Proton messenger

Attaching a pyridine group on the phosphine (e.g. PPh2Py) is also believed to enhance the catalytic activity in methyl methacrylate synthesis by picking up protons in solution and transferring them to the metal centre, which then uses them in the reaction [26], Such a process has been termed a proton messenger. 

It is assumed that a ligand protonated at the pyridyl group (P-NH" ), and mono-coordinated via phosphorus to the palladium center, fulfills a key role as a proton messenger , by bringing the proton into very close proximity with the coordination sphere at the palladium-alkenyl bond (Scheme 3). 

To be an effective proton messenger, several properties of the 2-pyridylphos-phine moiety are thought to be essential. These are ... 

Seuwen K, Ludwig MG, Wolf RM (2006) Receptors for protons or lipid messengers or both J Recept Signal Transduct Res 26 599-610... 

Interaction of the food with the gastric mucosal layer is the normal trigger for gastric cells to release gastrin, which is then carried by the bloodstream to the parietal cells. Calcium ions and cyclic AMP act as intracellular messengers in the transfer of the signal from the receptors to the proton pumps of parietal cells where the acid is generated. 

In higher eukaryotes messenger RNAs for chloroplast and mitochondrial proteins have been localized exclusively to free and not membrane-bound polysomes [4,68]. In yeast, the situation is not as clear a portion of the polysomes for some of the proteins is found to be bound to mitochondria [69,70], This may only reflect the speed at which the import process occurs in the different organisms. Most of the cytoplasmically-synthesized precursors have amino terminal extensions varying in size and nature. The size variation goes from no extension (cytochrome c and ATP-ADP translocator) to approximately 12000 Da (proteolipid of the proton... 

Hie final step in acid secretion in the parietal cell is the extrusion ( pumping") of prottms. The membrane pump, an H /K -ATPase. catalyzes the exchange of hydrogen ions for potassium ions. Inhibition of this proton pump acts beyond the site of action of second messengers (e.g.. Ca and cAMP) and is independent of the action of secretogogues histamine, gastrin, and acetylcholine. Thus, acid pump inhibitors block basal and stimulated secretion. 

Ach, HA, and gastrin stimulate acid secretion by activating specific receptors on the ba-solateral membrane of the parietal cell. Once bound to the respective G-protein-coupled receptor, second-messenger systems are activated. Ach and gastrin activate phospholipase C to catalyze the conversion of membrane-bound phospholipids to diacylglycerol and inositol triphosphate. The release of Ca from intracellular stores and the subsequent increase in cytoplasmic Ca + activates ATPase (proton pump). The binding of HA to the H2-receptor activates adenylate cyclase, resulting in an increase in cAMP, which activates the proton pump (11). 

Protonated cyclic GMP, which has an important biological role as a second messenger, has been studied by CPMAS NMR. ... 

---