Carbohydrate Metabolism 2 PDF Lecture Notes 2023-2024

University of Zakho Metabolism module/2023-2024 Session-3/Lecture-1 Carbohydrate metabolism 2 DR. SHAKIR ABDULRAHMAN JAMAL MBChB; DC(Path); FKBMS(Hematopathology) College of Medicine/University of Zakho shakir.abdulrahm...

University of Zakho Metabolism module/2023-2024 Session-3/Lecture-1 Carbohydrate metabolism 2 DR. SHAKIR ABDULRAHMAN JAMAL MBChB; DC(Path); FKBMS(Hematopathology) College of Medicine/University of Zakho [email protected] [email protected]  Learning Objectives 1) Pentose phosphate pathway (PPP) 2) Clinical condition of glucose 6-phosphate dehydrogenase deficiency (G6PD deficiency). 3) The biochemical basis of the signs and symptoms of G6PD deficiency. 4) The biochemical basis of lactose intolerance and galactosemia. 5) The role of pyruvate dehydrogenase in glucose metabolism. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 2 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL  LO-1 Pentose phosphate pathway (hexose monophosphate shunt) Storage form Hexokinase around 10% of glucose is metabolized through PPP, this is important in tissues such as liver, red cells and adipose tissues. Around 90 % 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 4 Pentose phosphate pathway LO-1 NADP NADP 6-PGD Synthesis of nucleotides (building blocks of DNA & RNA) 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 5 The major functions of PPP are to: LO-1 1-Produce NADPH in the cytoplasm. NADPH has many functions including: Reducing power for anabolic processes such as fatty acid synthesis, fatty acid elongation chain, cholesterol synthesis, neurotransmitter synthesis. In red blood cells it has an important role in maintaining free sulfydryl (SH) groups on cysteine residues in certain proteins including hemoglobin. In addition, it is involved in various detoxification mechanisms that protect cells against toxic chemicals. 2-Production of C5-ribose sugar for the synthesis of nucleotides, the building blocks from which DNA & RNA are made. The pathway therefore has a high activity in dividing tissues (e.g. bone marrow). 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 6  LO-1 Regulation of Pentose phosphate pathway The pathway is regulated by the activity of glucose 6-phosphate dehydrogenase enzyme ( the first enzyme, or rate limiting enzyme in the pathway). The activity of the enzyme is controlled by the NADP+/NADPH ratio in the cell, NADPH inhibiting and NADP+ activating the enzyme. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 7 Pentose phosphate pathway LO-1 NADP NADP 6-PGD Synthesis of nucleotides (building blocks of DNA & RNA) 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 8 Glucose 6-phosphate dehydrogenase deficiency (G6PD deficiency) LO-2 It is the commonest genetically determined enzyme deficiency, affecting around 400 million people in the world, widely distributed throughout Africa, southeast Asia and Mediterranean region. It is an X-linked recessive gene defect. The defect is caused by point mutations in the gene coding for glucose 6- phosphate dehydrogenase this result in reduced activity/stability of the enzyme in tissues such as the red blood cells and this lead to low levels of NADPH. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 9 Role of glucose 6-phosphate dehydrogenase in red blood cells LO-2 Diminished G6PD activity impairs the ability of the cell to form the NADPH, that is essential for the maintenance of the reduced glutathione ( GSH )pool. This results in a decrease in the cellular detoxification of free radicals and peroxides formed within the cell. RBC damage/Intravascular hemolysis H2O X H2O2 /Free radicals/ OXIDANT STRESS Metabolism, infections, drugs, certain foods….. Glucose GSSG GSH ( reduced glutathione) NADP NADPH Glucose 6 phosphate G6PD Pentose Shunt pathway (HMS) 10 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL  LO-3 GSH also helps maintain the reduced states of sulfhydryl groups in proteins, including hemoglobin. Oxidation of those sulfhydryl groups occurs ,Hemoglobin becomes cross-linked by disulphated bonds to form insoluble aggregates called Heinz bodies that attach to RBC membranes. Additional oxidation of membrane proteins causes RBCs to be rigid (less deformable), and they are removed from the circulation by macrophages in the spleen and liver, leading to hemolysis. 11 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL  LO-3 What about other tissues?? Why it is more severe in the RBCs?? Although G6PD deficiency occurs in all cells of the affected individual, it is most severe in RBCs, where the pentose phosphate pathway provides the only means of generating NADPH. The RBC has no nucleus or ribosomes and cannot renew its supply of the enzyme. Thus, RBCs are particularly vulnerable to enzyme variants with diminished stability. Other tissues have alternative sources for NADPH production (such as NADP+- dependent malate dehydrogenase [malic enzyme], that can form reduced glutathione. 