1 Introduction

The citric acid cycle, also known as the tricarboxylic acid (TCA) cycle or Krebs cycle, is a fundamental metabolic pathway that plays a crucial role in cellular respiration (Guo et al., 2022). Discovered by Hans Krebs in 1937, this cycle involves a series of enzyme-catalyzed chemical reactions that are pivotal for the aerobic oxidation of fuel molecules. Krebs identified that citrate is formed from oxaloacetate and acetyl-CoA, leading to the realization that this pathway is a cyclic sequence of reactions essential for energy production in aerobic organisms (Bodner, 1986; Choi et al., 2020). The TCA cycle is distinct from glycolysis in that it is a cyclic rather than a linear pathway, and it occurs within the mitochondria, tightly coupled with the electron transport chain and oxidative phosphorylation.

The citric acid cycle is central to energy metabolism, facilitating the controlled combustion of carbohydrates, fats, and proteins into carbon dioxide and water, while generating high-energy electron carriers NADH and FADH2. These carriers subsequently donate electrons to the electron transport chain, driving the production of ATP, the primary energy currency of the cell (Fernie et al., 2004; MacLean et al., 2023). Beyond its role in energy production, the TCA cycle is involved in various biosynthetic processes, providing precursors for amino acids, nucleotide bases, and other essential biomolecules (Iñigo et al., 2021). The cycle's intermediates also play significant roles in cellular signaling and regulation, influencing processes such as mitophagy, cellular stress responses, and metabolic reprogramming in response to environmental changes (Mccammon et al., 2003; Franco and Serrano-Marín, 2022; MacLean et al., 2023).

This study elucidates the central role of the citric acid cycle in energy metabolism, with a focus on its metabolic intermediates and regulatory mechanisms. By exploring the complex network of reactions and their regulation, the study aims to provide a comprehensive understanding of how the TCA cycle integrates with other metabolic pathways to maintain cellular homeostasis and respond to physiological demands. The study seeks to uncover new insights into the regulation of the TCA cycle by various factors, including enzyme activity, substrate availability, and cellular signaling pathways. This study may have significant implications for understanding metabolic diseases, developing therapeutic strategies, and advancing applications in metabolic engineering.

2 Metabolic Intermediates of the Citric Acid Cycle

2.1 Analysis of key intermediate

Citrate and isocitrate are key intermediates in the citric acid cycle, each with distinct structural characteristics that facilitate their roles in metabolism. Citrate, a tricarboxylate, is formed by the condensation of acetyl-CoA and oxaloacetate, catalyzed by citrate synthase (Granchi et al., 2019). This reaction releases CoA-SH and heat, producing citrate as the first intermediate of the cycle (Kumari, 2018). Citrate is then isomerized to isocitrate through a two-step process involving dehydration and rehydration, catalyzed by the enzyme aconitase, with cis-aconitate as an intermediate. The structural transformation from citrate to isocitrate is crucial for the subsequent steps of the cycle, as it prepares the molecule for oxidative decarboxylation (Kumari, 2018; Kynshi et al., 2021).

α-Ketoglutarate plays a pivotal role in the citric acid cycle, acting as a key intermediate in the oxidative decarboxylation process. It is formed from isocitrate through the action of isocitrate dehydrogenase, which first dehydrogenates isocitrate to oxalosuccinate and then decarboxylates it to α-ketoglutarate (Kumari, 2018). This intermediate is further oxidatively decarboxylated by the α-ketoglutarate dehydrogenase complex to form succinyl-CoA, a reaction that is unidirectional and crucial for the continuation of the cycle. Other intermediates, such as succinate, fumarate, and malate, also play significant roles in the cycle, contributing to the regeneration of oxaloacetate and the production of reducing equivalents for ATP synthesis (Patil et al., 2019; Sauer et al., 2020).

2.2 Role in energy production and biosynthesis

The citric acid cycle is central to cellular energy production, primarily through the generation of high-energy electron carriers NADH and FADH2. These carriers donate electrons to the electron transport chain, driving the production of ATP through oxidative phosphorylation (Guo et al., 2022). The cycle itself directly produces a small amount of ATP (or GTP) via substrate-level phosphorylation during the conversion of succinyl-CoA to succinate (Kumari, 2018). The intermediates of the cycle, therefore, play a crucial role in maintaining the flow of electrons and the production of ATP, which is essential for cellular energy homeostasis.

Beyond energy production, citric acid cycle intermediates serve as precursors for various biosynthetic pathways. For instance, α-ketoglutarate is a key precursor for the synthesis of amino acids such as glutamate, which can be further converted into other amino acids and neurotransmitters (Kumari, 2018). Citrate can be exported to the cytoplasm, where it is cleaved by ATP-citrate lyase to generate acetyl-CoA and oxaloacetate, providing building blocks for fatty acid and cholesterol synthesis (Patil et al., 2019). These biosynthetic roles highlight the versatility of citric acid cycle intermediates in supporting both energy metabolism and anabolic processes.

