cyclin-dependent kinase

Examples of cyclin-dependent kinase in the following topics:

• Regulation of the Cell Cycle at Internal Checkpoints

  • This transition, as with all of the major checkpoint transitions in the cell cycle, is signaled by cyclins and cyclin dependent kinases (CDKs).
  • Cyclins are cell-signaling molecules that regulate the cell cycle.
  • If the DNA has been correctly replicated, cyclin dependent kinases (CDKs) signal the beginning of mitotic cell division.

• Regulator Molecules of the Cell Cycle

  • Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints.
  • Like all kinases, Cdks are enzymes (kinases) that phosphorylate other proteins.
  • The levels of Cdk proteins are relatively stable throughout the cell cycle; however, the concentrations of cyclin fluctuate and determine when Cdk/cyclin complexes form.
  • Cyclin-dependent kinases (Cdks) are protein kinases that, when fully activated, can phosphorylate and activate other proteins that advance the cell cycle past a checkpoint.
  • To become fully activated, a Cdk must bind to a cyclin protein and then be phosphorylated by another kinase.

• Methods of Intracellular Signaling

  • The induction of a signaling pathway depends on the modification of a cellular component by an enzyme.
  • Various kinases are named for the substrate they phosphorylate.
  • The response to the increase in Ca2+ varies, depending on the cell type involved.
  • The main role of cAMP in cells is to bind to and activate an enzyme called cAMP-dependent kinase (A-kinase).
  • A-kinase regulates many vital metabolic pathways.

• Control of Catabolic Pathways

  • Whether a particular enzyme activity is released depends upon the energy needs of the cell (as reflected by the levels of ATP, ADP, and AMP).
  • The last step in glycolysis is catalyzed by pyruvate kinase.
  • Pyruvate kinase is also regulated by ATP (a negative allosteric effect).
  • Pyruvate dehydrogenase is also regulated by phosphorylation: a kinase phosphorylates it to form an inactive enzyme, and a phosphatase reactivates it.
  • The kinase and the phosphatase are also regulated.

• Plasma Membrane Hormone Receptors

  • Adenylyl cyclase catalyzes the conversion of ATP to cAMP. cAMP, in turn, activates a group of proteins called protein kinases, which transfer a phosphate group from ATP to a substrate molecule in a process called phosphorylation.
  • Further amplification occurs as protein kinases, once activated by cAMP, can catalyze many reactions.
  • The specific response of a cell to a lipid-insoluble hormone depends on the type of receptors that are present on the cell membrane and the substrate molecules present in the cell cytoplasm.

• The Energy-Releasing Steps of Glycolysis

  • The continuation of the reaction depends upon the availability of the oxidized form of the electron carrier NAD+.
  • In the seventh step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse reaction), 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP.
  • 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).

• Types of Receptors

  • The size and extent of each of these domains vary widely, depending on the type of receptor.
  • An example of this type of enzyme-linked receptor is the tyrosine kinase receptor.
  • The tyrosine kinase receptor transfers phosphate groups to tyrosine molecules.
  • Signaling molecules bind to the extracellular domain of two nearby tyrosine kinase receptors, which then dimerize.

• Regulating Protein Activity and Longevity

  • Because proteins are involved in every stage of gene regulation, the phosphorylation of a protein (depending on the protein that is modified) can alter accessibility to the chromosome, can alter translation (by altering transcription factor binding or function), can change nuclear shuttling (by influencing modifications to the nuclear pore complex), can alter RNA stability (by binding or not binding to the RNA to regulate its stability), can modify translation (increase or decrease), or can change post-translational modifications (add or remove phosphates or other chemical modifications).
  • The enzymes which are responsible for phosphorylation are known as protein kinases.
  • The addition of a phosphate group to a protein can result in either activation or deactivation; it is protein dependent.