Metabolism of Nutrients I (Carbohydrates) PDF

Metabolism of nutrients Konstantinos Ekmektzoglou MD, PhD, FEBGH Assistant Professor School of Medicine European University Cyprus October 2024 Carbohydrates Lipids Proteins Metabolism the sum of all energy-requiring and energy-consuming processes of the body Many factors con...

Metabolism of nutrients Konstantinos Ekmektzoglou MD, PhD, FEBGH Assistant Professor School of Medicine European University Cyprus October 2024 Carbohydrates Lipids Proteins Metabolism the sum of all energy-requiring and energy-consuming processes of the body Many factors contribute to overall metabolism lean muscle mass amount and quality of food consumed physical demands placed on the human body RELEASE OF ENERGY FROM FOODS Many of the chemical reactions in the cells are aimed at making the energy in foods available to the various physiological systems of the cell Muscle activity Secretion by the glands Maintenance of membrane potentials by the nerve and muscle fibers Synthesis of substances in the cells Absorption of foods from the GI tract And the list goes on…………. Coupled Reactions Free Energy ΔG All the energy foods can be oxidized in The amount of energy liberated by the cells ► large amounts of energy complete oxidation of a food released (heat) Usually expressed in terms of calories Energy needed by the physiologic per mole of substance processes of the cells is not heat but energy to cause ΔG liberated by complete oxidation of 1 mole (180 grams) of glc is 686,000 mechanical movement calories concentrate solutes effect other cell functions. To provide this energy, the chemical reactions must be “coupled” with the systems responsible for these physiologic functions Accomplished by special cellular enzymes and energy transfer systems Adenosine Triphoshate (ATP) Energy currency of the body ATP is present everywhere in the cytoplasm and nucleoplasm of all cells All physiological mechanisms that require energy for operation obtain it directly from ATP In turn, the food in the cells is gradually oxidized, and the released energy is used to form new ATP, thus always maintaining a supply of this substance All these energy transfers take place by means of coupled reactions Carbohydrates Lipids Proteins Central Role of Glucose in Carbohydrate Metabolism 80% Glc thus becomes the final common pathway for the transport of almost all carbohydrates to the tissue cells Where? 95% of all the monosaccharides that circulate in the blood are glc (the final conversion product) Transport of Glucose Through the Cell Membrane Before glc can be used by the body’s tissue cells, it must be transported through the cell membrane into the cellular cytoplasm Insulin Increases Facilitated Diffusion of Glucose Glc cannot easily diffuse through the pores of the cell membrane (due to tis high molecular weight of 180) Yet glc does pass to the interior of the cells with a reasonable degree of freedom by the mechanism of facilitated diffusion (carrier-mediated diffusion) HOWEVER IN THE GI TRACT The transport of glc IS DIFFERENT active sodium-glucose co-transport, in which active transport of sodium provides energy for absorbing glc against a concentration difference WHAT HAPPENS IMMEDIATELY UPON GLC ENTRY INTO CELLS? The phosphorylation of glucose is almost completely irreversible except in liver glucose phosphatase, when activated, can kidneys reverse the reaction Intestine WHAT HAPPENS IMMEDIATELY UPON GLC ENTRY INTO CELLS? glc can be used immediately stored for release of energy in the form of to the cell glycogen 5%-8% percent of their weight 1%-3% percent of their weight Glycogenesis—Formation of Glycogen Glycogenolysis—Breakdown of Stored Glycogen Activated by epinephrine or glucagon Release of Energy From Glucose - the Glycolytic Pathway Splitting Glucose to Form Pyruvic Acid By far the most important means of releasing energy from the glc molecule 10 successive chemical reactions Each step is catalyzed by at least one specific protein enzyme The end products of glycolysis are oxidized to provide energy A total of 38 moles of ATP for each mole of glc metabolized by the cells HOWEVER, THE NET GAIN IN ATP MOLECULES IS ONLY 2 MOLES FOR EACH MOLE OF GLC UTILIZED  WHAT HAPPENS NEXT TO PYRUVATE? Conversion of Pyruvic Acid to Acetyl Coenzyme A no ATP is formed BUT, but up to six molecules of ATP are formed when the 4H atoms are later oxidized  AND THEN WHAT? citric acid cycle OR tricarboxylic acid cycle OR Krebs cycle C Y C sequence of chemical reactions in which the L acetyl portion of acetyl-CoA is degraded to E CO2 and H WHERE? WHAT HAPPENS TO H ATOMS? Glycolysis – 4 H atoms a total of 24 H atoms are pyruvic acid to acetyl-CoA – 4 H atoms released for each original molecule of glc citric acid cycle – 16 H atoms 20 The remaining 4 H atoms used in the oxidative process WHAT HAPPENS TO CO2? CAN U SEE THE PROBLEM? WHERE IS THE ATP? Despite all the complexities of Glycolysis ► 2 ATP MOLECULES The citric acid cycle ► 2 ATP MOLECULES Dehydrogenation Decarboxylation ONLY 4 ATP MOLECULES!!!!!! for each molecule of glc metabolized SO WHERE DOES THE ENERGY COME FROM? Oxidative Phosphorylation Formation of Large Quantities of ATP by Oxidation of Hydrogen almost 90% of the total ATP created through glc metabolism is formed during subsequent oxidation of the hydrogen atoms that were released at early stages of glc degradation WHERE?  