Enzyme Kinetics and Mechanism of Action, Summaries of Medicine

Download Enzyme Kinetics and Mechanism of Action and more Summaries Medicine in PDF only on Docsity! Cabanilla, MED 2023 Overview of Metabolism and Provision of Metabolic Fuels A. Biomedical Importance Metabolism • interconversion of chemical compounds in the body • interrelationships of pathways cells to organ and organ to organ o Anabolic- synthesis of larger and more complex compounds from smaller precursors; endothermic (needs ATP) § Eg: protein from amino acids and the synthesis of triacylglycerol and glycogen from carbohydrates o Catabolic- breakdown of larger molecules (exothermic); Producing reducing equivalents. Mainly via respiratory chain (ATP) § E.g Oxidative reaction o Amphibolic: Crossroads; both anabolic and catabolic (E.g. TCA cycle) • essential for an understanding of abnormalities that underlie disease • metabolism in cells can not only perform their normal resting or basal metabolic functions but they can adapt to a changing environment • 70-kg adult human being requires about 8 to 12 MJ (1920-2900 kcal) from metabolic fuels each day, depend- ing on physical activity. • Caloric req increase when size of animal increase met by diet o Humans: Carbs (40-60%); lipids (30-40%); proteins (10-15%); alcohol • There is a constant requirement for metabolic fuels throughout the day. The resting or basal metabolic rate accounts for ~60% of daily energy expenditure in humans. o Physical activity -Total daily Energy expenditure increase metabolic rate by 40-50% over resting metabolic rate. o Not constant in 24 hours (average on equal ana and cata= net zero energy balance) o Calorie exceeds expenditureà stored as glycogen (liver or muscle) or TAG (adipose)à persistà obesity o Calorie is lower than expenditure: limited reserve of fat and CHOà oxidized AAà protein turnover increase that is normally recycledà protein synthesisà protein wasting, emaciationà death Overview of Substrate metabolism in fasting and Feasting § After an overnight fast the liver is the main source of glucose (~9 g/hr in humans) § glucose is derived from hepatic glycogen stores (via glycogenolysis) and synthesis of new glucose (via gluconeogenesis) § In humans over 60% (~6 g/hr) of the glucose released by the liver is metabolized by the central nervous system and red blood cells § Adipose tissue releases nonesterified fatty acids by hydrolysis of stored triglycerides § FA is oxidized to fuel heart, muscle and liver § liver can also synthesize ketone bodies from fatty acids to export to muscle and other tissues for oxidation; if reserves depleteà AA from net muscle protein breakdown (with prolonged fasting) + lactate via hydrolysis = gluconeogenesis à Glucose § Fed state o Ample supply of CHOà glucose consumer à stores as glycogen and small amount used for lipid synthesis o Glucose uptake by brain and RBC is unaltered o Release of FA from adipose tissues (Lipolysis) is suppressed and lenient in FA oxidation switch toglucose due to the decrease in fatty acid supply and increased glucose availability o Any that is not taken by liverà stored; in liver (glycogen) and in adipose (TAG) o Formation and mobilization of reserves of TAG and glycoge, and the extent of which cells will take up glucose are controlled by insulin and glucagon made by pancreas. o Because of absolute dependency of CNS on glucose, we have neuroendocrine system to protect low blood glucose level. B. Pathways to process the major products of our diet • Composition of the diet dictates the general metabolism by which the major products in our diet (CHO, lipids, and protein), are processed to their basic components suchs glucose, FA, glycerol, and AA. • If changes—> metabolic pathways can adapt to metabolize the nutrient • In ruminants, dietary cellulose is fermented by symbiotic micro- organisms to short-chain fatty acids (acetic, propionic, butyric), and metabolism in these animals is adapted to use these fatty acids as major substrates. • Products of digestion are oxidized to do to be ACETYL-COA, a common product which is then oxidized in TCA cycle. Carbohydrate Metabolism is centered on oxidaton & storage of glucose. Glucose is converted to glucose-6-phosphate using hexokinase and it will go to many fates. • pyruvate by the pathway of glycolysis-> pyruvate can be metabolized to acetyl-CoA-> citric acid cycle for complete oxidation to CO2 and H2O-> formation