What is the difference between krebs cycle and oxidative phosphorylation

Activated DCs and macrophages are characterized by a truncated TCA cycle that causes citrate levels to accumulate. An increased ACLY activity mediated histone acetylation and transcriptionally induced a subset of genes associated with cellular proliferation and production of chemokines These results indicate that a tight coordinated regulation and context-dependent metabolic control of histone acetylation is necessary to drive specific macrophages states.

In embryonic stem cell differentiation, as the cells acquire a more quiescent phenotype the levels of acetyl-CoA and histone acetylation decrease Additionally, mitochondrial respiratory inhibition in hematopoietic stem cells decreases histone acetylation, which is associated with a block in cellular differentiation Overall, these studies highlight that acetyl-CoA is not just a passive acetyl group donor but rather an important signaling molecule involved in the regulation of the activity of specific transcription factors and histone marks that ultimately dictate cellular functions by regulating gene expression.

In humans, 2-OGDD play a key role in physiologically important processes such as responses to hypoxia and chromatin modifications. Thus, we propose that it is the accumulation of inhibitors of 2-OGDD reactions, i. In normoxia, ROS released from mitochondria and accumulated levels of the metabolites succinate, fumarate and LHG can inhibit the activity of PHDs causing a pseudohypoxia state.

Histone methylation on single lysine K amino acid residues can activate or repress transcription depending on the particular residue and the level of methylation. DNA methylation modifies the chromatic structures and typically decreases gene expression by changing the structure of a single nucleotide. L- or DHG abundance is limited in normal tissues however they can reach to millimolar concentrations under certain pathologic conditions 40 , The isomer DHG is accumulated as a consequence of gain-of-function mutations in the cytosolic and mitochondrial isoforms of IDH 42 , Interestingly, IDH1 and IDH2 are the most frequently mutated metabolic genes in human cancer and are found in multiple tumor types, including gliomas, acute myelogenous leukemias AMLs , and myelodysplastic syndromes MDS 42 , 43 , 44 , The observation of DHG accumulation in these tumors prompted the community to use the term oncometabolite for the first time.

Additionally, DHG can be produced by the promiscuous activity of phosphoglycerate dehydrogenase PHGDH , an enzyme frequently overexpressed in cancer. In normal conditions, LDH catalyzes the interconversion of lactate and pyruvate. The ability of cells to increase LHG in hypoxic conditions to regulate histone methylation levels, including H3K9me3 and to reduce cellular reductive stress by inhibiting key metabolic pathways indicates an important physiological role of LHG 48 , This mechanism has been observed to specifically repress immunosuppressive genes when mitochondrial complex III is impaired.

Pathways for 2-HG removal are evolutionarily conserved. A deficiency in either of these two enzymes caused by germline transmission of homozygous mutations can lead to a disease known as 2-hydroxyglutaric acidurias 2-HGA.

DHGA is a rare disease, with symptoms, including macrocephaly, cardiomyopathy, mental retardation, hypotonia, and cortical blindness. LHGA is a rare neurodegenerative disorder that causes hypotonia, tremors, epilepsy, mental retardation, and psychomotor regression. Notably, it has been reported that children with LHGA developed medulloblastoma and glioblastoma multiforme, as well as Wilms tumor 53 , Moreover, increased LHG levels resulting from reduced expression of LHGDH were observed in renal cancer, which suggest a potential tumorigenic effect for this isomer as well All the promiscuous reactions leading to LHG production share in common the oxidation of NADH, a driver of the reactions when it accumulates.

Thus, inhibition of mitochondrial complex III in cancer cells has been shown to increase the production of 2-HG Noteworthy, fetal HSC kept their proliferative capacity in respiration-deficient conditions but were unable to differentiate.

More recently, it has been reported that loss of mitochondrial complex III in regulatory T cells Tregs resulted in a buildup of 2-HG Tregs are key mediators of immunological tolerance and homeostasis. In this study, mice lacking functional OXPHOS specifically in Tregs developed a lethal inflammatory disorder causing mice death between the third and the fourth week of life.

Interestingly, complex III-deficient Tregs lost their suppressive capacity while their proliferation and survival remained unaffected. The transcriptomic profile of complex III-deficient Tregs compared to wild-type showed differences in the expression of immune regulatory molecules while maintaining stable FOXP3 expression, the master regulatory transcription factor of Tregs.

In both contexts, in HSCs and Tregs, accumulated 2-HG levels seem to alter the cellular function, which is associated with hypermethylation of specific histone marks and DNA methylation. Interestingly, in Tregs the loci of differentially down-regulated genes exhibited DNA methylation, suggesting a potential role for 2-HG in contributing to the autoimmune pathological phenotype of these mice.