2.3 Intermediates as metabolic hubs

Citric acid cycle intermediates act as metabolic hubs, linking various metabolic pathways. For example, citrate links the citric acid cycle with fatty acid synthesis, while α-ketoglutarate connects with amino acid metabolism  (Kumari, 2018; Patil et al., 2019). Succinate and fumarate are involved in the regulation of immune cell functions, demonstrating the cycle's integration with cellular signaling pathways (Figure 1) (Patil et al., 2019). These cross-links ensure that the cycle is not only a central pathway for energy production but also a crucial node in the broader metabolic network.

The intermediates of the citric acid cycle also play significant regulatory roles in metabolism. For instance, citrate acts as an allosteric inhibitor of phosphofructokinase, a key enzyme in glycolysis, thereby linking the regulation of glycolysis and the citric acid cycle (Petillo et al., 2020). Similarly, succinate and fumarate have been shown to regulate immune cell functions, highlighting their role in cellular signaling and regulation (Patil et al., 2019). These regulatory functions underscore the importance of citric acid cycle intermediates in maintaining metabolic balance and responding to cellular needs. By understanding the structural characteristics, functional roles, and regulatory significance of citric acid cycle intermediates, we can appreciate their central role in energy metabolism and their broader impact on cellular function and health.

3 Enzymatic Regulation of the Citric Acid Cycle

3.1 Key enzymes and their regulation

Citrate synthase catalyzes the first step of the citric acid cycle, where the acetyl group from acetyl-CoA combines with oxaloacetate to form citrate, releasing CoA-SH and heat in the process (Kumari, 2018). This enzyme plays a crucial role in regulating the entry of carbon into the cycle and is subject to various regulatory mechanisms. For instance, citrate synthase activity can be modulated by the availability of substrates and feedback inhibition by its product, citrate (Kumari, 2018; Bergé et al., 2020). Additionally, citrate synthase has been implicated in non-enzymatic roles, such as regulating the bacterial cell cycle independently of its catalytic activity (Bergé et al., 2020).

Aconitase catalyzes the isomerization of citrate to isocitrate via cis-aconitate. This enzyme is sensitive to oxidative stress and can be inactivated by reactive oxygen species, which affects its iron-sulfur cluster (Kumari, 2018; Chen et al., 2020). Isocitrate dehydrogenase (IDH) catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate, producing NADH in the process. IDH is regulated by allosteric effectors and phosphorylation. For example, NADH and ATP act as inhibitors, while ADP and NAD+ serve as activators (Kumari, 2018; Chen et al., 2020; Igamberdiev, 2020).

3.2 Feedback inhibition and allosteric regulation

Feedback inhibition is a critical regulatory mechanism in the citric acid cycle. Citrate, the product of the reaction catalyzed by citrate synthase, inhibits the enzyme to prevent excessive accumulation of citrate and to balance the cycle's flux (Kumari, 2018; Bergé et al., 2020). Similarly, NADH, a product of several dehydrogenase reactions within the cycle, inhibits enzymes like isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase, ensuring that the cycle's activity is matched to the cell's energy needs (Kumari, 2018; Igamberdiev, 2020).

Allosteric regulation also plays a significant role in modulating the activity of citric acid cycle enzymes. For instance, isocitrate dehydrogenase is activated by ADP, which enhances its affinity for substrates, and inhibited by ATP and NADH, which signal sufficient energy levels in the cell (Kumari, 2018; Chen et al., 2020; Igamberdiev, 2020). This ensures that the cycle operates efficiently under varying metabolic conditions. Additionally, aconitase activity can be modulated by the redox state of the cell, linking its function to the overall oxidative stress response.

3.3 Impact of post-translational modifications on enzyme activity

Phosphorylation is a common post-translational modification that affects the activity of citric acid cycle enzymes. For example, isocitrate dehydrogenase can be phosphorylated, leading to its inactivation. This modification is reversible and allows for rapid regulation of the enzyme in response to cellular energy status (Kumari, 2018; Igamberdiev, 2020). The phosphorylation state of these enzymes is tightly controlled by specific kinases and phosphatases, which respond to various metabolic signals (Igamberdiev, 2020).

Beyond phosphorylation, other post-translational modifications such as acetylation, succinylation, and ubiquitination can also influence the activity of citric acid cycle enzymes. These modifications can alter enzyme stability, localization, and interaction with other proteins, thereby fine-tuning the cycle's function (Patil et al., 2019; Igamberdiev, 2020). For instance, succinylation of metabolic enzymes has been shown to affect their activity and is linked to the regulation of metabolic fluxes in response to nutrient availability.

