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21.12.2014
Although the absolute enzyme activities may differ by two or three orders of magnitude in different tissues, groups of related enzymes often maintain constant proportions to one another. Lactate is normally produced only by red blood cells and type 2B muscle fibres, which have very few mitochondria. Ketone bodies (acetoacetate, hydroxybutyrate & acetone) are produced in the liver during periods of rapid fat oxidation, when the rate of fat breakdown exceeds the capacity of the Krebs cycle to process the resulting acetyl CoA. Subcutaneous fat: the major fat depots are immediately beneath the skin, and packed around the viscera in the abdominal cavity. Great vessels above the heart: in healthy people about 70% of the blood normally resides within the veins, and red cells (erythrocytes) account for 40%-50% of the blood volume. Cardiac muscle: has the highest metabolic rate in the human body, and achieves the highest arteriovenous oxygen extractions. Colon: (large intestine) is mainly responsible for the resorption of water from the faeces.
Liver: is the major effector organ for the regulation of blood glucose and the processing of ingested food.
It is more efficient to concentrate major metabolic activities in a small number of tissues.
Use the mouse to point at the tissues illustrated on the right, to review details of their metabolic activities under different physiological conditions. Examples of key sensors with generalised metabolic effects: AMPK, glucose sensors in pancreas and brain, cytokine sensors in the brain. There are two different kinds of AMP, which is confusing for students because they are both involved in signalling. The first kind of AMP is ordinary linear 5' AMP which is formed through the myokinase reaction in the mitochondrial intermembrane space. This makes 5' AMP the ideal cellular emergency signal, indicating an emerging threat to the ATP supply long before serious trouble has developed. Both glycogen phosphorylase and phosphofructokinase are powerfully and directly activated by 5' AMP. In addition, 5' AMP binds to the protein kinase AMPK, favouring its activation by a variety of upstream kinases including the tumour suppressor gene LKB1. AMPK controls the overall balance between energy production and energy utilisation in all eukaryotic cells.
The following figure was compiled from two recent research papers [Canto et al (2009) and Rutter & Leclerc (2009)]. The second kind of AMP is 3'5' cyclic AMP, which is formed by adenyl cyclase in the plasmalemma under the control of G proteins.
Most cells can respond to a range of cytokines, but the hypothalamus is in a unique position, with overall responsibility for food and energy supplies, salt and water balance, temperature regulation, immune system activity and reproduction, and in ultimate command of both the immune system and the autonomic nervous system. These enzymes are often located at natural bottlenecks, and form the interface between major divisions of metabolism.
Pyruvate dehydrogenase: this important mitochondrial enzyme is the final "committed" step at the end of the glycolytic pathway. Oxoglutarate dehydrogenase (OGDH) in the Krebs cycle has an almost identical protein structure and reaction mechanism to PDH, but the regulation of OGDH and PDH are completely different. Pyruvate carboxylase is an intramitochondrial enzyme that is powerfully activated by acetyl-CoA, and in healthy people ensures that there are always sufficient Krebs cycle intermediates to meet the needs of the organism. Acetyl CoA carboxylase is one of the numerous enzymes that is phosphorylated and disabled by AMPK as part of its general strategy to switch off biosynthetic processes and switch on energy-yielding pathways. The most important processes that empty the Krebs cycle are gluconeogenesis in liver and kidney, and lipogenesis in liver and adipose tissue. Note: neurotransmitter release from synaptic vesicles can be very fast, where there is a premium on speed.
This seems so obvious that it is superfluous to mention it, except for the numerous misleading claims about regulatory systems that are actually close to equilibrium. PPAR-α stimulates transcription of fatty acid oxidation genes in mitochondria, peroxisomes and microsomes. PPAR-γ is necessary and sufficient for adipocyte differentiation from fibroblasts, but it also has roles in muscle and macrophages. CREB can be phosphorylated by cAMP-dependent protein kinase, after which it binds to an 8-nucleotide sequence termed the c-AMP response element (CRE) switching on transcription of gluconeogenic enzymes and PGC1. PGC1 is a transcriptional co-activator for PPAR-g that promotes mitochondrial gene expression in many tissues via NRF1 and NRF2. Leptin is a 16-kD secreted protein that acts mainly on the hypothalamus to inhibit food intake and stimulate energy expenditure. Adiponectin is a 33-kD secreted protein that stimulates energy metabolism, enhances insulin sensitivity, and reduces body weight.