Accumulated 2-HG due to defective mitochondrial function then has the ability to mediate cellular signaling by directly and specifically impacting the expression of immunosuppressive genes to regulate Tregs without affecting other biological outcomes of this T cells subset Fig.

This study adds more evidence on the interesting link between 2-HG levels and the regulation of cell fate in immune cells. Since the expression of 2-OGDDs likely is different among different tissues, the effects of 2-HG are likely to be context dependent.

Collectively, 2-HG, a metabolite that gets accumulated through abnormal metabolic functions, exert most of the effects through non-metabolic mechanisms. Succinate is a TCA cycle metabolite that has multiple intracellular functions, as well as organismal functions. Succinate is considered an oncometabolite that accumulates due to inactivating mutations in SDH.

Succinate accumulation has an impact in gene expression regulation and promotes tumorigenesis through two main mechanisms. Succinate is the product of 2-OGDD enzymes reactions and thus its accumulation inhibits these enzymes.

Consequently, changes in succinate have profound effects in histones and DNA methylation, changing the epigenetic landscape of the cells and gene expression Fig.

DNA hypermethylation was associated with the silencing of key genes involved in neuroendocrine differentiation favoring malignancy. Further studies will determine if analogously to the effects of acetyl-CoA fluctuations in histone acetylation, particular programs get activated to promote histones or DNA methylation at specific loci instead of global changes in the presence of high succinate levels.

The excess entry of electrons in the ubiquinone pool from complex II increases the propensity of electrons to go backward to complex I when the mitochondrial membrane potential is high. This triggers the generation of superoxide production from complex I. An independent study showed that E. The inhibition of complex II by treating mice with NPA, hampered the pro-inflammatory function of macrophages. Beyond its intracellular signaling role, succinate release has been shown to signal through the G-protein-coupled receptor succinate receptor 1 SUCNR1.

The ligand for this receptor was unknown up until when He et al. Since then, multiple signaling cascades have been described upon succinate binding to the receptor in various cell types, including dendritic cells and macrophages where it seems to contribute to their functionality in driving inflammation Taken together, these studies highlight that succinate oxidation is central and a main regulator of the immune effector cells functions.

Finally, it is worth noting that recent findings describe succinate as a systemic molecule that activates thermogenesis in brown adipocytes upon cold exposure In this condition, brown adipocytes exhibited a high avidity to uptake elevated circulating succinate that was then oxidized by SDH increasing ROS levels and UCP1 activity.

It will be interesting to see if additional TCA cycles metabolites can signal systemic responses in other contexts. Accumulation of fumarate to millimolar levels due to inactivating mutations of fumarate hydratase FH is found in the genetic disorder fumaric aciduria, as well as the hereditary leiomyomatosis and renal cell cancer HLRCC 73 , in which fumarate causes hypermethylation of DNA by inhibiting TET enzymes to trigger epithelial-mesenchymal transition EMT Fumarate is an electrophile and can cause succination of proteins, a process where fumarate binds and inactivates reactive thiol protein cysteine residues.

Other studies have shown that high levels of fumarate increase ROS signaling by binding glutathione 83 , Succination following fumarate accumulation has also been shown to cause the loss of the mitochondrial aconitase ACO2 activity, which is crucial for iron—sulfur cluster binding Finally, the iron-responsive element binding protein 2 IRP2 that promotes the transcription of transferrin and the family of iron—sulfur Fe-SA cluster biogenesis proteins have also been described to be succinated when fumarate accumulates in cancer cells Fumarate also functions as an immunomodulator by controlling chromatin modifications Fig.

Specifically, the accumulation of fumarate through glutamine anaplerosis in response to pro-inflammatory insults has been shown to be necessary for trained immunity and inflammation by inhibiting KDM5 histone demethylase activity Fumarate has also been shown to accumulate in LPS-activated macrophages 65 , Furthermore, fumarate derivatives like dimethyl fumarate DMF , a potent electrophile, can regulate T-cell functions DMF is currently being used in the clinic to treat autoimmune conditions, including multiple sclerosis MS and psoriasis.

Using an unbiased proteomic approach, Blewett et al. Endogenous fumarate was also found to inhibit GAPDH through succination in mouse and human macrophages. These results suggest that DMF modulate the activity of immune cells by negatively regulating glycolysis, a metabolic pathway required for the activation and proliferation of different subsets of T cells Itaconate is derived from the decarboxylation of the TCA cycle intermediate cis-aconitate. Accumulation of itaconate occurs in the lungs of mice infected with Mycobacterium tuberculosis 92 and in macrophages activated with LPS The immune-responsive gene 1 protein IRG1 is the enzyme responsible for itaconate production.