4 Integration of the Citric Acid Cycle with Other Metabolic Pathways

4.1 Connection with glycolysis and gluconeogenesis

Pyruvate serves as a crucial metabolite at the intersection of glycolysis and the citric acid cycle. It is produced in the cytosol through glycolysis and can be directed towards several metabolic fates. One primary pathway involves its conversion to acetyl-CoA by the pyruvate dehydrogenase complex, which then enters the citric acid cycle to facilitate energy production through oxidative phosphorylation (Zangari et al., 2020; Prochownik and Wang, 2021). Additionally, pyruvate can be carboxylated to oxaloacetate by pyruvate carboxylase, an anaplerotic reaction that replenishes citric acid cycle intermediates and supports gluconeogenesis (Hughey and Crawford, 2019; Roosterman and Cottrell, 2021).

The interplay between glycolysis and the citric acid cycle is tightly regulated to ensure metabolic flexibility and energy homeostasis. Glycolysis generates pyruvate, which can either be converted to lactate under anaerobic conditions or transported into mitochondria for further oxidation. The mitochondrial pyruvate carrier (MPC) is essential for pyruvate entry into the mitochondria, linking glycolysis to the citric acid cycle (Zangari et al., 2020). Furthermore, the regulation of key glycolytic enzymes and the pyruvate dehydrogenase complex ensures a balanced flow of carbon substrates between these pathways, adapting to cellular energy demands and metabolic states (Matschinsky and Wilson, 2019; Li et al., 2023).

4.2 Role in amino acid metabolism

The citric acid cycle is integral to amino acid metabolism through transamination reactions, where amino groups are transferred from amino acids to α-keto acids. This process is crucial for the synthesis and degradation of amino acids. For instance, the transamination of glutamate to α-ketoglutarate, a citric acid cycle intermediate, exemplifies the direct connection between amino acid metabolism and the citric acid cycle (Hughey and Crawford, 2019; Prochownik and Wang, 2021).

Amino acids can be converted into various intermediates of the citric acid cycle, facilitating their catabolism and integration into central energy metabolism. For example, alanine can be transaminated to pyruvate, which then enters the citric acid cycle as acetyl-CoA (Prochownik and Wang, 2021). Similarly, other amino acids such as aspartate and glutamate can be converted into oxaloacetate and α-ketoglutarate, respectively, feeding directly into the cycle and supporting its continuous operation.

4.3 Link with fatty acid oxidation

Fatty acid oxidation, or beta-oxidation, generates acetyl-CoA, which is a critical substrate for the citric acid cycle. This process occurs in the mitochondria, where fatty acids are broken down into acetyl-CoA units that enter the citric acid cycle, contributing to ATP production through oxidative phosphorylation. The integration of fatty acid oxidation with the citric acid cycle is essential for maintaining energy balance, especially during periods of low carbohydrate availability (Khan et al., 2022).

Citrate plays a dual role in metabolism by not only participating in the citric acid cycle but also serving as a shuttle for fatty acid synthesis. When energy levels are high, citrate is exported from the mitochondria to the cytosol, where it is cleaved by ATP-citrate lyase to generate acetyl-CoA and oxaloacetate. The acetyl-CoA produced in this manner is a precursor for fatty acid synthesis, linking the citric acid cycle to lipid biosynthesis and storage (Roosterman and Cottrell, 2021; Khan et al., 2022).

By integrating with glycolysis, amino acid metabolism, and fatty acid oxidation, the citric acid cycle serves as a central hub in cellular energy metabolism, ensuring a coordinated and efficient flow of metabolic intermediates and energy production.

5 Citric Acid Cycle and Cellular Energy Homeostasis

5.1 Role in ATP production

The citric acid cycle, also known as the tricarboxylic acid (TCA) cycle or Krebs cycle, is a central metabolic pathway that plays a crucial role in cellular energy production. It is responsible for the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide and water, while generating high-energy electron carriers NADH and FADH2 (Figure 2). These carriers subsequently donate electrons to the oxidative phosphorylation pathway, leading to the production of ATP, the primary energy currency of the cell (Guo et al., 2022). The efficient conversion of metabolic intermediates into ATP through the TCA is essential for maintaining cellular energy homeostasis and supporting various cellular functions (Matschinsky and Wilson, 2019).

5.2 Contribution to redox balance

The TCA is integral to maintaining cellular redox balance by regulating the levels of NADH and NADPH, which are crucial for various anabolic and catabolic processes. NADH, produced during the TCA, is a key electron donor in the mitochondrial electron transport chain, facilitating ATP production while also generating reactive oxygen species (ROS) as by-products. To mitigate the potential damage caused by ROS, cells employ antioxidant systems that rely on NADPH, another critical cofactor produced through metabolic pathways interconnected with the TCA (Shimizu and Matsuoka, 2019). The balance between NADH and NADPH is vital for cellular redox homeostasis, enabling cells to adapt to oxidative stress and maintain metabolic flexibility (Cho et al., 2020; Selinski and Scheibe, 2020).

