Tight binding of oxaloacetate to succinate dehydrogenase

Disordered-to-ordered transitions in assembly factors allow the complex II catalytic subunit to switch binding partners

Complex II (CII) activity controls phenomena that require crosstalk between metabolism and signaling, including neurodegeneration, cancer metabolism, immune activation, and ischemia-reperfusion injury. CII activity can be regulated at the level of assembly, a process that leverages metastable assembly intermediates. The nature of these intermediates and how CII subunits transfer between metastable complexes remains unclear. In this work, we identify metastable species containing the SDHA subunit and its assembly factors, and we assign a preferred temporal sequence of appearance of these species during CII assembly. Structures of two species show that the assembly factors undergo disordered-to-ordered transitions without the appearance of significant secondary structure. The findings identify that intrinsically disordered regions are critical in regulating CII assembly, an observation that has implications for the control of assembly in other biomolecular complexes.

Preventing mitochondrial reverse electron transport as a strategy for cardioprotection

In the context of myocardial infarction, the burst of superoxide generated by reverse electron transport (RET) at complex I in mitochondria is a crucial trigger for damage during ischaemia/reperfusion (I/R) injury. Here we outline the necessary conditions for superoxide production by RET at complex I and how it can occur during reperfusion. In addition, we explore various pathways that are implicated in generating the conditions for RET to occur and suggest potential therapeutic strategies to target RET, aiming to achieve cardioprotection.

Integrative Analysis of the Ethanol Tolerance of Saccharomyces cerevisiae

Ethanol (EtOH) alters many cellular processes in yeast. An integrated view of different EtOH-tolerant phenotypes and their long noncoding RNAs (lncRNAs) is not yet available. Here, large-scale data integration showed the core EtOH-responsive pathways, lncRNAs, and triggers of higher (HT) and lower (LT) EtOH-tolerant phenotypes. LncRNAs act in a strain-specific manner in the EtOH stress response. Network and omics analyses revealed that cells prepare for stress relief by favoring activation of life-essential systems. Therefore, longevity, peroxisomal, energy, lipid, and RNA/protein metabolisms are the core processes that drive EtOH tolerance. By integrating omics, network analysis, and several other experiments, we showed how the HT and LT phenotypes may arise: (1) the divergence occurs after cell signaling reaches the longevity and peroxisomal pathways, with CTA1 and ROS playing key roles; (2) signals reaching essential ribosomal and RNA pathways via SUI2 enhance the divergence; (3) specific lipid metabolism pathways also act on phenotype-specific profiles; (4) HTs take greater advantage of degradation and membraneless structures to cope with EtOH stress; and (5) our EtOH stress-buffering model suggests that diauxic shift drives EtOH buffering through an energy burst, mainly in HTs. Finally, critical genes, pathways, and the first models including lncRNAs to describe nuances of EtOH tolerance are reported here.

PCK1 dysregulation in cancer: Metabolic reprogramming, oncogenic activation, and therapeutic opportunities

The last few decades have witnessed an advancement in our understanding of multiple cancer cell pathways related to metabolic reprogramming. One of the most important cancer hallmarks, including aerobic glycolysis (the Warburg effect), the central carbon pathway, and multiple-branch metabolic pathway remodeling, enables tumor growth, progression, and metastasis. Phosphoenolpyruvate carboxykinase 1 (PCK1), a key rate-limiting enzyme in gluconeogenesis, catalyzes the conversion of oxaloacetate to phosphoenolpyruvate. PCK1 expression in gluconeogenic tissues is tightly regulated during fasting. In tumor cells, PCK1 is regulated in a cell-autonomous manner rather than by hormones or nutrients in the extracellular environment. Interestingly, PCK1 has an anti-oncogenic role in gluconeogenic organs (the liver and kidneys), but a tumor-promoting role in cancers arising from non-gluconeogenic organs. Recent studies have revealed that PCK1 has metabolic and non-metabolic roles in multiple signaling networks linking metabolic and oncogenic pathways. Aberrant PCK1 expression results in the activation of oncogenic pathways, accompanied by metabolic reprogramming, to maintain tumorigenesis. In this review, we summarize the mechanisms underlying PCK1 expression and regulation, and clarify the crosstalk between aberrant PCK1 expression, metabolic rewiring, and signaling pathway activation. In addition, we highlight the clinical relevance of PCK1 and its value as a putative cancer therapeutic target.

Cell-Permeable Succinate Increases Mitochondrial Membrane Potential and Glycolysis in Leigh Syndrome Patient Fibroblasts

Mitochondrial disorders represent a large group of severe genetic disorders mainly impacting organ systems with high energy requirements. Leigh syndrome (LS) is a classic example of a mitochondrial disorder resulting from pathogenic mutations that disrupt oxidative phosphorylation capacities. Currently, evidence-based therapy directed towards treating LS is sparse. Recently, the cell-permeant substrates responsible for regulating the electron transport chain have gained attention as therapeutic agents for mitochondrial diseases. We explored the therapeutic effects of introducing tricarboxylic acid cycle (TCA) intermediate substrate, succinate, as a cell-permeable prodrug NV118, to alleviate some of the mitochondrial dysfunction in LS. The results suggest that a 24-hour treatment with prodrug NV118 elicited an upregulation of glycolysis and mitochondrial membrane potential while inhibiting intracellular reactive oxygen species in LS cells. The results from this study suggest an important role for TCA intermediates for treating mitochondrial dysfunction in LS. We show, here, that NV118 could serve as a therapeutic agent for LS resulting from mutations in mtDNA in complex I and complex V dysfunctions.