12 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL Precipitating factors of acute hemolysis LO-3 Acute hemolytic episodes are precipitated by chemicals that reduce NADPH levels, making red cells more fragile and susceptible to hemolysis by free radicals, these includes: 1. Drugs that increase the oxidant stress (AAA)e.g. Antibacterial: Cotrimoxazole, Quinolones, sulphonamides…… Antipyretics: phenazopyridine, aspirin ( high doses)…… Antimalarials: primaquine….. 2. Infections. 3. Eating fava beans, in any forms which contains (divicine & isouramil). For this reason it is called favism. 4. Topical henna and ‫سماق‬. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL  Summary 10% LO-3 Glucose 6-phosphate Pentose phosphate pathway Glucose 6-phosphate dehydrogenase enzyme (rate limiting step) Hemolysis and hemolytic anemia Mutation in the gene enzyme activity/stability of G6PD Damage to RBC membrane Production of NADPH Accumulation of reactive oxygen intermediates inside the cells Reduced glutathione 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL  LO-4 Galactose metabolism Dietary lactose is hydrolysed by the digestive enzyme (lactase) to release glucose and galactose that are absorbed into the blood stream. Galactose is metabolised largely in the liver (some in kidney and gastrointestinal tract) by soluble enzymes. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 15  Lactase Dietary lactose +Glucose LO-4 reaction by UDP glucose The epimerase reaction is reversible enabling galactose to be synthesized from glucose via UDP glucose. This is important during lactation when breast tissue is synthesizing large amounts of lactose for milk production. 16 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 16 There are a number of clinical conditions that affect galactose LO-4 metabolism including Lactose Intolerance and Galactosaemia. In Galactosaemia individuals are unable to utilize galactose obtained from the diet because of a lack of the kinase or transferase enzyme. Inborn error of metabolism (Autosomal recessive disorders, incidence 1/30,000 - 1/100,0000) 1) Classical galactosemia is caused by a deficiency of galactose 1-phosphate uridyl transferase. It is more common, more serious as both galactose and galactose 1-phosphate accumulates in tissues. 2) Non-classical galactosemia: it is rare, due to deficiency of galactokinase, leading to galactosemia and galactosuria, but galactose 1-phosphate is not formed. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 17  Accumulation of galactose in tissues leads to its reduction to galactitol LO-4 by the activity of the enzyme aldose reductase This reaction depletes tissues NADPH. In the eye the lens structure is damaged, (cross linking of lens proteins by –S-S- bond formation) causing cataracts. In addition, there may be nonenzymatic glycosylation of the lens proteins because of the high concentration of galactose and this may contribute to the cataract formation. The accumulation of galactose and galactitol in the eye may lead to raised intra-ocular pressure (glaucoma) which if untreated may cause blindness. Accumulation of galactose 1-phosphate in tissues causes damage to the liver, kidney and brain and may be related to the sequestration of Pi making it unavailable for ATP synthesis. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 18 Lactose intolerance LO-4 is the inability to break down lactose due to ( lactase deficiency). Lactose is commonly found in dairy products, such as milk and yogurt. When this happens, the undigested lactose moves into the large intestine. The bacteria that are normally present in the large intestine interact with the undigested lactose and cause symptoms such as bloating, gas, vomiting and diarrhea, usually within 30 minutes to two hours after ingesting milk or other dairy products containing lactose. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL  LO-4 Summary of stage 2 catabolism of sugars 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 20  LO-5 Metabolism of pyruvate Pyruvate does not enter stage 3 of catabolism directly but is first converted to acetyl~CoA by the enzyme pyruvate dehydrogenase (PDH)( a multi-enzyme complex). The reaction requires the cofactors FAD, thiamine pyrophosphate and lipoic acid all of which act catalytically in the reaction. Thus, four B vitamins (pantothenic acid in CoA, niacin in NAD+, riboflavin in FAD and thiamine) are involved and the reaction is therefore very sensitive to vitamin B deficiency. 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 21 The PDH reaction cannot be reversed in the cell. LO-5 There are two major consequences of this: ✓ The loss of CO2 from pyruvate is irreversible. ✓ Acetyl-CoA cannot be reconverted to pyruvate and therefore cannot be converted to glucose by the process of gluconeogenesis. Cofactors: FAD, thiamine pyrophosphate and lipoic acid 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL GOOD LUCK ANY ????? 19/02/2024 DR.SHAKIR ABDULRAHMAN JAMAL 23