Adipocytes secrete pro-inflammatory cytokines such as TNF-α which may also help to explain the association between obesity and inflammatory arterial disease.
Muscle glucose uptake during exercise can make a useful contribution to overall glycaemic control in type 2 diabetes.
Cardiac muscle is specialised for long term sustained energy production from a wide range of substrates. Voluntary muscles contain a variety of fibre types which are specialised for particular tasks. Type 1 skeletal muscle or slow oxidative fibres have a slow contraction speed and a low myosin ATPase activity. Type 2A skeletal muscle or fast oxidative-glycolytic fibres have a fast contraction speed and a high myosin ATPase activity. Type 2B skeletal muscle or fast glycolytic fibres have a fast contraction speed and a high myosin ATPase activity.
These differences are nicely illustrated by the serial sections from rat diaphragm published by Gauthier and Lowey (1979) J.
A fiendishly complex system of DNA binding proteins regulates gene expression(know of CREB and PPARs but do not learn any details). Individual enzyme activities may be markedly raised or lowered in particular organs, or coded by tissue-specific genes with unique regulatory properties. This suggests that they are controlled as a unit, and their genes may share regulatory motifs. Cori cycle between liver and skeletal muscles, ketone metabolism, amino acid metabolism in starvation. It is almost all recycled into glucose by the liver, and the overall process is known as the Cori cycle.


Acetone is produced by the non-enzymic decarboxylation of acetoacetate and may sometimes be smelt on the breath in acute diabetes. Both are present in high concentrations in blood plasma, and they are the principal amino acids released from skeletal muscle during starvation as the muscle wastes away. Fat cells arise from the fibroblast lineage, and consist of a central lipid droplet, surounded by a thin skin of active cytoplasm. Red cells have neither nucleus nor mitochondria, and their metabolism is restricted to a sluggish anaerobic glycolysis producing lactate and ATP. It is capable of limited amino acid metabolism and during fasting it releases mainly alanine and glutamine for further processing by the liver and the small intestine. Soluble compounds are delivered to the liver via the portal vein, while dietary fats are handled by the lymphatic drainage and delivered as chylomicrons into the left subclavian vein. Resistant polysaccharides that escape digestion in the ileum are fermented by the colonic bacteria mainly to form butyrate, which can be absorbed and processed by the liver. It is also responsible for the synthesis of blood proteins, cholesterol and VLDL, ureogenesis, and the formation of bile. It is shown here in the relaxed position when the abdominal contents have compressed and emptied the lungs.
It is protected from infection and environmental stress by the blood:brain barrier, which also prevents the access of large particles such as chylomicrons, VLDL and free fatty acids bound to serum albumin. This arrangement has been selected in all species, including humans, because specialisation permits higher local substrate concentrations and more rapid catalysis.
First of all, write down what you think will happen, then review your answers from the screen. Myokinase (also known as adenylate kinase) is freely reversible and the equilibrium constant is close to 1.0 so that cytosolic [AMP] is normally very small, but increases with the square of [ADP] if anything goes wrong with the power supply.
When 5' AMP rises, cells abandon activities that are not essential for survival, and concentrate on restoring ATP production by switching on glycogen breakdown, and glycolysis.
This emergency switch over-rides numerous other factors that regulate these important enzymes. This is currently the subject of intense research activity, since it appears that LKB1 plays an important role in apoptosis and is required for the action of the oral hypoglycaemic drug metformin. It shows the central role of AMPK activation in three central planks of modern type 2 diabetes therapy: diet, exercise and metformin.
The "take home message" is that AMPK is intimately involved in the most effective treatments for type 2 diabetes, even though the precise mechanisms remain to be elucidated. It is massively involved in many kinds of hormone actions all over the body and is much more than a pure emergency signal. There are additional sensors in the hypothalamus, and also near the solitary tract in the medulla, which are outside the blood-brain barrier.