Oxaloacetic acid mediates ADP-dependent inhibition of mitochondrial complex II-driven respiration

We recently reported a previously unrecognized mitochondrial respiratory phenomenon. When [ADP] was held constant ("clamped") at sequentially increasing concentrations in succinate-energized muscle mitochondria in the absence of rotenone (commonly used to block complex I), we observed a biphasic, increasing then decreasing, respiratory response. Here we investigated the mechanism. We confirmed decades-old reports that oxaloacetate (OAA) inhibits succinate dehydrogenase (SDH). We then used an NMR method to assess OAA concentrations (known as difficult to measure by MS) as well as those of malate, fumarate, and citrate in isolated succinate-respiring mitochondria. When these mitochondria were incubated at varying clamped ADP concentrations, respiration increased at low [ADP] as expected given the concurrent reduction in membrane potential. With further increments in [ADP], respiration decreased associated with accumulation of OAA. Moreover, a low pyruvate concentration, that alone was not enough to drive respiration, was sufficient to metabolize OAA to citrate and completely reverse the loss of succinate-supported respiration at high [ADP]. Further, chemical or genetic inhibition of pyruvate uptake prevented OAA clearance and preserved respiration. In addition, we measured the effects of incremental [ADP] on NADH, superoxide, and H₂O₂ (a marker of reverse electron transport from complex II to I). In summary, our findings, taken together, support a mechanism (detailed within) wherein succinate-energized respiration as a function of increasing [ADP] is initially increased by [ADP]-dependent effects on membrane potential but subsequently decreased at higher [ADP] by inhibition of succinate dehydrogenase by OAA. The physiologic relevance is discussed.

Differential susceptibility of mitochondrial complex II to inhibition by oxaloacetate in brain and heart

Mitochondrial Complex II is a key mitochondrial enzyme connecting the tricarboxylic acid (TCA) cycle and the electron transport chain. Studies of complex II are clinically important since new roles for this enzyme have recently emerged in cell signalling, cancer biology, immune response and neurodegeneration. Oxaloacetate (OAA) is an intermediate of the TCA cycle and at the same time is an inhibitor of complex II with high affinity (K_d ~ 10^− 8 M). Whether or not OAA inhibition of complex II is a physiologically relevant process is a significant, but still controversial topic. We found that complex II from mouse heart and brain tissue has similar affinity to OAA and that only a fraction of the enzyme in isolated mitochondrial membranes (30.2 ± 6.0% and 56.4 ± 5.6% in the heart and brain, respectively) is in the free, active form. Since OAA could bind to complex II during isolation, we established a novel approach to deplete OAA in the homogenates at the early stages of isolation. In heart, this treatment significantly increased the fraction of free enzyme, indicating that OAA binds to complex II during isolation. In brain the OAA-depleting system did not significantly change the amount of free enzyme, indicating that a large fraction of complex II is already in the OAA-bound inactive form. Furthermore, short-term ischemia resulted in a dramatic decline of OAA in tissues, but it did not change the amount of free complex II. Our data show that in brain OAA is an endogenous effector of complex II, potentially capable of modulating the activity of the enzyme.

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Complex II in mitochondrial membranes is inhibited by tightly-bound oxaloacetate.

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Oxaloacetate binds to the heart enzyme during isolation.

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In brain a large fraction of Complex II is present in oxaloacetate-bound form.

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Short-time tissue ischemia does not affect the content of the free Complex II in brain.

Reactivity of the sulfhydryl groups of soluble succinate dehydrogenase

Soluble succinate dehydrogenase prepared by butanol extraction reacts with N-ethylmaleimide according to first-order kinetics with respect to both remaining active enzyme and the inhibitor concentration. Binding of the sulfhydryl groups of the enzyme prevents its alkylation by N-ethylmaleimide and inhibition by oxaloacetate. A kinetic analysis of the inactivation of alkylating reagent in the presence of succinate or malonate suggests that N-ethylmaleimide acts as a site-directed inhibitor. The apparent first-order rate constant of alkylation increases between pH 5.8 and 7.8 indicating a pKa value for the enzyme sulfhydryl group equal to 7.0 at 22 degrees C in 50 mM Tris-sufate buffer. Certain anions (phosphate, citrate, maleate and acetate) decrease the reactivity of the enzyme towards the alkylating reagent. Succinate/phenazine methosulfate reductase activity measured in the presence of a saturating concentration of succinate shows the same pH-dependence as the alkylation rate by N-ethylmaleimide. The mechanism of the first step of succinate oxidation, including a nucleophilic attack of substrate by the active-site sulfhydryl group, is discussed.