- Protein succinylation is an important post-translational modification (PTM) that regulates the function of proteins by adding a succinyl group to specific amino acid residues. One of the best-characterized forms is lysine succinylation, in which a succinyl group is attached to the amino group of a lysine residue. Protein succinylation has attracted considerable interest because it connects cellular metabolism with protein regulation and can influence metabolic enzymes, mitochondrial function, gene expression, cellular signaling, and disease processes.
- The discovery of lysine succinylation expanded the understanding of how metabolic intermediates can directly modify proteins. Like acetylation, malonylation, crotonylation, and lactylation, succinylation belongs to a broader group of lysine acylations. However, the succinyl group has distinct chemical properties, including the introduction of a negative charge under physiological conditions. This chemical change can have substantial effects on protein structure, enzyme activity, protein interactions, and other functional properties.
- A major connection between succinylation and metabolism is succinyl-CoA, a metabolic intermediate associated with the tricarboxylic acid (TCA) cycle and mitochondrial metabolism. Succinyl-CoA can serve as a source of the succinyl group for lysine succinylation. Because its abundance can change with metabolic conditions, nutrient availability, mitochondrial activity, and cellular stress, succinyl-CoA provides a potential link between metabolic state and protein modification.
- The relationship between succinyl-CoA metabolism and protein succinylation is therefore central to understanding this modification. Succinyl-CoA is generated within metabolic pathways and is involved in the TCA cycle, amino-acid metabolism, and other biochemical processes. Changes in the production, utilization, or compartmentalization of succinyl-CoA can influence the cellular succinylation landscape and potentially alter the activity of proteins involved in metabolism.
- Protein succinylation is particularly abundant in mitochondria, where many proteins involved in energy metabolism are located. Mitochondrial proteins can undergo extensive lysine succinylation, and modification of metabolic enzymes may affect their catalytic properties or interactions with other proteins. This has led to significant interest in mitochondrial protein succinylation as a mechanism for coordinating metabolic activity with cellular energy requirements.
- The chemical nature of succinylation distinguishes it from several other lysine modifications. Whereas acetylation neutralizes the positive charge of lysine, succinylation introduces a negatively charged group. This can produce substantial changes in the electrostatic properties of a protein and may influence its three-dimensional structure, molecular interactions, and enzymatic activity. Consequently, succinylation can have particularly strong effects on proteins when it occurs at functionally important lysine residues.
- The regulation of protein succinylation involves enzymes that add and remove succinyl groups. Enzymes or metabolic processes responsible for generating succinylation are sometimes described as succinylation writers, while enzymes that remove the modification are known as desuccinylases or erasers. Compared with more extensively characterized PTMs, the complete enzymatic machinery controlling succinylation remains an active area of investigation.
- Among the most important known regulators is SIRT5, a mitochondrial sirtuin with strong desuccinylase activity. SIRT5 can remove succinyl groups from lysine residues on a variety of mitochondrial proteins. Through this activity, SIRT5 helps regulate the balance between protein succinylation and desuccinylation and provides an important connection between mitochondrial metabolism and protein function.
- SIRT5-mediated desuccinylation is therefore a major area of research in protein succinylation biology. SIRT5 can regulate several types of lysine acylation, including succinylation and malonylation, which illustrates the interconnected nature of metabolic PTMs. Changes in SIRT5 activity can potentially influence multiple metabolic pathways simultaneously rather than affecting succinylation alone.
- Protein succinylation is a dynamic modification rather than a permanent chemical alteration. The balance between succinylation and desuccinylation can change in response to metabolic conditions, cellular stress, nutrient availability, and changes in enzyme activity. This dynamic behavior allows cells to adjust protein function as their metabolic requirements change.
- Although succinylation is strongly associated with mitochondrial proteins, it is not restricted to mitochondria. Non-histone protein succinylation has been identified in proteins located in different cellular compartments and involved in diverse processes. Succinylated proteins have been associated with metabolism, transcription, translation, cytoskeletal organization, signaling, and other cellular functions, suggesting that the biological effects of succinylation extend well beyond mitochondrial metabolism.