5.3 Impact on cellular response to energy stress

The TCA also plays a pivotal role in the cellular response to energy stress. Under conditions of energy deficiency, such as during intense physical activity or caloric restriction, the TCA can adjust its activity to optimize energy production and support cellular survival. For instance, the transcriptional coactivator PGC-1α is activated in response to energy stress and enhances the expression of genes involved in mitochondrial biogenesis and oxidative metabolism, thereby boosting the capacity for ATP production and ROS detoxification (Shelbayeh et al., 2023).

Additionally, metabolic intermediates of the TCA, such as succinate and fumarate, can act as signaling molecules that modulate cellular responses to stress, including inflammation and immune function (Patil et al., 2019). These adaptive mechanisms underscore the central role of the TCA in maintaining cellular energy homeostasis and resilience under varying metabolic conditions (Cani et al., 2019; Ghosh-Choudhary et al., 2020).

6 Citric Acid Cycle in Different Physiological and Pathological Conditions

6.1 Adaptation of the citric acid cycle during exercise

During exercise, the citric acid cycle (TCA) adapts to meet the increased energy demands of the body. A study on hepatic metabolism during exercise revealed that there is a pronounced hepatic uptake of lactate, pyruvate, various amino acids, and dicarboxylic acids, indicating a high demand for gluconeogenic substrates and an increase in anaplerotic reactions of the TCA. This adaptation ensures a continuous supply of energy substrates to support prolonged physical activity (Plomgaard et al., 2018). Additionally, medium-chain fatty acids such as caproic acid are taken up by the liver, highlighting their role in regulating gluconeogenesis and mitochondrial substrate oxidation during exercise.

6.2 Changes in the citric acid cycle in metabolic disorders

Metabolic disorders can significantly alter the function and regulation of the TCA. For instance, genetic variants in the TCA have been linked to colorectal cancer susceptibility. A study identified significant interactions between single nucleotide polymorphisms (SNPs) in the TCA and factors such as obesity, energy intake, and physical activity, which influence the risk of colorectal cancer. These findings suggest that individual differences in energy metabolism, influenced by genetic variations in the TCA, can contribute to the development of metabolic disorders and associated diseases (Cho et al., 2020). Furthermore, in veterans with Gulf War illness, relationships among TCA markers were found to shift, indicating possible alterations in bioenergetic pathways in this condition (Golomb et al., 2021).

6.3 Role in aging and neurodegenerative diseases

The TCA also plays a crucial role in aging and neurodegenerative diseases. As organisms age, the efficiency of the TCA can decline, leading to reduced energy production and increased oxidative stress. This decline is associated with various age-related diseases, including neurodegenerative disorders. The regulation of leukocyte function by TCA intermediates such as succinate, itaconate, citrate, and fumarate has been shown to mediate important cellular functions during infection and inflammation, which are processes often dysregulated in aging and neurodegenerative diseases (Patil et al., 2019). Additionally, the gut microbiome, which influences host metabolism through the production of metabolites that interact with the TCA, has been implicated in the regulation of systemic energy expenditure and may play a role in the aging process and the development of neurodegenerative diseases (Cani et al., 2019).

The citric acid cycle is a central metabolic pathway that adapts to various physiological conditions such as exercise, and its dysregulation is implicated in metabolic disorders, aging, and neurodegenerative diseases. Understanding these adaptations and changes can provide insights into the development of targeted therapies for these conditions.

7 Case Studies

7.1 Citric acid cycle dysfunction in mitochondrial diseases

Mitochondrial diseases often involve dysfunctions in the citric acid cycle (TCA), which is crucial for cellular energy production. Dysregulation in the TCA can lead to impaired energy metabolism, contributing to the pathophysiology of various mitochondrial disorders. For instance, mutations in genes encoding TCA enzymes can disrupt the cycle, leading to reduced ATP production and increased oxidative stress. This is evident in conditions such as mitochondrial myopathies and neurodegenerative diseases, where energy-demanding tissues like muscles and the brain are predominantly affected (Dai and Jiang, 2019; Luo et al., 2020).

The study found that key mitochondrial enzymes and mitochondrial DNA (mtDNA) mutations drive cancer development by disrupting the TCA cycle and oxidative phosphorylation. For example, mutations in the IDH1/2 genes lead to the accumulation of oncogenic metabolites, which interfere with normal cellular metabolism and promote tumor growth in gliomas and leukemia (Figure 3) (Luo et al., 2020). Additionally, mutations in the SDH and FH enzymes cause abnormal accumulation of metabolites, stabilizing HIF1α and creating a pseudo-hypoxic environment conducive to cancer progression. Furthermore, mitochondrial DNA mutations and defects in the electron transport chain exacerbate these dysfunctions, underscoring the central role of the TCA cycle in maintaining cellular energy homeostasis (Dai and Jiang, 2019; Luo et al., 2020).

