MB ChB Year 1: Nutrition and Energy

Lecture 23: Mobilisation of food reserves [1] - catecholamines & glucagon.

Dr John Illingworth

Click here to download the printed handout sheet for this lecture

In the first of two lectures on the mobilisation of food reserves, we will consider some of the short-term mechanisms that normally provide energy to the body. These are of particular importance in the metabolic adaptation to physical exercise, which may start suddenly and without warning. Many of them are based on catecholamine messengers and the autonomic nervous system, although the peptide hormone glucagon sometimes acts in a similar way. Direct delivery of neurotransmitters to particular target tissues via the autonomic nerves is of comparable importance to the systemic delivery of adrenalin via the blood.

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Point with the mouse to refresh your knowledge of the autonomic nervous system

eye tear mouth heart airways skin coeliac sup mes inf mes bladder female male adrenal kidney ureter liver spleen stomach duodenum pancreas ileum colon pancreas symp chain

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Signalling systems: G proteins, adenyl cyclase & phospholipase C.

Many different kinds of serpentine hormone receptor located in the plasmalemma signal via heterotrimeric G proteins, all of which require GTP. In the resting state the trimeric G protein contains bound GDP and is inactive. Contact with the hormone receptor plus ligand leads to the replacement of GDP by GTP and the dissociation of the trimer into free Ga subunits containing bound GTP and a pair of bg subunits. The Ga subunits may be either stimulatory or inhibitory (depending on the signalling system) but they all possess an intrinisic GTPase activity which self-terminates their action. Once the GTP has been hydrolysed, the Ga subunit must re-associate with a bg pair before the process can be repeated.

Gs and Gq are separate varieties of stimulatory Ga subunits activate two important signalling pathways: adenyl cyclase and phospholipase C.

Adenyl cyclase is an enzyme that is normally bound to the inner surface of the plasmalemma where it converts tiny amounts of ATP into 3'5' cyclic AMP. It is activated by Gs subunits. The cAMP is involved in b adrenergic signalling and in many other hormonal pathways. cAMP activates protein kinase A, and has numerous effects within cells, as explained below.

Phospholipase C (PLC) is also bound to the plasmalemma where it hydrolyses inositol containing phospholipids to yield the water soluble inositol triphosphate (IP3) and diacylglycerol (DAG), which is hydrophobic and remains within the membrane. PLC is activated by Gq subunits. IP3 and DAG are involved in a1 adrenergic signalling. IP3 releases calcium ions from intracellular stores, and these subsequently activate protein kinase C as explained below.

Gi is an inhibitory Ga subunit that blocks the action of adenyl cyclase and is involved in a2 adrenergic signalling systems.

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Protein kinases: PKA, PKB, PKC & PKG

Protein kinase A: (PKA) is the "original" cyclic AMP dependent protein kinase, that was first identified in connection with glycogen metabolism, and is now known to phosphorylate dozens of proteins involved in b adrenergic signalling. Some examples of these numerous proteins are discussed in greater detail below.

Protein kinase B: (PKB) is a phosphatidyl inositol 3,4,5 triphosphate (PIP3) dependent protein kinase, which is involved in insulin signal transduction. PIP3 is a phospholipid within the plasmalemma: do not confuse it with inositol triphosphate (IP3) which is a water soluble second messenger.

Protein kinase C: (PKC) is a calcium-dependent protein kinase that is normally bound to the inner surface of the plasmalemma and also requires diacylglycerol. It forms part of the IP3 signalling cascade.

Protein kinase G: (PKG) is a cyclic GMP dependent protein kinase which is believed to mediate many of the actions of cyclic GMP.

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Wrong kind of AMP...

There are two different kinds of AMP, which is confusing for students because they are both involved in signalling.

In the previous lecture we considered linear 5' AMP which is in equilibrium with ATP and ADP via the myokinase reaction in the mitochondrial intermembrane space, and which activates the important regulatory enzyme AMPK.

