Membrane-bound enzymes adenylate cyclase

Stimulus for phosphorylation is hormone interaction (glucagon or epinephrine) with a cellular receptor, which sends a signal via a G protein to the membrane-bound enzyme, adenylate cyclase. 

Cyclic AMP is synthesized by the membrane-bound enzyme adenylate cyclase which is present in almost all mammalian tissues. Hormonal regulation of adenylate cyclase is initiated by the hormone binding to cell surface receptors. The resulting hormone-receptor complex Interacts with either stimulatory regulatory proteins, (Ns), or inhibitory regulatory proteins, (Nl), which bind guanine nucleotides and Interact with the catalytic subunit (C) of adenylate cyclase to modulate the enzyme activity. 

IP3 and DAG are derived from the membrane lipid phosphatidylinositol 4,5-bisphosphate (which is a phosphorylated derivative of phosphatidylinositol see Topic El) by the action of phospholipase C which is also located in the plasma membrane and, like adenylate cyclase, is activated by G proteins (Fig. 5). One of the main actions of the polar IP3 is to diffuse through the cytosol and interact with Ca2+ channels in the membrane of the ER (Fig. 5), causing the release of stored Ca2+ ions which in turn mediate various cellular responses. The DAG produced by the hydrolysis of phosphatidylinositol 4,5-bisphosphate, along with Ca2+ ions released from the ER, activates protein kinase C, a membrane-bound enzyme that phosphorylates various target proteins, again leading to alterations in a variety of cellular processes (Fig. 5). 

When epinephrine binds to cells, it stays outside on the membrane-bound receptor. The second messenger, cyclic AMP, is made by the enzyme adenylate cyclase. 

The message is now carried by the alphas subunit which interacts with a membrane-bound enzyme called adenylate cyclase and switches it on. This enzyme then catalyses the synthesis of the secondary messenger—cyclic AMP—which moves into the cell s cytoplasm and proceeds to activate enzymes called protein kinases. 

Adenylate cyclase is a membrane-bound enzyme that, after stimulation by a G protein, catalyzes formation of cAMP from ATP (see below). cAMP, thus formed, stimulates activation of a kinase cascade that affects several metabolic pathways, such as glycogen metabolism (see here) and gluconeogenesis. Adenylate cyclase plays an important role in signal transduction. 

The Catalytic Site - The adenyl cyclase of rat liver plasma membranes is a membrane bound enzyme which catalyzes the conversion of ATP (or AMP-PNP see above) to cyclic AMP. A divalent cation, Mg+ or Mn, is required at the catalytic site, probably in the form of an ion-substrate complex, but the system is inhibited by Ca" ". Under standard assay conditions, with or without glucagon, the time course of enzyme activity is linear for at least 10 minutes and extrapolates to the origin. Activity is proportional to membrane concentration up to 1 mg of membrane protein per ml, ... 

Ptd-Ser is thought to play a key role for the activity of various membrane-bound enzymes. Recently, this lipid has been demonstrated to take part in the glucagon-responsiveness of adenylate cyclase in liver (Pohl et al., 1971) and heart (Levey, 1971), in the activation of tyrosine hydroxylase in rat brain striatal synapto-somes (Raese et al., 1976), in the increased turnover of dopamine in brain (Toffano Leon, this volume) and in other functional parameters. 

Figure 6.3. Mechanism of action of heterotrimeric G-proteins. Upon receptor occupancy, the Ga-subunit binds GTP in exchange for GDP, and then moves in the membrane until it encounters its target enzyme, shown here as adenylate cyclase (alternatively, a phospholipase). The activated target enzyme then becomes functional. Inherent GTPase activity within the a-subunit then hydrolyses bound GTP to GDP, and the a-subunit dissociates from its target enzyme (which becomes inactive) and rebinds the / - and ysubunits. Upon continued receptor occupancy, further catalytic cycles of GTP exchange and target enzyme activation may occur. The scheme shown is for a stimulatory G-protein (Got,), but similar sequences of events occur with inhibitory G-proteins (Gcx,) except that the interaction of the a-subunit with adenylate cyclase will result in its inhibition. The sites of action of pertussis and cholera toxins are shown.

Metabolism. The nucleotide cAMP (adenosine 3, 5 -cyclic monophosphate) is synthesized by membrane-bound adenylate cyclases [1] on the inside of the plasma membrane. The adenylate cyclases are a family of enzymes that cyclize ATP to cAMP by cleaving diphosphate (PPi). The degradation of cAMP to AMP is catalyzed by phosphodiesterases [2], which are inhibited by methylxanthines such as caffeine, for example. By contrast, insulin activates the esterase and thereby reduces the cAMP level (see p. 388). 

With regard to its effect on neurotransmitter function, alcohol increases adenylate cyclase activity, possibly via the membrane-bound G protein complex. The effect of alcohol on the secondary messenger system appears to depend on its location the noradrenaline-linked cyclase in the cortex seems to be directly affected by the drug, whereas the dopamine-linked enzyme in the basal ganglia appears to be altered by a combination of changes in the membrane fluidity, together with those in the G protein-cyclase complex. 

---