7.2 Analysis of the association between citric acid cycle genes and colorectal cancer

Colorectal cancer is a common malignant tumor worldwide, and energy metabolism plays a crucial role in its development. The citric acid cycle is central to cellular energy metabolism, and the polymorphisms in genes involved in this cycle may be associated with susceptibility to colorectal cancer. Cho et al. (2020) evaluated the association between the polymorphisms of citric acid cycle-related genes and the risk of colorectal cancer, as well as the interaction between these gene polymorphisms and energy balance factors such as obesity, physical activity, and energy intake. The researchers conducted a nested case-control study, selecting 3 523 colorectal cancer cases and 10 522 matched controls from the UK Biobank. They used conditional logistic regression models to assess the relationship between citric acid cycle gene polymorphisms and colorectal cancer risk. The study found that the rs35494829 locus in the SUCLG2 gene was significantly associated with the risk of colorectal cancer, particularly colon cancer. Additionally, significant interactions were observed between citric acid cycle genes and factors such as obesity, energy intake, and vigorous physical activity.

The close association between citric acid cycle gene polymorphisms and colorectal cancer risk, along with their interaction with energy balance factors, may provide new insights into the bioenergetic mechanisms of colorectal cancer (Yu et al., 2019; Cho et al., 2020). The findings offer important evidence for understanding the relationship between energy metabolism and the development of colorectal cancer, contributing to the formulation of personalized cancer prevention strategies.

7.3 Comparative analysis of citric acid cycle activity in different tissue types

The activity of the citric acid cycle varies significantly across different tissue types, reflecting their distinct metabolic demands and functions. For instance, highly oxidative tissues such as the heart and skeletal muscles exhibit robust TCA activity to meet their high energy requirements. In contrast, tissues with lower energy demands, such as adipose tissue, show relatively lower TCA activity (Zhelev et al., 2022).

This differential activity is also evident in pathological conditions. In cancerous tissues, the TCA is often reprogrammed to support anabolic processes and rapid cell division, whereas in metabolic diseases like obesity and diabetes, dysregulated TCA activity can contribute to altered energy homeostasis and metabolic dysfunction (Cho et al., 2020; Luo et al., 2020; Kim et al., 2022). Understanding these variations is crucial for developing targeted therapeutic strategies that address tissue-specific metabolic needs and dysfunctions.

8 Therapeutic Interventions Targeting the Citric Acid Cycle

8.1 Potential drug targets within the citric acid cycle

The citric acid cycle (TCA) is a central hub in cellular metabolism, making it an attractive target for therapeutic interventions. Several enzymes within the TCA have been identified as potential drug targets. For instance, succinate dehydrogenase and fumarate reductase are key enzymes that link the TCA to the respiratory chain, and inhibitors targeting these enzymes could disrupt both metabolic and respiratory processes in pathogens like Mycobacterium tuberculosis (Hards et al., 2019).

Additionally, enzymes such as isocitrate dehydrogenase, pyruvate dehydrogenase kinase, and α-ketoglutarate dehydrogenase have been identified as promising targets in cancer therapy, with agents like CPI-613 showing potential in clinical trials (Neitzel et al., 2020). The regulation of carnitine palmitoyltransferase I (CPT1A), a key enzyme in fatty acid oxidation, also presents a therapeutic opportunity, particularly in metabolic disorders and cancers (Schlaepfer and Joshi, 2020).

8.2 Nutritional interventions to modulate cycle activity

Nutritional interventions can significantly influence the activity of the citric acid cycle. For example, dietary components that affect the balance of glucose metabolism can modulate the TCA. Cinnamaldehyde, a compound found in cinnamon, has been shown to enhance the TCA by targeting α-enolase, thereby improving mitochondrial efficiency and reducing blood glucose levels (Zhang et al., 2020).

Furthermore, physical activity and a healthy diet have been recommended for the prevention of colorectal cancer, with genetic variants in the TCA interacting with obesity, energy intake, and physical activity to influence cancer risk (Cho et al., 2020). These findings suggest that lifestyle and dietary modifications can be effective strategies for modulating TCA activity and improving metabolic health.

8.3 Experimental therapies and clinical trials

Several experimental therapies and clinical trials are exploring the potential of targeting the citric acid cycle for therapeutic benefits. In cancer research, the reprogramming of cellular energy metabolism is a key focus, with the TCA playing a central role. Small molecule inhibitors targeting enzymes of the TCA, such as isocitrate dehydrogenase and pyruvate dehydrogenase kinase, are currently being tested in clinical trials for their effiTCAy in treating colorectal cancer (Neitzel et al., 2020). Additionally, the use of metabolic engineering to manipulate TCA flux has shown promise in enhancing the production of desired metabolites, which could have applications in both therapeutic and industrial settings (Kumar and Dubey, 2019). The regulatory functions of TCA intermediates, such as succinate and itaconate, in immune cell function also present potential translational applications for treating infections and inflammatory diseases (Patil et al., 2019).

