Thriving in Oxygen While Preventing ROS Overproduction: No Two Systems Are Created Equal.

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.

In Saccharomyces cerevisiae, withdrawal of the carbon source results in detachment of glycolytic enzymes from the cytoskeleton and in actin reorganization.

Metabolons are dynamic associations of enzymes catalyzing consecutive reactions within a given pathway. Association results in enzyme stabilization and increased metabolic efficiency. Metabolons may use cytoskeletal elements, membranes and membrane proteins as scaffolds. The effects of glucose withdrawal on a putative glycolytic metabolon/F-actin system were evaluated in three Saccharomyces cerevisiae strains: a WT and two different obligate fermentative (OxPhos-deficient) strains, which obtained most ATP from glycolysis. Carbon source withdrawal led to inhibition of fermentation, decrease in ATP concentration and dissociation of glycolytic enzymes from F-actin. Depending on the strain, inactivation/reactivation transitions of fermentation took place in seconds. In addition, when ATP was very low, green fluorescent protein-labeled F-actin reorganized from highly dynamic patches to large, non-motile actin bodies containing proteins and enzymes. Glucose addition restored fermentation and cytoskeleton dynamics, suggesting that in addition to ATP concentration, at least in one of the tested strains, metabolon assembly/disassembly is a factor in the control of the rate of fermentation.

The cytochrome c oxidase and its mitochondrial function in the whiteleg shrimp Litopenaeus vannamei during hypoxia.

Cytochrome c oxidase (COX), which is located in the inner membrane of mitochondria, is a key constituent of the electron transport chain that catalyzes the reduction of oxygen. The Pacific whiteleg shrimp Litopenaeus vannamei is constantly exposed to hypoxic conditions, which affects both the central metabolism and the mitochondrial function. The purpose of this study was to isolate shrimp mitochondria, identify the COX complex and to evaluate the effect of hypoxia on the shrimp mitochondrial function and in the COX activity. A 190 kDa protein was identified as COX by immunodetection techniques. The effect of hypoxia was confirmed by an increase in the shrimp plasma L-lactate concentration. COX activity, mitochondrial oxygen uptake and protein content were reduced under hypoxic conditions, and gradually restored as hypoxia continued, this suggests an adaptive mitochondrial response and a highly effective COX enzyme. Both mitochondrial oxygen uptake and COX activity were completely inhibited by KCN and sodium azide, suggesting that COX is the unique oxidase in L. vannamei mitochondria.

Effect of glycolysis inhibition on mitochondrial function in rat brain.

Inhibition of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase enhances the neural vulnerability to excitotoxicity both in vivo and in vitro through an unknown mechanism possibly related to mitochondrial failure. However, as the effect of glycolysis inhibition on mitochondrial function in brain has not been studied, the aim of the present work was to evaluate the effect of glycolysis inhibition induced by iodoacetate on mitochondrial function and oxidative stress in brain. Mitochondria were isolated from brain cortex, striatum and cerebellum of rats treated systemically with iodoacetate (25 mg/kg/day for 3 days). Oxygen consumption, ATP synthesis, transmembrane potential, reactive oxygen species production, lipoperoxidation, glutathione levels, and aconitase activity were assessed. Oxygen consumption and aconitase activity decreased in the brain cortex and striatum, showing that glycolysis inhibition did not trigger severe mitochondrial impairment, but a slight mitochondrial malfunction and oxidative stress were present.

The function of mitochondrial F(O)F(1) ATP-synthase from the whiteleg shrimp Litopenaeus vannamei muscle during hypoxia.

The effect of hypoxia and re-oxygenation on the mitochondrial complex F(O)F(1)-ATP synthase was investigated in the whiteleg shrimp Litopenaeus vannamei. A 660 kDa protein complex isolated from mitochondria of the shrimp muscle was identified as the ATP synthase complex. After 10h at hypoxia (1.5-2.0 mg oxygen/L), the concentration of L-lactate in plasma increased significantly, but the ATP amount and the concentration of ATPß protein remained unaffected. Nevertheless, an increase of 70% in the ATPase activity was detected, suggesting that the enzyme may be regulated at a post-translational level. Thus, during hypoxia shrimp are able to maintain ATP amounts probably by using some other energy sources as phosphoarginine when an acute lack of energy occurs. During re-oxygenation, the ATPase activity decreased significantly and the ATP production continued via the electron transport chain and oxidative phosphorylation. The results obtained showed that shrimp faces hypoxia partially by hydrolyzing the ATP through the reaction catalyzed by the mitochondrial ATPase which increases its activity.

Tumor cell energy metabolism and its common features with yeast metabolism.

During the last decades a considerable amount of research has been focused on cancer. A number of genetic and signaling defects have been identified. This has allowed the design and screening of a number of anti-tumor drugs for therapeutic use. One of the main challenges of anti-cancer therapy is to specifically target these drugs to malignant cells. Recently, tumor cell metabolism has been considered as a possible target for cancer therapy. It is widely accepted that tumors display an enhanced glycolytic activity and oxidative phosphorylation down-regulation (Warburg effect). Therefore, it seems reasonable that disruption of glycolysis might be a promising candidate for specific anti-cancer therapy. Nonetheless, the concept of aerobic glycolysis as the paradigm of tumor cell metabolism has been challenged, as some tumor cells use oxidative phosphorylation. Mitochondria are of special interest in cancer cell energy metabolism, as their physiology is linked to the Warburg effect. Besides, their central role in apoptosis makes these organelles a promising "dual hit target" for selectively eliminate tumor cells. Thus, it is desirable to have an easy-to-use and reliable model in order to do the screening for energy metabolism-inhibiting drugs to be used in cancer therapy. From a metabolic point of view, the fermenting yeast Saccharomyces cerevisiae and tumor cells share several features. In this paper we will review these common metabolic properties and we will discuss the possibility of using S. cerevisiae as an early screening test in the research for novel anti-tumor compounds used for the inhibition of tumor cell metabolism.