phosphorylation

Examples of phosphorylation in the following topics:

• Termination of the Signal Cascade

  • Signal cascades convey signals to the cell through the phosphorylation of molecules by kinases.
  • A major component of cell signaling cascades is the phosphorylation of molecules by enzymes known as kinases.
  • Phosphorylation adds a phosphate group to serine, threonine, and tyrosine residues in a protein, changing their shapes, and activating or inactivating the protein .
  • In protein phosphorylation, a phosphate group is added to residues of the amino acids serine, threonine, and tyrosine.

• Fermentation Without Substrate-Level Phosphorylation

  • Even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as sugars are readily available for consumption (a phenomenon known as the Crabtree effect).
  • Fermentation is important in anaerobic conditions when there is no oxidative phosphorylation to maintain the production of ATP (adenosine triphosphate) by glycolysis.
  • For example, even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as sugars are readily available for consumption (a phenomenon known as the Crabtree effect).

• Methods of Intracellular Signaling

  • Various kinases are named for the substrate they phosphorylate.
  • Phosphorylation of serine and threonine residues often activates enzymes.
  • Phosphorylation may activate or inactivate enzymes; the reversal of phosphorylation, dephosphorylation by a phosphatase, will reverse the effect.
  • It phosphorylates serine and threonine residues of its target proteins, activating them in the process.
  • Enzymes known as kinases phosphorylate PI to form PI-phosphate (PIP) and PI-bisphosphate (PIP2).

• The Energy-Requiring Steps of Glycolysis

  • The first step in glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars.
  • Hexokinase phosphorylates glucose using ATP as the source of the phosphate, producing glucose-6-phosphate, a more reactive form of glucose.
  • This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins.
  • The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase.
  • The first half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which is then split into two three-carbon molecules.

• Two-Component Regulatory Systems

  • Signal transduction occurs through the transfer of phosphoryl groups from adenosine triphosphate (ATP) to a specific histidine residue in the histidine kinases (HK).
  • The level of phosphorylation of the response regulator controls its activity.
  • Some HK are bifunctional, catalysing both the phosphorylation and dephosphorylation of their cognate RR.
  • Here a hybrid HK autophosphorylates and then transfers the phosphoryl group to an internal receiver domain, rather than to a separate RR protein.
  • The phosphoryl group is then shuttled to histidine phosphotransferase (HPT) and subsequently to a terminal RR, which can evoke the desired response.

• Chemiosmosis and Oxidative Phosphorylation

  • The production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation.
  • In oxidative phosphorylation, the hydrogen ion gradient formed by the electron transport chain is used by ATP synthase to form ATP.

• ATP in Metabolism

  • Phosphorylation by ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium.
  • Sometimes phosphorylation of an enzyme leads to its inhibition.
  • For example, the pyruvate dehydrogenase (PDH) complex could be phosphorylated by pyruvate dehydrogenase kinase (PDHK).
  • Phosphorylation refers to the addition of the phosphate (~P).
  • In phosphorylation reactions, the gamma phosphate of ATP is attached to a protein.

• Respiration and Proton Motive Force

  • The products of the Krebs cycle include energy in the form of ATP (via substrate level phosphorylation), NADH, and FADH2.
  • The initial phosphorylation of glucose is required to destabilize the molecule for cleavage into two pyruvate.
  • With the help of phosphofructokinase, an additional ATP can be used to turn phosphorylate fructose 6-phosphate into fructose 1, 6-diphosphate.
  • Fructose 1, 6-diphosphate then splits into two phosphorylated molecules with three carbon chains that later degrades into pyruvate.
  • The overall process of creating energy in this fashion is termed oxidative phosphorylation.

• Regulator Molecules of the Cell Cycle

  • To be fully active, the Cdk/cyclin complex must also be phosphorylated in specific locations.
  • Like all kinases, Cdks are enzymes (kinases) that phosphorylate other proteins.
  • Phosphorylation activates the protein by changing its shape.
  • The proteins phosphorylated by Cdks are involved in advancing the cell to the next phase. .
  • As the cell increases in size, Rb is slowly phosphorylated until it becomes inactivated.

• The Energy-Releasing Steps of Glycolysis

  • The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate.
  • (This is an example of substrate-level phosphorylation. ) A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-phosphoglycerate is formed.
  • The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme in this case is named for the reverse reaction of pyruvate's conversion into PEP) and results in the production of a second ATP molecule by substrate-level phosphorylation and the compound pyruvic acid (or its salt form, pyruvate).
  • The second half of glycolysis involves phosphorylation without ATP investment (step 6) and produces two NADH and four ATP molecules per glucose.