Cytosolic Respiration PDF

22 Cytosolic Respiration : ILOs By the end of this lecture, students will be able to 1. Correlate carbohydrate intake to production of energy 2. Deduce how energy production differs among different cells and different cellular compartments 3. Correlate regulation of glycolysi...

22 Cytosolic Respiration : ILOs By the end of this lecture, students will be able to 1. Correlate carbohydrate intake to production of energy 2. Deduce how energy production differs among different cells and different cellular compartments 3. Correlate regulation of glycolysis to energy production ❖ What is Glycolysis? And why is it important? - Is a sequence of reactions occuring in the cytoplasm for the oxidation of Glucose to two molecules of pyruvic acid (3-carbon molecule) under aerobic conditions; or lactate under anaerobic conditions providing energy (as ATP) and intermediates for other metabolic pathways. - The unique ability to function in presence or absence of oxygen makes glycolysis the only source of energy in RBCs (as they lack mitochondria) and when performing physically- demanding tasks, the anaerobic glycolysis serves as the primary energy source for the muscles. - The glycolytic pathway may be considered as the preliminary step before complete oxidation. - It provides carbon skeletons for amino acid synthesis and the glycerol portion of fat. - It occurs as follows: Page 1 of 4  Net gain of 8 ATP produced Under anaerobic conditions NADH+H+ (at step 6) is re oxidized via lactate formation. This allows glycolysis to proceed in the absence of oxygen. Page 2 of 4  Clinical Implications: Pyruvate kinase deficiency: As mature RBCs are completely dependent on glycolysis for ATP production. Pyruvate kinase deficiency leads to decreased ATP production. This results in hemolytic anemia, with the severe form requiring regular transfusions. Severity depends both on the degree of enzyme deficiency. ❖ Regulation of the Glycolytic Pathway The regulatory enzymes of the glycolytic pathway are the 3 irreversible enzymes: hexokinase, phosphofructokinase (PFK-1), and pyruvate kinase (PK). A- Hormonal regulation: 1- Covalent Modification: - Insulin secreted in fed state activates key enzymes of glycolysis by dephosphorylation. - Glucagon secreted in fasting inhibits key enzymes of glycolysis by phosphorylation. 2- Induction/Repression: - Insulin leads to induction of key enzymes of glycolysis while glucagon represses them B- Allosteric Regulation: Page 3 of 4 Inhibitors of glycolysis: 1. Mercury inhibits glyceraldehyde-3-P dehydrogenase by binding to the enzyme’s active site. This will inhibit glycolysis to proceed leading to cell death. The most common cause of mercury poisoning is from eating polluted or wrongly preserved seafood. 2. Fluoride combines with Mg2+ as Mg fluoride, Mg is essential for the activity of enolase enzyme, therefore fluoride interferes with enolase activity. For this reason fluoride has been used for years as a rodenticide (to kill rodents) and a pesticide. It also explains why the FDA requires that all fluoride toothpastes should carry a warning that If more than used for brushing is accidentally swallowed, medical help should be seeked right away. ❖ What happens after the 2 pyruvates are produced? Pyruvate produced from glycolysis (under aerobic conditions) is then transported to the mitochondria via a special transporter where it is converted to Acetyl Co-A by “Pyruvate Dehydrogenase Complex”. (PDH) (see Aerobic VS Anaerobic). Page 4 of 4 24 Aerobic VS Anaerobic respiration : ILOs By the end of this lecture, students will be able to 1. Clarify conditions and end products of aerobic vs anaerobic respiration. 2. Discuss organs and cells utilizing each type of respiration 3. Describe the shuttles linking cytosolic and mitochondrial energy production - As we discussed before, glycolysis pathway is unique in that it occurs in both aerobic and anaerobic conditions. But definitely there are many differences between glycolysis occurring in aerobic and anaerobic cells. To know these differences we should answer the following questions ❖ Which cells utilize the aerobic respiration and which utilize the anaerobic? - Anaerobic respiration occurs in tissues that are poorly perfused either under normal conditions for example, the lens and cornea of the eye and the kidney medulla, or due to pathological causes. - Exercising muscles: During exercise, demand for oxygen by working muscle increases in proportion to the level of work performed. This results in a higher demand to deliver necessary oxygen to these active tissues; the oxygen supplied is not enough for these active cells therefore they utilize anaerobic respiration - Cells lacking mitochondria as RBCs utilize anaerobic respiration even in presence of good oxygen supply. - Under normal conditions in the presence of good oxygen supply all other tissues utilize aerobic respiration. ❖ What happens if glycolysis occurs in cells utilizing aerobic respiration? - If glycolysis occurs under aerobic conditions the 2 NADH+H+ molecules produced by the action of glyceraldehyde 3 phosphate dehydrogenase will be shuttled to the inner mitochondrial membranes where they will be fully oxidized producing 3 ATP each. - The net energy production from aerobic glycolysis will therefore be 8 ATP. - The 2 Pyruvate molecules produced by the end step of glycolysis by the action of pyruvate kinase enzyme will then be transported to the mitochondria via a special transporter where it is converted to Acetyl Co-A - The conversion of pyruvate to acetyl CoA is an irreversible reaction performed by the action of “Pyruvate