IRG1 was consequently renamed as cis-aconitate decarboxylase after this discovery. IRG1 loss of function in macrophages infected with Salmonella enterica had reduced aconitate levels and decreased anti-bactericidal activity The anti-bactericidal properties of itaconate are derived from its ability to inhibit isocitrate lyase, a key enzyme of the glyoxylate shunt, a pathway required for the survival of many parasites especially under poor glucose conditions.

Initially, itaconate was shown to inhibit SDH thus limiting succinate oxidation and this was assumed to be the primary mechanism by which itaconate limits inflammation 95 , Recent studies indicate that itaconate activates pathways downstream of the antioxidant transcription factor NRF2, as well as NRF2 independent mechanisms that contribute to its anti-inflammatory properties in activated macrophages.

Interestingly, derivatives of itaconate such as octyl-itaconate or dimethyl-itaconate, which are strong electrophiles, diminish pro-inflammatory cytokine production in vitro and in vivo 97 , These results significantly extended the knowledge of itaconate biology and raised the possibility of using itaconate derivatives as a therapeutic agent in autoimmune diseases.

However, since high levels of itaconate might cause a B12 deficiency, the safety of long-term exposures to itaconate needs to be evaluated In the past two decades, mitochondrial biology has undergone a renaissance partly due to the appreciation that mitochondria have important biological functions beyond ATP and macromolecules production.

Indeed, mitochondria have evolved from passive to active players in determining cell fate and function. Mechanistically, TCA cycle metabolites have been demonstrated to control transcription factors and chromatin modifications to change cell function and fate. However, in many contexts the molecular details of how changes in TCA cycle metabolites abundance affect the expression of specific genes remains to be elucidated.

Future investigations might also discover additional mechanisms by which TCA cycle metabolites exert signaling functions beyond post-translational modifications. Emerging evidence indicates that beyond cell autonomous functions, TCA cycle metabolites control physiology through non-cell autonomous functions.

Discovering systemic effects of metabolites and their role in communicating different parts of the body will be of much interest for the field in the upcoming years. A fascinating development is the use of derivatives of TCA cycle metabolites to ameliorate inflammatory diseases in humans. We hope to see more of the recent findings of TCA cycle signaling effects being translated into the clinic. Going forward, we predict that TCA cycle metabolites will continue to shed new light on biology, physiology, and diseases.

Chandel, N. Evolution of mitochondria as signaling organelles. Cell Metab. Liu, X. Induction of apoptotic program in cell-free extracts: requirement for dATP and cytochrome c.

Cell 86 , — Mitochondrial reactive oxygen species trigger hypoxia-induced transcription. Natl Acad. USA 95 , — Herzig, S. AMPK: guardian of metabolism and mitochondrial homeostasis. Cell Biol. West, A. Mitochondrial DNA in innate immune responses and inflammatory pathology. DeBerardinis, R. The electron transport chain then generates additional ATPs by oxidative phosphorylation. The citric acid cycle also produces 2 ATP by substrate phosphorylation and plays an important role in the flow of carbon through the cell by supplying precursor metabolites for various biosynthetic pathways.

The citric acid cycle involves 8 distinct steps, each catalyzed by a unique enzyme. You are not responsible for knowing the chemical structures or enzymes involved in the steps below. They are included to help illustrate how the molecules in the pathway are manipulated by the enzymes in order to to achieve the required products. The mitochondria would be unable to generate new ATP in this way, and the cell would ultimately die from lack of energy.

This is the reason we must breathe to draw in new oxygen. In the electron transport chain, the free energy from the series of reactions just described is used to pump hydrogen ions across the membrane. Hydrogen ions diffuse through the inner membrane through an integral membrane protein called ATP synthase Figure 4.

This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient from the intermembrane space, where there are many mutually repelling hydrogen ions to the matrix, where there are few. This flow of hydrogen ions across the membrane through ATP synthase is called chemiosmosis. Chemiosmosis Figure 4. The result of the reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms.

These atoms were originally part of a glucose molecule. At the end of the electron transport system, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen ions attract hydrogen ions protons from the surrounding medium, and water is formed.

The electron transport chain and the production of ATP through chemiosmosis are collectively called oxidative phosphorylation. The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species. Another source of variance stems from the shuttle of electrons across the mitochondrial membrane.

The NADH generated from glycolysis cannot easily enter mitochondria. Another factor that affects the yield of ATP molecules generated from glucose is that intermediate compounds in these pathways are used for other purposes. Glucose catabolism connects with the pathways that build or break down all other biochemical compounds in cells, and the result is somewhat messier than the ideal situations described thus far.

For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction. However, the kinetics of the internal processes has been modelled using numerical tools. We also show that the Krebs cycle and oxidative phosphorylation together can be combined in a similar fashion - a black box with a few inputs and outputs. The Octave script is flexible and customisable for any chosen set-up for this model.