acetylation

Examples of acetylation in the following topics:

• Acetyl CoA to CO2

  • The acetyl carbons of acetyl CoA are released as carbon dioxide in the citric acid cycle.
  • Acetyl CoA links glycolysis and pyruvate oxidation with the citric acid cycle.
  • In the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups.
  • For each molecule of acetyl CoA that enters the citric acid cycle, two carbon dioxide molecules are released, removing the carbons from the acetyl group.
  • Describe the fate of the acetyl CoA carbons in the citric acid cycle

• Breakdown of Pyruvate

  • After glycolysis, pyruvate is converted into acetyl CoA in order to enter the citric acid cycle.
  • In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes to become acetyl Coenzyme A (acetyl CoA).
  • The conversion of pyruvate to acetyl CoA is a three-step process .
  • The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA.
  • The remaining two carbons are then transferred to the enzyme CoA to produce Acetyl CoA.

• Connecting Lipids to Glucose Metabolism

  • The synthesis of cholesterol starts with acetyl groups, which are transferred from acetyl CoA, and proceeds in only one direction; the process cannot be reversed.
  • Fatty acids are catabolized in a process called beta-oxidation that takes place in the matrix of the mitochondria and converts their fatty acid chains into two carbon units of acetyl groups, while producing NADH and FADH2.
  • The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric acid cycle as it combines with oxaloacetate.

• Citric Acid Cycle

  • Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of the mitochondria.
  • The first step is a condensation step, combining the two-carbon acetyl group (from acetyl CoA) with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate.
  • CoA is bound to a sulfhydryl group (-SH) and diffuses away to eventually combine with another acetyl group.
  • In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule.
  • Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle.

• Epigenetic Alterations in Cancer

  • Histone proteins that surround that region lack the acetylation modification (the addition of an acetyl group) that is present when the genes are expressed in normal cells.
  • Because these changes are temporary and can be reversed (for example, by preventing the action of the histone deacetylase protein that removes acetyl groups, or by DNA methyl transferase enzymes that add methyl groups to cytosines in DNA) it is possible to design new drugs and new therapies to take advantage of the reversible nature of these processes.

• Connecting Proteins to Glucose Metabolism

  • When deaminated, amino acids can enter the pathways of glucose metabolism as pyruvate, acetyl CoA, or several components of the citric acid cycle.
  • The carbon skeletons of certain amino acids (indicated in boxes) are derived from proteins and can feed into pyruvate, acetyl CoA, and the citric acid cycle.

• The lac Operon: An Inducer Operon

  • The lac operon encodes the genes necessary to acquire and process the lactose from the local environment, which includes the structural genes lacZ, lacY, and lacA. lacZ encodes β-galactosidase (LacZ), an intracellular enzyme that cleaves the disaccharide lactose into glucose and galactose. lacY encodes β-galactoside permease (LacY), a membrane-bound transport protein that pumps lactose into the cell. lacA encodes β-galactoside transacetylase (LacA), an enzyme that transfers an acetyl group from acetyl-CoA to β-galactosides.

• Transcriptional Enhancers and Repressors

  • The corepressor can repress transcriptional initiation by recruiting histone deacetylase, which catalyzes the removal of acetyl groups from lysine residues.

• Control of Catabolic Pathways

  • If more energy is needed, more pyruvate will be converted into acetyl CoA through the action of pyruvate dehydrogenase.
  • If either acetyl groups or NADH accumulate, there is less need for the reaction and the rate decreases.

• Epigenetic Control: Regulating Access to Genes within the Chromosome

  • They are chemical modifications (phosphate, methyl, or acetyl groups) that are attached to specific amino acids in the protein or to the nucleotides of the DNA.
  • When unmodified, the histone proteins have a large positive charge; by adding chemical modifications, such as acetyl groups, the charge becomes less positive.