The citric acid cycle offers multiple avenues for therapeutic intervention, from drug targeting of key enzymes to nutritional and lifestyle modifications, as well as experimental therapies currently under investigation. These strategies highlight the central role of the TCA in energy metabolism and its potential for improving health outcomes across various diseases.

9 Concluding Remarks

The citric acid cycle (TCA), also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a cornerstone of cellular energy metabolism. It serves as a central hub connecting various metabolic pathways, facilitating the conversion of carbohydrates, fats, and proteins into energy. The cycle begins with the reaction between oxaloacetate and acetyl-CoA, leading to the production of key intermediates that are crucial for both energy generation and biosynthesis of essential biomolecules. The TCA not only generates ATP through oxidative phosphorylation but also produces NADH and FADH2, which are vital for the electron transport chain. Additionally, intermediates of the TCA play significant roles in regulating immune responses and cellular functions, highlighting its multifaceted importance in cell metabolism.

Future research should focus on the intricate regulatory mechanisms of the TCA and its broader implications in health and disease. One promising area is the manipulation of TCA flux through metabolic engineering to enhance the production of desired metabolites, which could have significant industrial and therapeutic applications. Additionally, understanding the genetic variants of TCA enzymes and their interactions with lifestyle factors such as diet and physical activity could provide insights into disease susceptibility, particularly in conditions like colorectal cancer. Another critical avenue is exploring the non-enzymatic roles of TCA enzymes, such as citrate synthase, in cell cycle regulation and development, which could uncover novel therapeutic targets for bacterial infections and cancer.

The regulation of the citric acid cycle is paramount for maintaining cellular energy homeostasis and supporting various physiological functions. The cycle's ability to integrate and respond to cellular energy demands underscores its central role in metabolism. Moreover, the emerging understanding of TCA intermediates in regulating immune responses and cell cycle progression highlights the complexity and versatility of metabolic regulation. As research continues to unravel the multifaceted roles of the TCA, it becomes increasingly clear that metabolic regulation is not just about energy production but also about maintaining cellular and organismal health. The ongoing exploration of these regulatory mechanisms holds great promise for developing innovative strategies to treat metabolic disorders and improve overall health.

Acknowledgments

Authors extend sincere gratitude to the two anonymous peer reviewers for their contributions during the evaluation of this manuscript.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

References

Bergé M., Pezzatti J., González-Ruiz V., Degeorges L., Mottet-Osman G., Rudaz S., and Viollier P., 2020, Bacterial cell cycle control by citrate synthase independent of enzymatic activity, eLife, 9: e52272.

https://doi.org/10.7554/eLife.52272

Bodner G., 1986, Metabolism: Part II. the tricarboxylic acid (TCA), citric acid, or krebs cycle, Journal of Chemical Education, 63: 673.

https://doi.org/10.1021/ED063P673

Cani P., Hul M., Lefort C., Depommier C., Rastelli M., and Everard A., 2019, Microbial regulation of organismal energy homeostasis, Nature Metabolism, 1: 34-46.

https://doi.org/10.1038/s42255-018-0017-4

Chen Y., Cai G., Xia B., Wang X., Zhang C., Xie B., Shi X., Liu H., Lu J., Zhang R., Zhu M., Liu M., Yang S., Zhang D., Chu X., Khan R., Wang Y., and Wu J., 2020, Mitochondrial aconitase controls adipogenesis through mediation of cellular ATP production, The FASEB Journal, 34: 6688-6702.

https://doi.org/10.1096/fj.201903224RR

Cho S., Song N., Choi J., and Shin A., 2020, Effect of citric acid cycle genetic variants and their interactions with obesity, physical activity and energy intake on the risk of colorectal cancer: results from a nested case-control study in the UK Biobank, Cancers, 12(10): 2939.

https://doi.org/10.3390/cancers12102939

Choi I., Son H., and Baek J., 2020, Tricarboxylic acid (TCA) cycle intermediates: regulators of immune responses, Life, 11(1): 69.

https://doi.org/10.3390/life11010069

Dai W., and Jiang L., 2019, Dysregulated mitochondrial dynamics and metabolism in obesity, diabetes, and cancer, Frontiers in Endocrinology, 10: 570.

https://doi.org/10.3389/fendo.2019.00570

Fernie A., Carrari F., and Sweetlove L., 2004, Respiratory metabolism: glycolysis, the TCA cycle and mitochondrial electron transport, Current Opinion in Plant Biology, 7(3): 254-261.

