Systems, Cells, Molecules and Genes Lecture 2 PDF

Systems, Cells, Molecules and Genes (and back again) Lecture 2 Dr Clare Rollie  Aims - 2 Discuss how the direction and speed of reaction can be controlled in a metabolic pathway. When this goes wrong - Appreciate the prevalence and...

Systems, Cells, Molecules and Genes (and back again) Lecture 2 Dr Clare Rollie  Aims - 2 Discuss how the direction and speed of reaction can be controlled in a metabolic pathway. When this goes wrong - Appreciate the prevalence and severity of diabetes. Give an example of altered carbohydrate metabolism in another animal species. Link to Padlet for anonymous questions Where we got to yesterday...  Phosphorylation Control of Glucose Metabolism This is a key metabolic control process. Multiple enzymes are phosphorylated or dephosphorylated in response to insulin or glucagon. We will look at a single step - but recognise that others are controlled by similar kinds of mechanisms. Glycogen Synthase/Glycogen Phosphorylase  INSULIN Glucose-1-phosphate ON OFF Glycogen Synthase Glycogen Phosphorylase Glycogen  Glucagon Glucose-1-phosphate OFF ON Glycogen Synthase Glycogen Phosphorylase Glycogen Glucagon  OFF Glucagon ON Example of hormone causing phosphorylation of BOTH enzymes  but it turns one ‘off,’ and one ‘on’ ON INSULIN OFF  Insulin Glucagon Increased activity of glycogen Decreased activity of glycogen synthase, reduced activity of synthase, increased activity of glycogen phosphorylase = net glycogen phosphorylase = net store of glycogen. breakdown of glycogen. Gluconeogenesis in the liver is Gluconeognesis in liver is suppressed. increased Turns ON glycolysis; turns OFF Turns OFF glycolysis; turns ON gluconeogenesis. gluconeogenesis. Turns ON glycolytic enzyme gene Turns OFF glycolytic enzyme gene expression, turns OFF expression, turns ON gluconeogenic enzyme gene gluconeogenic enzyme gene expression expression Blood glucose levels fall Blood glucose levels rise  Insulin Glucagon Increased activity of glycogen Decreased activity of glycogen synthase, reduced activity of synthase, increased activity of glycogen phosphorylase = net glycogen phosphorylase = net store of glycogen. breakdown of glycogen. Gluconeogenesis in the liver is Gluconeognesis in liver is suppressed. increased Turns ON glycolysis; turns OFF Turns OFF glycolysis; turns ON gluconeogenesis. gluconeogenesis. Turns ON glycolytic enzyme gene Turns OFF glycolytic enzyme gene expression, turns OFF expression, turns ON gluconeogenic enzyme gene gluconeogenic enzyme gene expression expression Blood glucose levels fall Blood glucose levels rise Revision question Why does phosphorylation cause a change in enzyme activity?  General themes: Biosynthetic and degradative pathways are almost always distinct. This means that both pathways can be thermodynamically favourable. The rates of metabolic pathways are governed by the activities of key enzymes (and not by mass action). Glycogen formation/breakdown is a perfect example of this.  General themes: Biosynthetic and degradative pathways are almost always distinct. This means that both pathways can be thermodynamically favourable. The rates of metabolic pathways are governed by the activities of key enzymes (and not by mass action). Glycogen formation/breakdown is a perfect example of this. Specific hormones induce specific event in cells/tissues. Allosteric modulation of enzyme activity. Reversible covalent modification is a well-used example. In cases where the direction of a metabolic pathway has to be reversed, the pathway is controlled at an irreversible step. Irreversible step What do enzymes do to the energy landscape of a reaction? What is a rate determining step? Enzymes lower activation energy for reactions…. The rate determining step is the slowest step of a chemical reaction that determines the speed (rate) at which the overall reaction proceeds. The rate determining step can be compared to the neck of a funnel. This is true in metabolic pathways. Often represents a key control point (logical).  Do not learn The rate limiting steps in the glycolytic this, you will pathway are: (i) the phosphorylation not be tested of glucose by hexokinase or on this figure. glucokinase; (ii) the phosphorylation of fructose-6-phosphate to form fructose-1,6-bisphosphate by bisphosphate fructose-6-phosphate kinase. These are the rate limiting The generation of fructose-1,6- steps –red arrows bisphosphate by phosphofructokinase-1 is a key Primary control points regulatory point in the pathway and is also the rate-limiting step. Because: They are essentially The reaction is coupled to the irreversible hydrolysis of ATP and is, in essence, irreversible. Hence a different pathway must be used to do the reverse conversion during gluconeogensis.  