Vets 111/Biov 111 Cell Signalling-2 Secondary messengers the cyclic AMP intracellular signalling system
The classical secondary messenger model of intracellular signalling A cell surface receptor binds the signal molecule (the primary messenger ). Occupation of the receptor is transduced into an intracellular signal by activation of an effector enzyme. This enzyme makes a secondary messenger that diffuses away from the occupied receptor. The secondary messenger interacts with target proteins that promote activation/inactivation of protein kinases and/or protein phosphatases. Changes in the phosphorylation state of the protein substrates (e.g. enzymes, transcription factors) for these signal-sensitive protein kinases/phosphatases bring about changes in cellular activity.
Ligand Receptor Transducer Effector Enzyme Membrane Cell Interior Secondary Messenger ACTIVATION Primary Protein Kinase PHOSPHORYLATION ACTIVATION A B C Other Protein Kinase(s) PHOSPHORYLATION D E F Fig. 1 The classical secondary messenger signalling system The classical secondary messenger mechanism
The cyclic AMP secondary messenger system Adrenaline, acting on a -adrenergic receptor (e.g. in skeletal muscle) or glucagon acting on a glucagon receptor (e.g. in the liver), leads to the activation of adenylate cyclase with the consequent production of a secondary messenger - cyclic AMP. The -adrenergic receptor a 7 pass (or 7 transmembrane) receptor.
The cyclic AMP secondary messenger system is the basis of many hormonal effects. Hormone cell Physiological effects Glucagon Liver Glycogen breakdown increased, gluconeogenesis increased. Adrenaline Adipose ( 1 -receptor) Lipolysis increased, fatty acid synthesis inhibited. Adrenaline Heart ( 1 -receptor) Increase in heart rate and contractile force. Vasopressin Kidney (V 2 -receptor) Sodium and water reabsorption stimulated. ACTH Adrenal cortex Glucocorticoid synthesis stimulated. Thyrotropin Thyroid gland Thyroid hormone synthesis stimulated. Parathormone Bone Bone resorption stimulated.
The role of G-proteins in cyclic AMP signalling Heterotrimeric ( ) guanine-nucleotide binding proteins (G proteins) couple seven-pass hormone receptors to the enzymes in the plasma membrane responsible for the generation of secondary messengers (e.g. cyclic AMP). Occupation of the receptor causes activation of the G protein, which, in turn, regulates the activity of the plasma membrane enzyme (adenylate cyclase).
Enzymic synthesis and degradation of cyclic AMP Cyclic AMP is formed by the cyclization of ATP. The enzyme adenylate cylase is responsible for this reaction. Cyclic AMP is hydrolyzed to 5'- AMP by specific phosphodiesterases (PDEs).
Cyclic AMP-dependent protein kinase (PK-A) The enzyme is a tetramer. Two catalytic (C) subunits are bound with two regulatory R subunits to give an overall R 2 C 2 structure. The R subunits possess two non-identical, high affinity binding sites for cyclic AMP. Occupation of first site leads to a conformational change and the exposure of a second binding site on each R subunit. Binding to the second site leads to the dissociation of the complex and the release of free catalytically active C subunits.
PK-A is able to phosphorylate a wide range of target proteins. PK-A phosphorylates and activates the enzyme phosphorylase kinase that, in turn, is responsible for phosphorylation and activation of glycogen phosphorylase. PK-A catalytic subunit complexed with PKI(5-24) and ATP analogue.
Adrenaline -Receptor Membrane Gs Adenylate Cyclase AMP PDE cyclic AMP ACTIVATION Protein Kinase A PHOSPHORYLATION ACTIVATION A B C Phosphorylaae Kinase PHOSPHORYLATION D Glycogen Phosphorylase F Fig. 2 The cyclic AMP signalling system in liver The cyclic AMP secondary messenger system.
Cyclic AMP-dependent control of glycogen metabolism: activation of glycogen phosphorylase and inhibition of glycogen synthase. (Glycogen phosphorylase is ACTIVATED when phosphorylated. Glycogen synthase is INACTIVATED when phosphorylated.)
How does insulin counteract the effects of glucagon and adrenaline on glycogen metabolism?
Receptor tyrosine kinases Insulin and many growth factors such as epidermal growth factor (EGF) bind to the extracellular component of transmembrane receptors that have tyrosine kinase domains within their intracellular component. This tyrosine kinase activity can phosphorylate target proteins on specific tyrosine residues. In the case of insulin, the receptor is a disulfide-bonded dimer of polypeptide chains. The Insulin receptor
How does insulin counteract the effects of camp-dependent control of glycogen metabolism? Insulin triggers a cascade leading to the activation of protein phosphatase 1, which results in the stimulation of glycogen synthesis and inhibition of its breakdown. The activated receptor tyrosine kinase activates an insulin-sensitive protein kinase (serine-specific) that phosphorylates the glycogenbinding subunit of protein phosphatase 1 leading to activation of the protein phosphatase.
Summary The cyclic AMP signalling system is the classical secondary messenger pathway. Occupation of a receptor is transduced into adenylate cyclasemediated intracellular cyclic AMP production by the G-protein cycle. Accumulated cyclic AMP is able to bind to, and activate, the cyclic AMP-dependent protein kinase. This enzyme increases the phosphorylation state of specific target proteins (e.g. enzymes, transcription factors). The actions of adenylate cyclase and cyclic AMP-dependent protein kinase are opposed by phosphodiesterase enzymes and protein phosphatases. These enzymes attenuate the signalling activity of the cyclic AMP signalling pathway.