https://doi.org/10.1016/J.PBI.2004.03.007

Franco R., and Serrano-Marín J., 2022, The unbroken Krebs cycle. Hormonal‐like regulation and mitochondrial signaling to control mitophagy and prevent cell death, BioEssays, 45(3): 2200194.

https://doi.org/10.1002/bies.202200194

Ghosh-Choudhary S., Liu J., and Finkel T., 2020, Metabolic regulation of cell fate and function, Trends in Cell Biology, 30(3): 201-212.

https://doi.org/10.1016/j.tcb.2019.12.005

Golomb B., Koslik H., Han J., Guida A., Hamilton G., snd Kelley R., 2021, A pilot study of bioenergetic marker relationships in gulf war illness: phosphocreatine recovery vs. citric acid cycle intermediates, International Journal of Environmental Research and Public Health, 18(4): 1635.

https://doi.org/10.3390/ijerph18041635

Granchi D., Baldini N., Ulivieri F., and Caudarella R., 2019, Role of citrate in pathophysiology and medical management of bone diseases, Nutrients, 11(11): 2576.

https://doi.org/10.3390/nu11112576

Guo D., He H., Meng Y., Luo S., and Lu Z., 2022, Determiners of cell fates: the tricarboxylic acid cycle versus the citrate-malate shuttle, Protein & Cell, 14: 162-164.

https://doi.org/10.1093/procel/pwac026

Hards K., Adolph C., Harold L., McNeil M., Cheung C., Jinich A., Rhee K., and Cook G., 2019, Two for the price of one: attacking the energetic-metabolic hub of mycobacteria to produce new chemotherapeutic agents, Progress in Biophysics and Molecular Biology, 152: 35-44.

https://doi.org/10.1016/j.pbiomolbio.2019.11.003

Hughey C., and Crawford P., 2019, Pyruvate carboxylase wields a double-edged metabolic sword, Cell Metabolism, 29(6): 1236-1238.

https://doi.org/10.1016/j.cmet.2019.05.013

Igamberdiev A., 2020, Citrate valve integrates mitochondria into photosynthetic metabolism, Mitochondrion, 52: 218-230.

https://doi.org/10.1016/j.mito.2020.04.003

Iñigo M., Deja S., and Burgess S., 2021, Ins and outs of the TCA cycle: the central role of anaplerosis, Annual Review of Nutrition, 41: 19-47.

https://doi.org/10.1146/annurev-nutr-120420-025558

Khan A., Salehi H., Alexia C., Valdivielso J., Bozic M., Lopez-Mejia I., Fajas L., Gerbal-Chaloin S., Daujat-Chavanieu M., Gitenay D., and Villalba M., 2022, Glucose starvation or pyruvate dehydrogenase activation induce a broad, ERK5-mediated, metabolic remodeling leading to fatty acid oxidation, Cells, 11(9): 1392.

https://doi.org/10.3390/cells11091392

Kim Y., Lee W., Chung S., Yu B., Lee Y., and Hong J., 2022, Metabolic alterations of short-chain fatty acids and TCA cycle intermediates in human plasma from patients with gastric cancer, Life Sciences, 309: 121010.

https://doi.org/10.2139/ssrn.4195321

Kumar P., and Dubey K., 2019, Citric acid cycle regulation: back bone for secondary metabolite production, In: Gupta V.K., and Pandey A., eds., New and Future Developments in Microbial Biotechnology and Bioengineering, Elsevier, pp.165-181.

https://doi.org/10.1016/B978-0-444-63504-4.00013-X

Kumari A., 2018, Citric Acid Cycle, In: Kumari A., ed., Sweet Biochemistry, Academic Press, pp.7-11..

https://doi.org/10.1016/B978-0-12-814453-4.00002-9

Kynshi B., Sachu M., and Syiem M., 2021, Modulation in isocitrate dehydrogenase activity under citrate enrichment affects carbon and nitrogen fixations in the cyanobacterium Nostoc muscorum Meg 1, Biochimie, 186: 94-104.

https://doi.org/10.1016/j.biochi.2021.03.018

Li C., Liu F., Shen Y., Tian Y., and Han F., 2023, Research progress on the mechanism of glycolysis in ovarian cancer, Frontiers in Immunology, 14: 1284853.

https://doi.org/10.3389/fimmu.2023.1284853

Luo Y., Ma J., and Lu W., 2020, The significance of mitochondrial dysfunction in cancer, International Journal of Molecular Sciences, 21(16): 5598.

https://doi.org/10.3390/ijms21165598

MacLean A., Legendre F., and Appanna V., 2023, The tricarboxylic acid (TCA) cycle: a malleable metabolic network to counter cellular stress, Critical Reviews in Biochemistry and Molecular Biology, 58: 81-97.