In this lecture we will consider 3'5' cyclic AMP, which is formed by adenyl cyclase in the plasmalemma under the control of G proteins.

ATP 3'5' cyclic AMP + pyrophosphate

Cyclic AMP activates protein kinase A (PKA). It is involved in a very wide variety of hormone actions. It is not just for catecholamines and it is much more complex than a pure emergency signal.

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Catecholamine receptors:

It is possible to distinguish four main classes of catecholamine receptor through their differing sensitivities to agonists and inhibitors. These receptor proteins are the products of different genes, and differ in their signalling mechanisms. Receptors for the peptide hormone glucagon also signal via the cyclic AMP system, so that many of the metabolic effects of glucagon are very similar to the catecholamines.

receptor (& mechanism) a1
(IP3 / DAG)
(less cAMP)
(more cAMP)
(more cAMP)

blood vessels










GI non sphincter





GI sphincter





bladder detrusor





bladder sphincter










seminal tract





iris (radial)





ciliary muscle






pacemaker cells





Purkinje fibres





ventricular muscle












K+ release








lipolysis (β3)





less release

more release




less release



salivary gland


K+ release


amylase secretion








mast cells





less release

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Mobilising lipase in adipose tissue

Sympathetic nerves and circulating adrenalin activate mobilising lipase in adipose tissue, by the classical mechanism involving b3 receptors, Gs, adenyl cyclase, cAMP and PKA. In white adipose tissue this results in the release of free fatty acids and glycerol into the bloodstream, but in brown adipose tissue the free fatty acids uncouple the mitochondria and the lipids are oxidised in situ. Other tissues (e.g. heart and type 1 muscle fibres) with internal fat stores also mobilise triglyceride for local use under these conditions. The glycerol released from triglyceride stores is metabolised in the liver and becomes an important source of blood glucose.

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Glycogen synthase and glycogen phosphorylase

These two enzymes catalyse a potentially futile cycle, and are regulated in a reciprocal fashion. Both enzymes can be converted between active and less active forms using a system of protein kinases.

Hormones such as glucagon and adrenalin that promote glycogen breakdown act partly via cyclic AMP and protein kinase A. PKA phosphorylates and activates another protein kinase called phosphorylase b kinase, which in turn activates glycogen phosphorylase. This results in glycogen breakdown to yield glucose 1 phosphate. At the same time, PKA also phosphorylates and inactivates glycogen synthase, which prevents the futile cycling of glucose 1 phosphate back into glycogen via UDP-glucose.

Phosphorylase b kinase also contains a calcium-binding subunit called calmodulin. In addition to the b effects mediated via cAMP, a1 agonists (and several other signals) also promote glycogen breakdown by raising the intracellular calcium concentration, and activating phosphorylase by the calmodulin route.

Insulin antagonises the actions of adrenalin and promotes glycogen synthesis. As previously explained in lecture 20, phosphorylated insulin receptor substrate 1 (IRS-1) activates PI-3 kinase, leading to the synthesis of the phospholipid PIP3 or phosphatidyl inositol 3,4,5 trisphosphate. This lipid indirectly activates protein kinase B, which converts glycogen synthase kinase 3 (GSK3) from the active to the non-active form. Phosphorylated GSK3 is not able to phosphorylate and inactivate glycogen synthase, leading to increased glycogen deposition in the presence of insulin.

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Phosphofructokinase regulation

Phosphofructokinase (PFK1) illustrates many features of eukaryotic regulation. This was one of the first allosteric enzymes to be discovered and it is a major control point for the glycolytic pathway. The enzyme is allosterically activated by 5'AMP and inhibited by citrate. This helps to ensure a stable energy supply and to control the conversion of carbohydrates into Krebs cycle intermediates and fats, as explained in detail below. [It is also inhibited by ATP, but this is of doubtful physiological significance.]

