acetyl CoA

(noun)

a molecule that conveys the carbon atoms from glycolysis (pyruvate) to the citric acid cycle to be oxidized for energy production

Related Terms

Examples of acetyl CoA 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

    • Cholesterol contributes to cell membrane flexibility and is a precursor to steroid hormones.
    • 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.
    • Triglycerides, a form of long-term energy storage in animals, are made of glycerol and three fatty acids.
    • 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.

  • Connecting Proteins to Glucose Metabolism

    • Proteins are a good example of this phenomenon.
    • Proteins are hydrolyzed by a variety of enzymes in cells.
    • The remaining atoms of the amino acid result in a keto acid: a carbon chain with one ketone and one carboxylic acid group.
    • 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.

  • 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.
    • CoA binds the succinyl group to form succinyl CoA.
    • 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.

  • The lac Operon: An Inducer Operon

    • A major type of gene regulation that occurs in prokaryotic cells utilizes and occurs through inducible operons.
    • The lac operon is a typical inducible operon.
    • To do so, the cAMP–CAP protein complex serves as a positive regulator to induce transcription.
    • 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.
    • The lac operon uses a two-part control mechanism to ensure that the cell expends energy producing β-galactosidase, β-galactoside permease, and thiogalactoside transacetylase (also known as galactoside O-acetyltransferase) only when necessary.

  • Control of Catabolic Pathways

    • Dephosphorylation by a phosphatase reactivates it.
    • 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.
    • When more ATP is needed, as reflected in rising ADP levels, the rate increases. α-Ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA, a subsequent intermediate in the cycle, causing a decrease in activity.
    • Greater ATP consumption by a cell is indicated by a buildup of ADP.

  • ATP in Metabolism

    • Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use as needed.
    • The core of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group.
    • The addition of a phosphate group to a molecule requires energy.
    • This reaction leads to inhibition of PDH and its inability to convert pyruvate into acetyl-CoA.
    • A + enzyme + ATP→[ A enzyme −P ] B + enzyme + ADP + phosphate ion

  • Epigenetic Alterations in Cancer

    • Epigenetic alterations are as important as genetic mutations in a cell's transformation to cancer.
    • In cancer cells, the DNA in the promoter region of silenced genes is methylated on cytosine DNA residues in CpG islands, genomic regions that contain a high frequency of CpG sites, where a cytosine nucleotide occurs next to a guanine nucleotide .
    • 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.
    • Indeed, many researchers are testing how a silenced gene can be switched back on in a cancer cell to help re-establish normal growth patterns.
    • In cancer cells, silencing genes through epigenetic mechanisms is a common occurrence.

  • Transcriptional Enhancers and Repressors

    • They can be located upstream of a gene, within the coding region of the gene, downstream of a gene, or may be thousands of nucleotides away.
    • Whereas DNA is generally depicted as a straight line in two dimensions, it is actually a three-dimensional object.
    • Therefore, a nucleotide sequence thousands of nucleotides away can fold over and interact with a specific promoter .
    • A corepressor is a protein that decreases gene expression by binding to a transcription factor that contains a DNA-binding domain.
    • The corepressor can repress transcriptional initiation by recruiting histone deacetylase, which catalyzes the removal of acetyl groups from lysine residues.