Split of each H atom into a H ion and an electron Electrons combine dissolved O2 of the fluids with H2O molecules to form hydroxyl ions H and hydroxyl ions combine with each other to form H2O  Summary of ATP Formation During the Breakdown of Glucose Glycolysis ► 4 ATP molecules are formed 2 ATP molecules are expended net gain of 2 ATP molecules to cause the initial phosphorylation of glc to get the process going Citric acid cycle ► 2 ATPs molecule are formed (each glc molecule splits into two pyruvic acid molecules so 2 revolution of the cycle), of two more molecules of ATP. Glc breakdown ► 24 H atoms are released during glycolysis and the citric acid cycle. 20 H atoms ► oxidative phosphorylation ► 3 ATP molecules per two H atoms metabolized ► 30 ATP molecules Remaining H atoms 4 ►are released by their dehydrogenase into the chemiosmotic oxidative schema in the mitochondrion beyond the first state. 2 ATP molecules are usually released for every two hydrogen atoms oxidized ► 4 ATP molecules TOTAL of 38 ATP molecules formed for each glc molecule degraded to CO2 and H2O.  456,000 calories of energy can be stored in the form of ATP 686,000 calories are released during the complete oxidation of each gram-molecule of glc This outcome represents an overall maximum efficiency of energy transfer of 66% The remaining 34% of the energy becomes heat and, therefore, cannot be used by the cells to perform specific functions Effect of ATP and ADP Cell Concentrations in Controlling Glycolysis and Glucose Oxidation Continual release of energy from glc when the cells do not need energy would be an extremely wasteful process SO HOW IS glycolysis and the subsequent oxidation of H atoms are continually controlled in accordance with the need of the cells for ATP? multiple feedback control mechanisms Phosphofructokinase when ATP is used by the cell to energize the different physiological functions in the cell, the newly formed ADP and AMP turn on the energy processes again, and ADP and AMP are almost instantly returned to the ATP state Anaerobic Glycolysis When O2 becomes unavailable or insufficient ► oxidative phosphorylation cannot take place HOWEVER a small amount of energy can still be released to the cells by the glycolysis stage of carbohydrate degradation, because the chemical reactions for the breakdown of glc to pyruvic acid do not require O2 extremely wasteful process only 24,000 calories of energy are used to form ATP for each molecule of glc ► 3% of the total energy in the glc molecule BUT LIFESAVING!!!!!! Anaerobic Glycolysis IS THIS THE ONLY WAY FOR ENERGY PRODUCTION? pentose phosphate pathway responsible for as much as 30% of the glucose breakdown in the liver and even more than this in fat cells provides energy independently of all the enzymes of the citric acid cycle alternative pathway for energy metabolism when certain enzymatic abnormalities occur in cells Glucose Conversion to Glycogen or Fat When glc is not immediately required for energy, the extra glc that continually enters the cells is either stored as glycogen or converted into fat Glc is preferentially stored as glycogen until the cells have stored as much glycogen as they can—an amount sufficient to supply the energy needs of the body for only 12 to 24 hours When the glycogen-storing cells (primarily liver and muscle cells) approach saturation with glycogen, the additional glucose is converted into fat in liver and fat cells and is stored as fat in the fat cells Gluconeogenesis Formation of Carbohydrates From Proteins (amino acids) and Fats (glycerol portion) When? When the body’s stores of carbohydrates decrease below normal Why ? Gluconeogenesis is especially important in preventing an excessive reduction in the blood glc concentration during fasting – BRAIN RUNS ON GLUCOSE!!!!! 25% of the liver’s glucose production Where? during fasting is from gluconeogenesis How is Gluconeogenesis regulated? Diminished carbohydrates in the cells and decreased blood glc are the basic stimuli that increase the rate of gluconeogenesis