Liver (L-type) and muscle (M-type) PFK subunits form mixed tetramers, but the genes are on separate chromosomes and their relative importance varies in different tissues. The remainder of this discussion applies mainly to liver cells, where the gluconeogenic pathway may be in operation, via fructose-1,6-bisphosphatase (FBPase). This enzyme is also present in muscle tissue, but its activity is very much lower. PFK and FBPase catalyse a potential futile cycle, and appropriate regulatory mechanisms are required.

In addition to the allosteric regulation by 5'AMP and citrate, PFK is strongly activated by fructose-2,6-bisphosphate which is produced by a second enzyme called phosphofructokinase-2 (PFK2). Conversely, fructose-1,6-bisphosphatase is inhibited by fructose-2,6-bisphosphate, which consequently sets the switch in favour of glycolysis, and blocks gluconeogenesis.

PFK2 is known as "tandem enzyme" because it also possesses fructose-2,6-bisphosphatase activity. The ratio between the two antagonostic enzyme activities is controlled by adrenalin and glucagon via adenyl cyclase, 3'5' cyclic AMP and protein kinase A. Phosphorylation of tandem enzyme converts it into the fructose-2,6-bisphosphatase form. This enables adrenalin and glucagon to switch on gluconeogenesis in liver tissue and stablise the blood glucose concentration.

The residual expression of FBPase in non-gluconeogenic tissues such as skeletal muscle suggests that some FBPase activity is important for normal PFK regulation. Newsholme has suggested that a low level of futile cycling may be important for effective regulation at low glycolytic fluxes, and allows the pathway to be controlled with much smaller excursions in the concentrations of the allosteric effectors.

Insulin increases PFK gene expression in the longer term and PFK activity is depressed in type 1 diabetics. Fructose bisphosphatase and PFK are reciprocally regulated, and corticosteroids antagonise the insulin effects, although the detailed molecular mechanisms have yet to be elucidated.

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Pyruvate kinase and PEP carboxykinase

These enzymes also catalyse a potential futile cycle, although it is a more circuitous process than the examples considered above. PEPCK is not expressed in most extra-hepatic tissues, so the "problem" is largely confined to the liver, where the regulation incorporates some special features that are described below.

As with phosphofructokinase (described above) this system in liver has two stable states, which depend on cyclic AMP:

  1. A glycolytic - lipogenic state, dominated by insulin, with LOW cyclic AMP. This converts surplus blood glucose into fat after feeding.

  2. A lipolytic - gluconeogenic state, dominated by adrenalin and glucagon, with HIGH cyclic AMP. This makes new glucose during fasting.

Insulin reinforces the glycolytic - lipogenic state by inducing the key cytosolic enzymes involved in fat synthesis: not only fatty acid synthase itself, but all the ancilliary enzymes such as ATP:citrate lyase, acetyl CoA carboxylase and "malic" enzyme that supply essential cofactors for the synthetic process.

The pyruvate kinase variant expressed in liver tissue has an extended amino acid sequence that is a substrate for protein kinase A. Phosphorylation inactivates liver pyruvate kinase, but the isoenzyme in other tissues is not regulated in this way. Phosphorylation in the presence of cyclic AMP ensures that all the phosphoenolpyruvate generated by liver PEPCK will be diverted into the gluconeogenic pathway, where it can contribute to blood glucose, instead of being converted into fat or oxidised via the Krebs cycle, as would otherwise occur.

In addition, expression of the cytosolic PEPCK gene is strongly and rapidly induced by cyclic AMP. This involves a nuclear protein called CREB (cyclic AMP response element binding protein) which is a substrate for protein kinase A. When phosphorylated, CREB binds to specific DNA sequences upstream of the cAMP regulated genes, and switches on transcription. This ensures that key cytosolic enzymes in the gluconeogenic pathway will be produced in appropriately larger amounts when hepatic glucose production is stimulated by cyclic AMP.

Human liver contains a second mitochondrial isoenzyme of PEPCK. This is the product of a separate gene, located in the nucleus but delivering its ultimate product to the mitochondria, which is not regulated in the same way as the cytosolic version.

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