Dehydrogenase Complex” (PDH). - The Pyruvate Dehydrogenase Complex is actually 3 enzymes complexed together responsible for the combined dehydrogenation and decarboxylation (oxidative decarboxylation) of pyruvate to acetyl-CoA (which enters the CAC). - For this enzyme complex to function it needs the action of five different coenzymes which are: Page 1 of 5  - Thiamine pyrophosphate (TPP) - Lipoic Acid - Coenzyme A - Flavin adenine dinucleotide (FAD) - Nicotinamide adenine dinucleotide (NAD). Regulation of Pyruvate Dehydrogenase: Allosteric Inhibition by elevated acetyl CoA and NADH+H Covalent modification: dephosphorylated form is the active one by the action of insulin Mercury can inhibit PDH. Arsenic poisoning is due to inhibition of lipoic acid which is essential for PDH action. Clinical implications: Pyruvate dehydrogenase deficiency is a rare condition with severe health effects. Therefore, many affected individuals do not survive past childhood. However, alteration in the action of PDH is more commonly due deficiencies of thiamine (TPP) or niacin (NAD) these conditions are usually due to severe nutritional deficiencies or alcohol abuse The condition is characterized by lactic acidosis, which can cause severe respiratory problems. It also affects the neurological and mental ability since the brain cells are unable to produce sufficient ATP (via the CAC cycle) if the PDH activity is decreased. - Therefore, under aerobic conditions complete glucose oxidation can produce up to 38 ATP, 8 ATP from aerobic glycolysis, 6 ATP from the 2 NADH+H+ produced by PDH enzyme complex acting on 2 pyruvates and 24 ATP from oxidation of 2 Acetyl CoA in CAC. - Another fate for Pyruvate produced by glycolysis under aerobic conditions might be Carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase - Pyruvate carboxylase is a biotin-dependent reaction. This irreversible reaction is important because it replenishes the CAC cycle intermediate Page 2 of 5  Clinical Implications: Biotin deficiency (vitamin B7) even in mild cases would affect the energy production by the cells leading to several manifestations including hair loss, thin nails, conjunctivitis or even some neurological manifestations. Biotin deficiency usually occurs due to dietary absence of the vitamin or consuming raw egg whites over months. ❖ What if glycolysis reactions occurred in anaerobic cells? - In this case glycolysis will produce only 2 ATP molecules and the 2 pyruvate molecules produced will not be converted to acetyl CoA alternatively they will be converted to Lactate by the action of Lactate Dehydrogenase enzyme (LDH) in a reversible reaction. - N.B: NADH produced by glycolysis will be used up by the LDH enzyme, there will be no energy production from NADH oxidation in ETC. Therefore, the net energy production in anaerobic respiration is only 2 ATP molecules. - Lactate formation in muscle: In exercising skeletal muscle, NADH production by glycolysis exceeds the oxidative capacity of the electron transport chain (ETC). This results in an elevated NADH/NAD+ ratio, favoring reduction of pyruvate to lactate by LDH. - Therefore, during intense exercise, lactate accumulates in muscle, causing a drop in the intracellular pH, potentially resulting in cramps. - The lactic acid then enters “Cori cycle”: Lactate which is formed during anaerobic oxidation of glucose in muscles and in RBCs diffuses to the blood then to the liver. In the liver lactate is oxidized to pyruvate (as the reaction is reversible), which can be converted to glucose again. Glucose goes back to tissues and is reutilized for production of energy. Page 3 of 5  Clinical Implications: Lactic acidosis: Elevated concentrations of lactate in the plasma, termed lactic acidosis, occur when there is generalized decrease in tissue perfusion, such as with severe hypotension, myocardial infarction and hemorrhage, or when an individual is in shock. The failure to bring adequate amounts of Oxygen to the tissues results in impaired oxidative phosphorylation and decreased ATP synthesis. To survive, the cells rely on anaerobic glycolysis for generating ATP, with accumulation of lactic acid as the end product. This will lead to fast, shallow breathing, disorientation, a general feeling of discomfort, muscle pain or cramping, and unusual sleepiness, tiredness, or weakness. The onset of lactic acidosis might be rapid and occur within minutes or hours, or gradual, happening over a period of days. If left untreated “lactic acidosis” can result in severe and life-threatening complications. ❖ How is NADH+H produced in the cytoplasm transported to the mitochondria? - The inner mitochondrial membrane lacks a NADH transporter, and NADH produced in the cytosol cannot directly enter the mitochondrial matrix. - However, NADH molecules are transported from the cytosol to the mitochondria using substrate shuttles. - In the glycerol 3-phosphate shuttle: NADH used to convert dihydroxyacetone phosphate by cytosolic glycerol 3-phosphate dehydrogenase glycerol 3-phosphate. The glycerol 3-phosphate can pass to the mitochondria where it is converted back to DHAP but producing FADH2 instead of the utilized NADH. Therefore, the glycerol 3-phosphate shuttle results in the synthesis of two ATP for each cytosolic NADH oxidized. Page 4 of 5 - This contrasts with the malate shuttle, in which oxaloacetate uses NADH+H to be converted to malate. Malate passes to the mitochondria where it is reconverted to oxaloacetate producing NADH+H, thereby yielding three ATP for each cytosolic NADH oxidized. - Both shuttles move molecules from cytoplasm to mitochondria but not the opposite. Page 5 of 5