At the same time, using O2 as the final acceptor in respiratory chains increased ATP production manifold. Thus, both O2 and ROS were strong drivers of evolution, as species optimized aerobic metabolism while developing ROS-neutralizing mechanisms. The first line of defense is preventing ROS overproduction and two mechanisms were developed in parallel: 1) Physiological uncoupling systems (PUS), which increase the rate of electron fluxes in respiratory systems. 2) Avoidance of excess [O2]. However, it seems that as avoidance efficiency improved, PUSs became less efficient. PUS includes branched respiratory chains and proton sinks, which may be proton specific, the mitochondrial uncoupling proteins (UCPs) or unspecific, the mitochondrial permeability transition pore (PTP). High [O2] avoidance also involved different strategies: 1) Cell association, as in biofilms or in multi-cellularity allowed gas-permeable organisms (oxyconformers) from bacterial to arthropods to exclude O2. 2) Motility, to migrate from hypoxic niches. 3) Oxyregulator organisms: as early as in fish, and O2-impermeable epithelium excluded all gases and only exact amounts entered through specialized respiratory systems. Here we follow the parallel evolution of PUS and O2-avoidance, PUS became less critical and lost efficiency. In regard, to proton sinks, there is fewer evidence on their evolution, although UCPs have indeed drifted in function while in some species it is not clear whether PTPs exist.

Hydrogen sulfide (H2S) is a gas produced endogenously in organisms from the three domains of life. In mammals, it is involved in diverse physiological processes, including the regulation of blood pressure and its effects on memory. In contrast, in unicellular organisms, the physiological role of H2S has not been studied in detail. In yeast, for example, in the winemaking industry, H2S is an undesirable byproduct because of its rotten egg smell; however, its biological relevance during fermentation is not well understood. The effect of H2S in cells is linked to a posttranslational modification in cysteine residues known as S-persulfidation. In this paper, we evaluated S-persulfidation in the Saccharomyces cerevisiae proteome. We screened S-persulfidated proteins from cells growing in fermentable carbon sources, and we identified several glycolytic enzymes as S-persulfidation targets. Pyruvate kinase, catalyzing the last irreversible step of glycolysis, increased its activity in the presence of a H2S donor. Yeast cells treated with H2S increased ethanol production; moreover, mutant cells that endogenously accumulated H2S produced more ethanol and ATP during the exponential growth phase. This mechanism of the regulation of metabolism seems to be evolutionarily conserved in other yeast species, because H2S induces ethanol production in the pre-Whole-Genome Duplication species Kluyveromyces marxianus and Meyerozyma guilliermondii. Our results suggest a new role of H2S in the regulation of the metabolism during fermentation.