Metabolic Regulation by Protein Lysine Acetylation
Protein acetylation is a key role in regulation of transcription in the nucleus. What more important about protein acetylation is its function in regulating metabolic pathways. For example, glycolysis, gluconeogenesis, the tricarboxylic acid (TCA) cycle, urea cycle, fatty acid metabolism, and glycogen metabolism in liver tissue.
What is protein acetylation
Acetylation is a reaction which introduces an acetyl functional group into a chemical compound. Typically, protein lysine acetylation is the most important pathway which regulates the metabolic pathways, by acetylating lysine residue in enzyme, enzymes are able to regulate metabolix pathways. In living cells, acetylation occurs as a co-translational and post-translational modification of proteins. In liver tissue, acetyl-coenzyme A (acetyl-CoA), a high energy molecule, act as the acetyl group donor to the lysine acetylation.
Lysine Acetylation in fatty acid oxidation
Enoyl-coenzyme A hydratase/3-hydroxyacyl-coenzyme A (EHHADH) is an important enzyme which catalyze two steps in fatty acid oxidation. There are four acetylated lysine residues been identified in EHHADH, which are Lys165, Lys171, Lys346 and Lys584. Immunoprecipitation of ectopically expressed FLAG-tagged EHHADH and Western blotting with antibody to acetyllysine confirmed that EHHADH was indeed acetylated(Zhao, et al). In order to explore the effect of acetylation on fatty acid oxidation. Isobaric tags are used, which is TSA and NAM. TSA and NAM treatment increased all the four lysine residues’ acetylation. Consistently, corresponding unacetylated peptide was decreased. Scientists treat TSA and NAM to Chang Human Liver cells doubled the activity of EHHADH, which indicates that acetylation of EHHADH would increase fatty acid oxidation pathway. In order to confirm the result, site-directed mutagenesis was used and the four lysine residue was replaced by glutamine, TSA and NAM can no longer acetylated lysine residues and EHHADH is no longer regulated.
Lysine Acetylation in TCA cycle
TCA cycle includes seven enzymes, and all of them can be acetylated. Malate dehydrogenase (MDH) was studied. Again, four lysine residues are identified: Lys185, Lys301, Lys307 and Lys314. Again, TSA and NAM treatment was performed and wild-type MDH’s activity was increased while mutant MDH4KR’s activity wasn’t changed, which indicate that lysine acetylation can stimulate the TCA cycle. Another method can be performed for studying lysine acetylation. When MDH was treated with high concentration of glucose, MDH acetylation was increased by 60%, and the activity of MDH was increased, which again, confirm that lysine acetylation stimulates the TCA cycle. This result matches our logic, as we eat more glucose, glycolysis and TCA cycle are stimulated because we need to consume these glucose to created ATP, which is energy.
Lysine Acetylation in urea cycle
Urea cycle is coupled with TCA cycle, thus studying of urea cycle is also a good way to understand the effect of lysine acetylation on metabolic pathways. ASL, an enzyme in urea cycle, was studied. High glucose concentration is treated to ASL, the acetylation of ASL decrease the activity of ASL by 50%, which indicated that lysine acetylation inhibit the urea cycle. On the other hand, CobB (one of the amino acids) is treated to ASL, which deacetylates the ASL, experiment results show that ASL activity is increased. The dual regulation of ASL indicate that lysine acetylation of ASL will inhibit the urea cycle. The result make sense as we invest more glucose, in order to maintain body osmotic, we need to conserve water, which means that our body should perform minimum amount of urea cycle.
Lysine Acetylation in Gluconeogenesis
Gluconeogenesis, the opposite of glycolysis, is also studied for understanding lysine acetylation in metabolism. The enzyme Phosphoenolpyruvate carboxykinase 1 (PEPCK1) is studied for gluconeogenesis. There are three lysine residues which were identified: Lys70, Lys71, Lys594. Treatment of both high glucose and TSA/NAM acetylated PEPCK1 and the amount of PEPCK1 was decreased, and decrease the stability of PEPCK1. Which indicates that lysine acetylation inhibits the gluconeogenesis pathway. To confirm our result, mutated form of PEPCK 1(PEPCK13KR, replace all the lysine by arginine) was examined, and the gluconeogenesis is no longer regulated by high glucose and TSA/NAM treatment, which confirm our result. The result aligns with our logic, that when we investigate a lot of glucose to our body, we would like to perform glycolysis instead of gluconeogenesis.
Protein Lysine acetylation is one of the important way to regulate metabolic pathways. These regulation intend to balance of energy in organisms and osmosis. When we infuse a lot of sugar, we are willing to consume these sugar to keep osmotic pressure of our body and make energy from glucose, thus TCA cycle fatty acid oxidation is stimulated by protein lysine acetylation; it also inhibit Urea cycle and gluconeogenesis so that we can conserve water to keep osmotic pressure and prevent the synthesis of glucose.
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|Pathway||Fatty Acid Oxidation||TCA Cycle||Urea Cycle||Gluconeogenesis|
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1. Shimin Zhao, et al. (2010) Regulation of Cellular Metabolism by Protein Lysine Acetylation. Science Vol 327