14062076.ppt
- 1. Click to edit Master title style • Edit Master text styles • Second level • Third level • Fourth level • Fifth level Heme Metabolism (synthesis) Dr. Dalia Abdelwahab & Dr. Marian Maher Lecturers of Medical Biochemistry and Molecular Biology
- 2. Click to edit Master title style • Edit Master text styles • Second level • Third level • Fourth level • Fifth level Lecture 1
- 3. Click to edit Master title style • Edit Master text styles • Second level • Third level • Fourth level • Fifth level Intended Learning Outcomes ILOs 1. Describe the structure of Porphyrins 2. List important heme-proteins 3. Describe steps of heme biosynthesis 4. Identify the regulation of heme synthesis
- 4. Structure Of Porphyrins They are cyclic compounds formed from: 4 pyrrole rings linked by methenyl bridges 4 methyl , 2 propionyl and 2 vinyl groups connected to the pyrrole rings
- 5. They constitute a system of conjugated double bonds which absorb light Porphyrinogens are reduced forms of porphyrins .They contain methyl (- CH2-) rather than methenyl bridges (-HC=). So, they have no conjugated double bonds and colorless .
- 6. Porphyrins form complexes with metal ions , that bind to Nitrogen of pyrrole rings.: Iron porphyrins (Heme), Magnesium porphyrins (Clorophyll).
- 7. Haemoglobin Myoglobin Cytochromes of electron transport chain (cyt aa3,cyt.c) cytochrome P450 Catalse and Peroxidase (degradation of H2O2), , Tryptophan pyrrolase (Oxidation of tryptophan), Cytoplasmic guanyl cyclase (activated by NO). Some important heme-proteins
- 8. Biosynthesis of Heme The principal tissues involved in heme biosynthesis are mainly the bone marrow and the liver. Heme biosynthetic pathway is partly mitochondrial and partly cytosolic. The reactions are Irreversible.
- 9. Steps: The initial reaction and the last three steps in the formation of porphyrins occur in mitochondria, whereas the intermediate steps of the biosynthetic pathway occur in the cytosol (A pyrrol) (tetrapyrrol) porphobilinogen synthase hydroxymethylbilane synthase
- 10. Glycine and succinyl coenzyme A that condense to form ALA by mitochondrial ALA synthase (ALAS) . This reaction requires pyridoxal phosphate (PLP) It is the committed and rate-limiting step in porphyrin biosynthesis. There are two isoforms of ALAS 1 and 2. Erythroid tissue produces only ALAS2. TCA cycle COASH
- 11. By Zn-containing ALA dehydratase (porphobilinogen synthase) It is extremely sensitive to inhibition by heavy metal ions e.g. lead that replace the zinc . This inhibition cause the elevation in ALA and the anemia seen in lead poisoning. (cytosolic enzyme) porphobilinogen synthase
- 12. The condensation of four porphobilinogens produces the linear tetrapyrrole, hydroxymethylbilane, porphobilinogen deaminase
- 13. Hydroxymethylbilane, which is isomerized and cyclized by uroporphyrinogen III Synthase to produce the asymmetric uroporphyrinogen III.
- 14. This cyclic tetrapyrrole undergoes decarboxylation of its acetate groups, generating coproporphyrinogen III. These reactions occur in the cytosol.
- 15. CH2 CH (IX) (IX) enters the mitochondrion 2 CO2 inhibited by lead Corpropophrynogen
- 18. Porphyrin ring present in all of the following except: A. peroxidase B. Xanthine oxidase C. Tryptophan pyrrolase D. catalase E. guanyl cyclase MCQ
- 19. An amino acid required for porphyrin synthesis is: A. proline B. serine C. glycine D. histidine E. alanin MCQ
- 20. ALA synthase is the rate limiting regulatory enzyme 1. Excess heme is converted to hemin by oxidation of Fe2+to Fe3+. Hemin decreases the amount of ALAS1 by repressing transcription of its gene (act as aporepressor), increasing degradation of its messenger RNA, and decreasing import of the enzyme into mitochondria. Regulation of Heme Synthesis
- 21. 2. In erythroid cells, ALAS2 is controlled by the availability of intracellular iron.
- 22. 3. Substances metabolized by cytochrome P450 increases the activity of ALAS1 in liver e.g. barbiturates , alcohol and carcinogens. Drugs metabolized by cytochrome P450 monooxygenase in the liver . Synthesis of cytochrome P450 Consumption of heme a component of cytochrome P450 proteins. The concentration of heme in liver Cells. The synthesis of ALAS1(derepression)
- 23. ALA dehydratase and ferrochelatase inhibited by Lead poisoning lead lead
- 24. -Aminolevulinic acid synthase activity: A. in liver is frequently decreased in individuals treated with drugs, such as the barbiturate phenobarbital. B. catalyzes a rate-limiting reaction in porphyrin biosynthesis. C. requires the coenzyme biotin. D. is strongly inhibited by heavy metal ions such as lead. E. occurs in the cytosol. MCQ
- 25. Click to edit Master title style • Edit Master text styles • Second level • Third level • Fourth level • Fifth level Lecture 2
- 26. Click to edit Master title style • Edit Master text styles • Second level • Third level • Fourth level • Fifth level Intended Learning Outcomes ILOs 1. Define Porphyrias 2. Identify clinical manifestations of porphyrias 3. List different types of porphyrias 4. Describe the treatment of porphyrias
- 27. Loss of function mutations in ALAS2 result in X-linked sideroblastic anemia. Low levels of ALA2 cause no porphyria (only anemia) no known defect of ALA1 Disorders of Heme Synthesis 1) sideroblastic anemia
- 28. A group of diseases collectively called porphyrias are associated with abnormalities in biosynthesis of heme. They are characterized by accumulation and excretion of porphyrins or porphyrin precursors. Disorders of Heme Synthesis ( Porphyrias) Hydroxymethylbylane ALA dehydratase porphyria (protoporphyria)
- 29. “Porphyria” refers to the purple color caused by pigment-like porphyrins in the urine of some patients • One common feature of the porphyrias is a decreased synthesis of heme. In the liver, heme normally functions as a repressor of the gene for ALAS1. Therefore, the absence of this end product results in an increase in the synthesis of ALAS1 (derepression). This causes an increased synthesis of intermediates that occur prior to the genetic block. • The accumulation of these toxic intermediates is the major pathophysiology of the porphyrias.