of ATP in the process of oxidative phosphorylation Cabanilla, MED 2023 • Pyruvate can provide for intermediates in TCA Cycle that can provide the carbon skeletons for synthesis of non essential amino acid. • Acetyl-coa from pyruvate is a precursor for FA synthesis and cholesterol (steroid hormones) • Anaerobic: pyruvate is converted to lactate as end product. Glucose and its metabolites also take part in other processes. • can be stored as a polymer called glycogen in skeletal muscle and liver • can be diverted to the pentose phosphate pathway (alternative to glycolysis) : source of reducing equivalents (NADPH) for fatty acid synthesis; source of ribose for nucleotide and nucleic acid synthesis • glycolytic pathway triose phosphate intermediates can give rise to the glycerol moiety of triacylglycerols • Some tissues can synthesize glucose from precursors such as lactate, amino acids, and glycerol by the process of gluconeogenesis (important when CHO is low or inadequate) Lipid Metabolism is Corcerned Mainly with FA & Cholesterol. • Long chain FA are either from dietary lipid or from acetyl-coa (from CHO or AA in lipogenesis) • FA may be oxidized to acetyl-CoA (β-oxidation) or esterified with glycerol, forming triacylglycerol as the body’s main fuel reserve • Stored TCA can be mobilized (lipolysis) -> release of nonesterified FA and glycerol Acetyl-CoA formed by β-oxidation of fatty acids may undergo three fates: 1. Oxidized to CO2 + H2O via the citric acid cycle 2. Synthesis of cholesterol and other steroids 3. Synthesize ketone bodies (acetoacetate and 3-hydroxybutyrate) in the liver Much of AA Metabolism involves in Transamination • Amino acids are required for protein synthesis • Essential amino acids: diet bcoz cannot be synthesized • Nonessential or dispensable amino acids: supplied in the diet, can also be formed from metabolic intermediates by transamination using the amino group from other amino acids • If carbon backbone is used for other processes, the alpha amino nitrogen must be removed by deamination, metabolized to urea in the liver, and excreted in kidney. The carbon skeleton remained maybe: 1. oxidized to CO2 IN TCA cycle; 2. Used for glucose synthesis via gluconeogenesis; and 3. fatty acid or ketone bodies • Several amino acids are also the precursors of other com- pounds, for example, purines, pyrimidines, hormones such as epinephrine and thyroxine, and neurotransmitters. C. Metabolic pathways at organ & cellular level. • Whole organism level: substrates are moved between organs that can either add or remove substrate to the blood perfusing organ. • Concentration of substrate entering and leaving can be measured to help describe how it moves between organs. • Depending on pathway: it could occur in cytosol (glycolysis) or be compartmentalized in subcellular organelles (TCA in mitochondria). The anatomical location of an organ & the blood circulation integrates metabolism. • When food is ingested, its either: o Taken up directly by portal vein to the liver. The taken up amount may be allowed to pass in systemic circulation. o packaged and secreted to lymphatic system, where substrates coalesce into common thoracic duct, bypasses the liver, and drains its content to systemic circulation. • AA and glucose from digestion à absorbed via hepatic portal vein • Glucose in fed state o 10-15% of absorbed glucose is taken up by liver and majority is used to synthesize glycogen via glycogenesis o some in FA synthesis via lipogenesis o remainder is broken down to glycolysis to generate pyruvate that can be oxidized in the citric acid cycle for pyruvate oxidation • glucose not taken up by the liver is oxidized by the brain and many other tissues including skeletal muscle • Between meals, the liver rapidly switches to a producer of glucose. It is the primary source of glucose in the fasted setting which is from 2 sources: o stored glycogen (glycogenolyis) o Other metabolites like lactate glycerol o AA via gluconeogenesis. Cabanilla, MED 2023 § Extracellular lipoprotein lipase is synthesized and activated in response to insulinà non-esterified FA is taken up by tissuesà used for synthesis of TAG while glycerol remains in the bloodstreamà taken up by liver for gluconeogenesis and glycogen synthesis or lipogenesis. § FA remaining in the bloodstream are taken up by the liver and re-esterified. § Remnants of chylomicron are cleared by the liver and the