- The discovery and characterization of succinylated proteins have been greatly accelerated by mass spectrometry-based proteomics. Modern proteomic approaches can identify large numbers of succinylated proteins and individual succinylation sites. Large-scale succinylome analysis has revealed extensive protein succinylation across different organisms, tissues, and physiological conditions and has helped researchers investigate how succinylation changes during development, stress, aging, and disease.
- The succinylome can vary significantly according to cellular metabolic state. Nutrient availability, mitochondrial function, metabolic flux, and disease-associated changes can alter the abundance of succinylated proteins. Comparing succinylation profiles between normal and pathological conditions can therefore provide insights into metabolic pathways that have been disrupted or adapted.
- One of the most important biological roles proposed for protein succinylation is the regulation of metabolic enzymes. Succinylation can modify enzymes involved in glycolysis, the TCA cycle, fatty-acid metabolism, amino-acid metabolism, and other metabolic pathways. Modification of these enzymes may change their activity and thereby influence the movement of metabolites through interconnected biochemical pathways.
- The relationship between succinylation and the tricarboxylic acid cycle is particularly important. Succinyl-CoA is an intermediate of the TCA cycle, placing the metabolite directly within a central energy-producing pathway. Changes in TCA-cycle activity can therefore influence the availability of succinyl-CoA and potentially affect protein succinylation. Conversely, succinylation of metabolic enzymes may influence the activity of pathways that generate or consume metabolic intermediates, creating potential feedback mechanisms.
- Mitochondrial energy metabolism is consequently one of the major areas in which succinylation has been studied. Proteins involved in oxidative phosphorylation, the TCA cycle, fatty-acid oxidation, amino-acid metabolism, and other mitochondrial pathways can undergo succinylation. Through these modifications, cells may be able to coordinate mitochondrial protein activity with changes in metabolic conditions.
- Succinylation is also closely related to amino-acid metabolism. Several metabolic pathways can influence the production or utilization of succinyl-CoA and related intermediates. Succinylation of enzymes involved in amino-acid metabolism may therefore provide a regulatory connection between nutrient utilization and protein activity.
- The modification may also affect fatty-acid metabolism and lipid homeostasis. Mitochondrial fatty-acid oxidation depends on coordinated activity among numerous enzymes, and succinylation of these proteins may influence metabolic flux. The relationship between succinylation, acetylation, malonylation, and other lysine acylations further illustrates the complexity of metabolic regulation.
- Protein succinylation also has potential roles in epigenetic regulation. Histones and other nuclear proteins can undergo succinylation, raising the possibility that changes in cellular metabolism may influence chromatin structure and gene expression. Because succinylation introduces a negatively charged group onto lysine, histone succinylation may have distinct effects on chromatin compared with acetylation and other histone modifications.
- Histone succinylation is an emerging area of research within epigenetics. Succinylation of histone lysine residues may influence nucleosome structure, DNA-histone interactions, chromatin accessibility, and transcriptional regulation. The precise functions of individual histone succinylation sites are still being investigated, and the biological significance of this modification may depend on cellular and genomic context.
- Succinylation can interact with other histone modifications and epigenetic marks. Acetylation, methylation, phosphorylation, ubiquitination, crotonylation, lactylation, malonylation, and other modifications can occur within overlapping regulatory networks. Competition for lysine residues and interactions between modification-specific enzymes and reader proteins may contribute to complex patterns of gene regulation.
- The broader crosstalk between lysine acylations is particularly relevant to succinylation. Acetyl-CoA, malonyl-CoA, succinyl-CoA, crotonyl-CoA, and other acyl-CoA metabolites are generated through interconnected metabolic pathways. Changes in metabolism can therefore potentially influence several PTMs at the same time. Understanding succinylation requires considering it as part of this wider metabolic modification network.
