Adrenaline Phosphorylation

Heterologous desensitisation refers to the desensitisation of the response to one agonist by the application of a different agonist. For example, desensitisation of a response to adrenaline by application of 5-HT is mediated by protein kinase A or protein kinase C because these kinases can phosphorylate receptors which are not occupied by agonist. Phosphorylation disrupts the receptor-G-protein interaction and induces the binding of specific proteins, arrestins which enhance receptors internalisation via clathrin-coated pits. Thus desensitisation of G-protein-coupled receptors results in a decrease in the number of functional receptors on the cell surface. 

Fig. 2. An adrenaline molecule (1) binds to its binding site on the extracellular site of an adrenaline receptor (2). Thereby, the exchange of GDP by GTP in the Ga subunit of a hetero-trimeric G protein (3) is induced, followed by the dissociation of the Ga and Gpr subunits. G now binds and stimulates its effector adenylate cyclase (4), which produces cyclic AMP (5) from ATP (6). This second messenger starts a cascade of enzymatic reactions, which alter the behavior of the cell via several phosphorylation steps... 

Reversible phosphorylation is the main control mechanism of liver phosphorylase allosteric effects being much less pronounced. This is in contrast with muscle phosphorylase, which is also controlled by phosphorylation, stimulated by an adrenaline-... 

Furthermore, as well as CaCM-induced phosphorylation, MLCK is also subject to control via a cAMP-dependent protein kinase, PKA. Phosphorylated MLCK binds CaCM only weakly, thus contraction is impaired. This explains the relaxation of smooth muscle when challenged with adrenaline (epinephrine), a hormone whose receptor is functionally linked with adenylyl cyclase (AC), the enzyme that generates cAMP from ATP. 

Figure 12.5 Effector mechanism activation of a membrane-bound phospholipase. An example is activation of a membrane-bound phospholipase which hydrolyses phosphatidylinositol bisphosphate (PIP2) and results in the formation of the two messengers, inositol trisphosphate (IP3) and diacylglycerol (DAG). Messenger IP3 binds to a receptor on the endoplasmic reticulum that results in release of Ca ions into the cytosol. DAG, which remains within the membrane, activates protein kinase-C at the membrane surface. When the kinase leaves the membrane, it is unclear how it remains active or loss of activity is prevented, so that it can phosphorylate proteins in the cytosol or even the nucleus. An example is adrenaline binding to the a-receptor in the liver, in which Ca ions stimulate glycogenolysis.

The activation of protein kinase A in heart leads to phosphorylation of a number of proteins which together result in changes that increase cardiac output (Chapter 22). An important clinical effect of adrenaline. 

The best investigated is the desensitization of the adrenaline receptor type P2 and of rhodopsin. Rhodopsin has the function of a light receptor in the process of vision, ft receives light signals and conducts them to the relevant G-protein, transducin. The key reaction in desensitization of both systems is the phosphorylation of the receptor at the cytoplasmic side by specific protein kinases. 

Activation of Gs or Gi proteins results in stimulation or inhibition, respectively, of adenylyl cyclase which catalyses the formation of cyclic adenosine monophosphate (cAMP) from ATP The cAMP binds to protein kinase A (PKA), which mediates the diverse cellular effects of cAMP by phosphorylating substrate enzymes, thereby increasing their activity. Among the responses mediated by cAMP are increases in contraction of cardiac and skeletal muscle and glycogenolysis in the liver by adrenaline (epinephrine). Because a single activated receptor can cause the conversion of up to 100 inactive Gs proteins to the active form, and each of these results in the synthesis of several hundred cAMP molecules, there is a very considerable signal amplification. For example, adrenaline concentrations as low as 10-10 M can stimulate the release of glucose sufficient to increase... 

The existence of cAMP as a compound mediating the action of adrenaline and glucagon on glycogen phosphorylase was first recognized in 1956 by Sutherland.168169 However, for many years most biochemists regarded cAMP as a curiosity and the regulatory chemistry of phosphorylase as an unusual specialization. That view was altered drastically when cAMP was found to function as a second messenger in the action of over 20 different hormones. Phosphorylation by... 

In response to energy demands, the fatty acids of stored triacylglycerols must be mobilised for use by peripheral tissues. This release is controUed by a complex series of interrelated cascades that result in the activation of hormone-sensitive lipase. In adipocytes this stimulus can come from glucagon, adrenaline (epinephrine) or 8-corticotropin. These hormones bind cell-surface receptors that are coupled to the activation of adenyl cyclase the resultant increase in intracellular cAMP leads to activation of cAMP-dependent protein kinase, which in turn phosphorylates and activates hormone-sensitive lipase (Figure 5.5). 

Figure 6.5 Regulation of HMG-CoA reductase. HMG-CoA reductase is active in the dephospho-rylated state phosphorylation (inhibition) is catalysed by reductase kinase, an enzyme whose activity is also regulated by phosphorylation by reductase kinase kinase. Hormones such as glucagon and adrenalin (epinephrine) negatively affect cholesterol biosynthesis by increasing the activity of the inhibitor of phosphoprotein phosphatase-1, PPI-1, (by raising cAMP levels) and so reducing the activation of HMG-CoA reductase. Conversely, insulin stimulates the removal of phosphates (and lowers cAMP levels), and thereby activates HMG-CoA reductase activity. Additional regulation of HMG-CoA reductase occurs through an inhibition of synthesis of the enzyme by elevation in intracellular cholesterol levels.

Adrenalin (epinephrin) stimulates phosphorylation and inactivation in adipose tissue. 

Many hormones, such as epinephrine (adrenaline), alter the activities of enzymes by stimulating the phosphorylation of the hydroxyl amino acids serine and threonine phosphoserine and phosphothreonine are the most ubiquitous modified amino acids in proteins. Growth factors such as insulin act by triggering the phosphorylation of the hydroxyl group of tyrosine residues to form phosphotyrosine. The phosphoryl groups on these three modified amino acids are readily removed thus they are able to act as reversible switches in regulating cellular processes. The roles of phosphorylation in signal transduction will be discussed extensively in Chapter 14. 

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