Up to this point it is always possible to get back to glucose, but once through PDH there is no return. It is therfore necessary to have mechanisms for filling and emptying the cycle to cope with metabolic demands.
It has an almost identical mechanism to pyruvate carboxylase and produces malonyl CoA which is required for the biosynthesis of fats. It is only possible to regulate reactions that would otherwise proceed spontaneously with a large negative DG.
It is in any case incomplete because new regulatory enzymes are being discovered every day. Tissue selective gene expression, and the selective responses to cytokines depend on specific DNA binding proteins which regulate gene expression. Peroxisome proliferators are a diverse group of oils and lipids, used for example as drugs, plasticisers and industrial chemicals, which were found to increase the number of peroxisomes in cells. It is the nuclear receptor for fibrates, an important class of lipid-lowering drugs that are used to treat hypercholesterolaemia. It is the nuclear receptor for thiazolidinediones, a new class of oral anti-diabetic drugs.
In particular it stimulates expression of the mitochondrial uncoupling protein UCP1 and genes from the gluconeogenic pathway. It has a volatile glycogen store and it can switch rapidly between glycolysis to gluconeogenesis. The free fatty acids needed to make the triglyceride component in VLDL may be synthesised in the liver from carbohydrate precursors, or they may be derived from circulating free fatty acids bound to albumin.
They can be activated by many other tissues (including brain) and oxidised via acetyl-CoA to generate ATP. Genetic defects in human leptin production are extremely rare, but when present cause severe hereditary obesity. It is structurally related to collagen and forms triple helices and high molecular weight adducts. Such patients are strongly encouraged to lose weight through a combination of diet and increased exercise, with drug treatment and insulin only as the last resort. They are progressively recruited when additional effort is required, but are still very resistant to fatigue. These cells are thin (high surface to volume ratio) with a good capillary supply for efficient gas exchange. These are large cells with a poor surface to volume ratio and their limited capillary supply slows the delivery of oxygen and removal of waste products. It is mechanically difficult to get fats across the blood ? brain barrier, because the chylomicrons are too big, and free fatty acids are stuck to serum albumin. Type 2B muscle fibres are only recruited during very strenuous exercise, when blood lactate concentrations may rise sharply.
Hydroxybutyrate is an "honorary" ketone, because it is chemically related to acetoacetate, but it is in fact a secondary alcohol. Most of the other amino acids are converted into alanine and glutamine within the muscle fibres. In the fed state adipocytes respond strongly to circulating insulin with enhanced glucose uptake, associated with rapid glycolysis and the synthesis of triglyceride, thereby helping to stabilise the blood glucose concentration. At least three muscle fibre types are recognised in clinical practice, whose proportions vary in different muscles.
During fasting the ileum helps to convert glutamine released from skeletal muscle into blood glucose. There is a further advantage where most tissues are resting most of the time, because the available cardiac output, and hepatic support services (for example, lactate re-cycling) can be focussed towards "mission critical" activities which receive the lion's share of the available blood supply.


AMPK is also involved in obesity and weight regulation, and is activated by by the adipocyte hormone adiponectin described below.
Among many other enzymes, it phosphorylates and inactivates HMG-CoA reductase and acetyl CoA carboxylase (described below). Some neurons depolarise, while others hyperpolarise in response to changes in circulating glucose.
The key information is provided by insulin (size of last meal), leptin (size of food reserves), interleukin-1 (immune system activity) plus a host of less important signals that collectively report the overall status of the organism. Malonyl CoA also inhibits carnitine palmitoyl transferase 1 (CPT1, see lecture 19) and helps to control the rate of fat oxidation. If a process has almost reached equilibrium regulation will make no difference to the outcome. It is here for reference, and lists some of the important regulatory enzymes governing metabolism in mammalian liver, and the types of control that are observed. There may be literally thousands of the these proteins, which are themselves expressed in a tissue selective fashion. They often have branched chains or other unusual chemical structures and some of them cause cancers. Adiponectin binds to cell surface receptors which ultimately stimulate AMPK-kinase and thereby phosphorylate and activate AMPK.