https://doi.org/10.1080/10409238.2023.2201945

Matschinsky F., and Wilson D., 2019, The central role of glucokinase in glucose homeostasis: a perspective 50 years after demonstrating the presence of the enzyme in islets of langerhans, Frontiers in Physiology, 10: 148.

https://doi.org/10.3389/fphys.2019.00148

Mccammon M., Epstein C., Przybyla-Zawislak B., McAlister-Henn L., and Butow R., 2003, Global transcription analysis of Krebs tricarboxylic acid cycle mutants reveals an alternating pattern of gene expression and effects on hypoxic and oxidative genes, Molecular Biology of the Cell, 14(3): 958-972.

https://doi.org/10.1091/MBC.E02-07-0422

Neitzel C., Demuth P., Wittmann S., and Fahrer J., 2020, Targeting altered energy metabolism in colorectal cancer: oncogenic reprogramming, the central role of the TCA cycle and therapeutic opportunities, Cancers, 12(7): 1731.

https://doi.org/10.3390/cancers12071731

Patil N., Bohannon J., Hernandez A., Patil T., and Sherwood E., 2019, Regulation of leukocyte function by citric acid cycle intermediates, Journal of Leukocyte Biology, 106: 105-117.

https://doi.org/10.1002/JLB.3MIR1118-415R

Petillo A., Abruzzese V., Koshal P., Ostuni A., and Bisaccia F., 2020, Extracellular citrate is a trojan horse for cancer cells, Frontiers in Molecular Biosciences, 7: 593866..

https://doi.org/10.3389/fmolb.2020.593866

Plomgaard P., Hu C., Hansen J., Zhao X., Hoene M., Wang X., Secher N., Haering H., Lehmann R., Xu G., and Weigert C., 2018, Metabolic activity of the liver during exercise—a metabolomics approach, Diabetes, 67(Supplement_1): 1856-P.

https://doi.org/10.2337/db18-1856-P

Prochownik E., and Wang H., 2021, The metabolic fates of pyruvate in normal and neoplastic cells, Cells, 10(4): 762.

https://doi.org/10.3390/cells10040762

Roosterman D., and Cottrell G., 2021, Rethinking the citric acid cycle: connecting pyruvate carboxylase and citrate synthase to the flow of energy and material, International Journal of Molecular Sciences, 22(2): 604.

https://doi.org/10.3390/ijms22020604

Sauer D., Trebesch N., Marden J., Cocco N., Song J., Koide A., Koide S., Tajkhorshid E., and Wang D., 2020, Structural basis for the reaction cycle of DASS dicarboxylate transporters, eLife, 9: e61350.

https://doi.org/10.7554/eLife.61350

Schlaepfer I., and Joshi M., 2020, CPT1A-mediated fat oxidation, mechanisms and therapeutic potential, Endocrinology, 161(2): bqz046.

https://doi.org/10.1210/endocr/bqz046

Selinski J., and Scheibe R., 2020, Central metabolism in mammals and plants as a hub for controlling cell fate, Antioxidants & Redox Signaling, 34: 1025-1047.

https://doi.org/10.1089/ars.2020.8121

Shelbayeh O., Arroum T., Morris S., and Busch K., 2023, PGC-1α is a master regulator of mitochondrial lifecycle and ROS stress response, Antioxidants, 12(5): 1075.

https://doi.org/10.3390/antiox12051075

Shimizu K., and Matsuoka Y., 2019, Redox rebalance against genetic perturbations and modulation of central carbon metabolism by the oxidative stress regulation, Biotechnology Advances, 37(8): 107441.

https://doi.org/10.1016/j.biotechadv.2019.107441

Yu T., Wang Y., Fan Y., Fang N., Wang T., Xu T., and Shu Y., 2019, CircRNAs in cancer metabolism: a review, Journal of Hematology & Oncology, 12: 1-10.

https://doi.org/10.1186/s13045-019-0776-8

Zangari J., Petrelli F., Maillot B., and Martinou J., 2020, The multifaceted pyruvate metabolism: role of the mitochondrial pyruvate carrier, Biomolecules, 10(7): 1068.

https://doi.org/10.3390/biom10071068

Zhang W., Gao J., Shen F., Ma X., Wang Z., Hou X., Hao E., Hou Y., and Bai G., 2020, Cinnamaldehyde changes the dynamic balance of glucose metabolism by targeting ENO1, Life Sciences, 258: 118151.

https://doi.org/10.1016/j.lfs.2020.118151

Zhelev Z., Aoki I., Lazarova D., Vlaykova T., Higashi T., and Bakalova R., 2022, A “weird” mitochondrial fatty acid oxidation as a metabolic “secret” of cancer, Oxidative Medicine and Cellular Longevity, 2022(1): 2339584.

https://doi.org/10.1155/2022/2339584