- Protein succinylation has also attracted interest in cancer biology. Cancer cells undergo extensive metabolic reprogramming, including changes in mitochondrial metabolism, amino-acid utilization, glycolysis, and lipid metabolism. These changes can alter the availability of metabolic intermediates and may consequently affect protein succinylation. Abnormal succinylation could potentially influence tumor-cell metabolism, proliferation, survival, and adaptation to metabolic stress.
- The tumor microenvironment may further influence protein succinylation. Cancer cells interact with immune cells, stromal cells, and other cell populations in an environment characterized by changing nutrient availability and metabolic conditions. Alterations in metabolites and mitochondrial activity may affect succinylation patterns in both tumor and non-tumor cells, potentially contributing to interactions between metabolism and disease progression.
- Succinylation is also being investigated in inflammation and immune regulation. Immune-cell activation requires substantial metabolic remodeling, and mitochondrial metabolism can change dramatically during inflammatory responses. Changes in succinyl-CoA metabolism and protein succinylation may therefore influence immune-cell function, inflammatory signaling, and cellular adaptation.
- Another important area is the potential role of succinylation in oxidative stress and cellular stress responses. Mitochondrial dysfunction can alter metabolic flux and the production of reactive oxygen species. Because succinylation modifies many mitochondrial proteins, changes in succinylation may form part of the cellular response to mitochondrial and metabolic stress.
- Protein succinylation has also been studied in aging and age-related changes in metabolism. Mitochondrial function, metabolic flux, and the activity of enzymes that regulate protein acylation can change with age. Alterations in succinylation may therefore contribute to age-associated changes in protein function and metabolic homeostasis. However, determining whether such changes are causal or secondary remains an important research challenge.
- The biological consequences of succinylation depend heavily on site-specific modification. A single protein can contain multiple succinylation sites, but not every site necessarily has the same functional importance. Succinylation of a catalytic lysine may directly influence enzyme activity, whereas modification at another site may affect protein interactions or stability. Identifying and experimentally characterizing individual sites is therefore essential.
- Modern quantitative succinylome profiling allows researchers to compare succinylation patterns across different physiological and pathological conditions. When combined with metabolomics, transcriptomics, proteomics, and functional biochemical studies, these approaches can reveal relationships between metabolic changes and protein regulation. Such integrated approaches are increasingly important for understanding the biological significance of succinylation.
- A major challenge is determining exactly how changes in succinyl-CoA levels translate into changes in protein succinylation. Metabolite concentration alone may not determine modification levels because enzyme activity, subcellular compartmentalization, substrate accessibility, protein turnover, and desuccinylase activity also contribute. Understanding the regulation of succinylation therefore requires analysis of multiple interconnected factors.
- Another important question concerns the existence and specificity of succinylation reader proteins. Reader proteins recognize particular PTMs and help translate them into biological outcomes. Compared with some other modifications, the molecular mechanisms by which proteins recognize and interpret succinylated lysine residues remain less completely characterized. Identifying these readers could substantially improve our understanding of how succinylation affects cellular signaling and chromatin regulation.
- Protein succinylation also has potential implications for therapeutic research. Enzymes and metabolic pathways controlling succinylation or desuccinylation could potentially be targeted to alter disease-associated protein functions. SIRT5 is of particular interest because of its central role in mitochondrial desuccinylation. However, because SIRT5 regulates multiple lysine acylations, manipulating its activity may have broad effects on cellular metabolism.
- Overall, protein succinylation represents an important connection between cellular metabolism, mitochondrial function, protein regulation, and epigenetics. Its dependence on succinyl-CoA provides a mechanism through which metabolic conditions can influence protein activity and cellular behavior. At the same time, succinylation of histones and non-histone proteins suggests that its functions extend beyond metabolic enzymes and may include regulation of gene expression, signaling, and cellular responses.