Energy landscape of both favourable Mechanism is quite distinct Glycolysis Do not learn this, you will not be tested on this figure. It’s not just phosphorylation that can control the activity on an enzyme Enzyme can be controlled by allosteric interactions with other molecules Often some of the other molecules and intermediates in the downstream pathway Molecules which potentiate one direction (glycolysis) are often negative regulators of the other direction (gluconeogenesis) Main message: Allosteric regulation can be used to superimpose other control pathways on top of a metabolic pathway  Insulin and Glucagon Both are polypeptide hormones released from the pancreas. Both bind specific receptors enriched in muscle, liver and fat cells but have opposing actions. I will use these as a paradigm for – Coordination of metabolism – Regulated membrane trafficking (insulin release, GLUT4 trafficking) – Receptor structure and function – Cell signalling processes  Global Epidemic of Diabetes Diabetes is a growing and massive silent epidemic that has the potential to cripple health services in all parts of the world. -Director of Diabetes Action Now ~415 million sufferers worldwide from diabetes >6 x estimates of 10 years ago. This figure is likely to more than double by 2050. >4 million deaths (9% global total) can be attributed to diabetes each year. Diabetes is a risk factor for other disease – COVID-19 Dysregulation of glucose homeostasis Cost to UK (4 million diabetics) is £1million per hour Warning – graphic image  Kidney failure Stroke Macular degernaation Foot ulcers Fatty liver Atherosclerosis  The underlying nature of diabetes Type 1 Type 2 – Insulin resistance – β-cell destruction – β-cell dysfunction – Autoimmune or idiopathic 10% 90%  Diabetes Mellitus Type-1 diabetes is caused by destruction of pancreatic β-cells due to an autoimmune process or unknown aetiology (idiopathic). Insulin is not produced. Type-2 diabetes results from a defect in insulin action, almost always with insulin resistance as a root cause. Mechanism poorly understood. Risk factors include: obesity, sedentary lifestyle, age, diet and GENETICS Inflammation  Genetics, Ethnicity and Diabetes Half of all South Asian, Black African and African Caribbean people in the UK will develop Type-2 diabetes by age 80. For Europeans, the figure is 20%. South Asian men are typically 5 years younger on diagnosis and have increased risk of all complications compared to other ethnic groups. Kevin O’Dell lectures will go into genetic and environmental interactions and disease  How can we cure/treat the disease? First we need to understand the biology. In humans and also in other  Mouse models of diabetes Important in studying/manipulating metabolic pathways and drug testing. Most involve obese mice. Single gene mutation – usually in leptin (Lepob/ob) Leptin signals satiety – mutations cause hyperphagia and hyperglycaemia. Homework – come up with some pros and cons of using this mouse model as a study system to learn more about diabetes in humans. Post on the forum or in tomorrow’s lecture  Hibernation – a state of altered metabolism Hibernating animals store massive amounts of fat each autumn. These serve as the main source of metabolic fuel over-winter. Fats are energy rich and have the advantage of generating metabolic water as they are catabolized. This is entrained by the length of daylight.  Cycles of high calorie intake (20,000/day), obesity, and long periods with no exercise In humans would predispose to T2D Why not in bears? Bears can turn on/off insulin resistance to maintain blood sugar. Insulin and glucose levels in blood remain stable all year. Gene expression changes responsible. 8 key genes identified (Akt), some not previously associated with glucose homeostasis. Homologs of these genes in humans? Migratory birds Some birds fly huge distances during their annual migration, often over water requiring constant flight for up to 60 h or more at speeds approaching 40 kph. This is made possible by accumulation of large fat deposits that then are efficiently and selectively mobilised during the flight. Hummingbirds have a high sugar diet and high blood sugars – remain ‘healthy’ Migratory birds Some birds accumulate up to 0.15g of trigyceride/day/g body weight. For a typical human, this would represent 10kg/day! The birds are obese prior to migration! But flight muscles increase in size markedly too. Studying metabolics profile may help us understand metabolic disease and diabetes in humans.  Summary direction and speed of reaction can be controlled in a metabolic pathway – Irreversible, rate limiting step – Reciprocal regulation of enzymes – allosteric modification When this goes wrong – disease Need to understand insulin production and signalling to work on a cure – animals as models