remaining TAG is exported with VLDL o Some excess may also be used in lipogenesis and TAG synthesis. It also inhibits intracellular lipolysis and release of non-esterified FA by adipose tissue o Chylomicrons: products of lipid digestion that enter the circulation; largest plasma lipoprotein; rich in TAG • In healthy individuals o catabolism and anabolism are equal in 24-hr period à whole body proteins are constant o protein synthesis is not constant: ↓ in fasting, ↑ in feeding in response to insulin o Cachexia: ↑ protein catabolism rate o Protein synthesis is energy expensive process (20% of resting E expenditure and 9% in fasting state) Metabolic Fuel Reserves are Mobilized in Fasting State • Plasma non-esterified FA ↑ in fasting, and rise a little in starvation. If prolonged, plasma concentration of ketone bodies ↑. • In fasting: as conc of glucose in portal blood from small intestine ↓, insulin secretion ↓, and skeletal muscle and adipose tissue take up less glucose. • ↑ glucagon secretion by alpha cells of the pancreas: o inhibits glycogen synthesis o activates glycogen phosphorylase o mobilize glycogen stores. o G-6-P is hydrolyzed by G-6-P phosphatase o Glucose is released in the bloodstream for use of brain and RBC • Muscle cannot contribute in plasma glucose concentration due to: o Lack of G-6-P phosphatase o Primary use is to provide a source of G-6-P phosphate and pyruvate for the muscle itself o But: acetyl-coA is formed by oxidation of FAà inhibits pyruvate dehydrogenaseà accumulation of pyruvate § Most are transaminated to alanineà AA arise § Alanine, lactate, and keto acid by transamination are exported from muscle and processed in the liver by gluconeogenesis o Muscle preferentially takes up non-esterified FA in a fasting state but this cannot supply its energy requirement by beta- oxidation o As fasting is prolongedàmore acetyl-coA can be oxidizedà used in ketone bodies synthesis (major metabolic fuels for heart and skeletal muscle and up to 20% of brain energy needs) • In adipose tissue: ↓ insulin and ↑ in glucagon lead to: o Inhibition of lipogenesis o Inactivation and internalization of lipoprotein lipase o Activation of intracellular hormone-sensitive lipase o This leads to the release of glycerol and non-esterified FA used by the liver, heart, and skeletal muscle H. CLINICAL ASPECTS • Extreme protein catabolism and not replaced (esp. essential AA)à Death • Cachexia o release of cytokines in response to tumors and disease o increased metabolic rate (in a state of starvation) • Fetus: high demand for glucose • Lactose synthesis in lactation can lead to ketosis • Ketogenic diets o used to treat intractable sustained relief with antiepileptic drugs and are not surgical epilepsy o carbohydrate intake is below the needs of the body à gluconeogenesis and high rate of fat oxidation o provides ketone bodies to the brain to support energy requirements not met by glucose • Diabetes o 3 common forms § T1DM § T2DM § Gestational Diabetes o T1DM § Autoimmune destruction of beta cellsà near complete loss of insulin secretion o T2DM § A combination of impaired sensitivity of tissue to insulin and impaired insulin secretion o Gestational diabetes § During pregnancy o Inadequate management can lead to organ dysfunction Cabanilla, MED 2023 § Blindness § Renal and CVS diseases § Fetal complications o In poorly controlled, patients can be severely hypoglycemic as: § Lack of or inadequate insulin secretion to restrain hepatic glucose production in fasted state § Impaired stimulation uptake of glucose by hepatic and peripheral tissues in fed state o Severe hyperglycemia can lead to diabetic ketoacidosisà medical emergency • Absence of insulinà o fuels gluconeogenesis o lipolysis in adipose tissues amplified by sympathetic NS and glucagon o increase FA delivery to the liver • excess glucagon: o increase FA supply to increase ketone bodies formation o utilization of ketone bodies may be impaired due to lack of oxaloacetate o amplify increase in circulating ketones (acetoacetate and 3-hydroxybutyrate: strong acids) o can result to coma from acidosis, increase osmolarity of extracellular fluid, and dehydration Reference: Kennelly, P. J., Botham, K. M., McGuinness, O. P., Rodwell, V. W., & Weil, P. A. (2023). Chapter 14: Overview of Metabolism & the Provision of Metabolic Fuels. In Harper’s Illustrated Biochemistry (32nd ed., pp. 132–146). essay, Mcgraw-Hill Education.