Bioscience Evidence 2024, Vol.14, No.3, 110-121 http://bioscipublisher.com/index.php/be 111 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
RkJQdWJsaXNoZXIy MjQ4ODYzMg==