Muscle can partially degrade amino acids to Krebs cycle intermediates, but has no urea cycle, so it must transaminate the end products and tends to export alanine and glutamine to the blood. The pattern of gene expression within each voluntary muscle cell is governed by the firing pattern of its single motor neurone.
In the figure above, the left hand section was stained for the mitochondrial enzyme succinate dehydrogenase, the centre panel shows direct immunofluorescence against "fast" type myosin, and the right hand section was stained for alkali-stable ATPase activity (i.e. Under these conditions some of the surplus lactate may be oxidised by highly aerobic tissues, such as heart muscle. The preferred substrates are free fatty acids and ketones, but heart muscle can also use triglycerides, lactate and even glucose if insulin is present. Type 1 fibres are rich in mitochondria and myoglobin and have an aerobic fat- or ketone-based metabolism. At least some of these nerve cells, which depolarise in response to rising glucose, appear to use the same sensing mechanism as β-cells, in that they rely on glucokinase and ATP-sensitive potassium channels to initiate signalling.
PDH is a huge multi-enzyme complex and the reaction mechanism involves about half of the B group vitamins (see Nelson & Cox, page 569) Which B vitamins, exactly? Consider a large hydro-electric scheme: do we put the sluice gates in the concrete dam, or in the middle of the lake, or downstream on the way out to the sea?
Their common feature is that they are difficult to oxidise using the conventional mitochondrial pathways, and need a preliminary softening up in the peroxisomes. Contrast this with chylomicrons, which are produced by intestinal cells and are only found in the circulation after a fatty meal. Motor neurones branch within their target muscle and thereby control several muscle fibres, called a motor unit.
They are built for aerobic metabolism and can use either glucose or fats as a source of energy.
Ketone bodies cannot be metabolised any further by the liver, but they are a useful source of energy for extra-hepatic tissues (including brain) during starvation. In the fasting state adipocytes respond to circulating glucagon and adrenalin (and to noradrenalin released from local nerves) with rapid lipolysis catalysed by hormone-sensitive lipase followed by the release of free fatty acids and glycerol into the blood.
They are fatigue resistant, have a slow twitch speed and are recruited first during physical activity.
Leptin has signifcant effects on the male and female reproductive systems, and may be involved in the menarche and in hypothalamic amenorrhea due to strenuous exercise or low weight. Adiponectic receptors also increase PPAR-α but the detailed signalling pathways are unknown.
Muscles need insulin to use glucose at low work loads, but the requirement is relaxed during vigorous exercise. The high precision eye muscles have only a few fibres in each motor unit, but the muscles in your back have thousands. These are general purpose muscle fibres which give the edge in athletic performance, but they are more expensive to operate than type 1. Notice the differences in the fibre diameters, which correlates with their requirements for efficient gas and substrate exchange.
Type 2A fibres have a similar aerobic metabolism, but they are fast twitch, fatigue-resistant fibres which can use either fats or glucose. One major effect is a massive outpouring of adrenalin if blood glucose concentration falls, but they are also able to directly control hepatic metabolism via autonomic nervous connections. Adiponectin can be taken up by the brain, which responds by reducing food intake and increasing whole body energy expenditure. If the blood concentration of ketone bodies exceeds the renal threshold (ketonuria) during acute diabetes then the resulting disturbance of salt and water balance may prove rapidly fatal.
They give the edge to athletic performance and are recruited in addition to the type 1 fibres when more effort is demanded. Adiponectin opposes some of the biochemical effects of TNF-α and has anti-inflammatory actions on blood vessel walls.
Type 2B fibres are fast contracting anaerobic fibres that are only recruited when a maximum effort is required. Leptin has a variety of other functions, including the regulation of hematopoiesis, angiogenesis, bone formation, wound healing, and the immune and inflammatory response. The leptin receptor belongs to the cytokine receptor family and has a single-transmembrane-domain. Adiponectin output normally falls with increasing fat cell mass in otherwise healthy patients.
Receptors are widely distributed and occur in alternatively spliced forms, which may mediate different actions. This effect may contribute to the insulin resistance and inflammation associated with obesity, but adiponectin is unexpectedly low in anorexic and bulimic patients.



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