- 30. • The porphyrias are classified as 1-erythropoietic (Congenital erythropoietic porphyria & Protoporphyria (erythropoietic porphyria)). 2-hepatic (chronic or acute) depending on whether the enzyme deficiency occurs in the erythropoietic cells of the bone marrow or in the liver.
- 31. • Most of porphyrias are 1-inherited as autosomal dominant manner except congenital erythropoietic porphyria (recessive). 2- acquired porphyrias can result from lead poisoning. The toxic effect of lead is due to inhibition of ferrochelatase and ALA dehydratase.
- 32. Individuals with an enzyme defect prior to the synthesis of the tetrapyrroles manifest abdominal and neuropsychiatric signs. Clinical manifestations: Whereas those with enzyme defects leading to the accumulation of tetrapyrrole intermediates show photosensitivity that is, their skin itches and burns (pruritus) when exposed to visible light.
- 33. Enzyme deficiency early in pathway before formation of tetrapyroles : only abdominal pains and neuro-psychiatric symptoms are present as in acute intermittent porphyria which is characterized by deficiency of the enzyme porphobilinogen deaminase
- 34. Enzyme deficiency late in pathway after formation of tetrapyroles & porphyrinogen : Porphyrinogens are oxidized to their corresponding porphyrins which react with molecular oxygen to form reactive oxygen species that cause oxidative damage membranes and cause the release of destructive enzymes from lysosomes. photosensitivity (skin inflammation damage with ultimate disfigurement and scaring.) in response to visible light, urine is red Porphyria cutanea tarda which is the (most common type). Due to low levels of Uroporphyrinogen decarboxylase
- 35. Symptoms of the acute hepatic porphyrias (as ALA dehydratase deficiency porphyria, acute intermittent porphyria, hereditary coproporphyria, and variegate porphyriaare) often precipitated by drugs that cause induction of cytochrome P450 e.g. steroids , alcohol, Phenobarbital ,are contraindicated for porphyria patients because they precipitate attacks.
- 36. The severity of symptoms of porphyrias can be diminished by intravenous injection of hemin which decreases synthesis (represses) ALA synthase. Avoidance of sunlight and ingestion of B-carotenes (anti-oxidant), are helpful in photosensitivity. Treatment
- 37. MCQ A. Repression of ALA synthase B. Derepression of ALA synthase C. Rerepression of ALA synthase D. MiRNA mediated Biochemical basis of precipitation of porphyria by barbiturates is
- 38. MCQ Most common porphyria is due to deficiency of A. PBG deaminase B. Uroporphyrinogen decarboxylase C. Ferrocheletase D. Coproporphyrinogen oxidase E.ALA synthase
- 39. Lecture 3
- 40. Catabolism of Heme After RBCs reach the end of their life span ( average 120 days), they are phagocytosed by reticulo- endothelial cells of liver, spleen and bone marrow. Hemoglobin is degraded first into globin and heme
- 41. 1-Oxidation of Heme : a- Heme is degraded by microsomal enzyme; of the reticuloendothelial cells of liver, spleen and bone marrow which requires molecular oxygen and NADPH b- It catalyzes the cleavage of α methenyl bridge between the pyrrole rings I and II to from biliverdin with the release of carbon monoxide CO. c- Iron is liberated from heme d- In mammals, biliverdin is further reduced to bilirubin by NADPH – dependent biliverdin reductase.
- 42. 2- Transport of Bilirubin: Bilirubin is slightly soluble in plasma. It transported to the liver by non covalent binding to albumin forming hemobilirubin (unconjugated or indirect bilirubin). Liver uptake bilirubin by carrier mediated transport. 3- Uptake of bilirubin by liver: The solubility of bilirubin increase by conjugating with 2 molecules of glucuronic acid to form cholebilirubin (conjugated or direct bilirubin). This reaction is catalyzed by glucuronosyl transferase enzyme
- 43. Extended Modular Program 43 But where do Gulcuronic acid come from???
- 44. Uronic acid pathway Occur in liver Oxidize glucose (C6) to Obtain glucuronic acid Not ATP
- 46. 1) Detoxification of some compounds • steroids, bilirubin and some drugs • Glucuronic acid is a highly polar molecule so, it has the ability to be conjugated with less polar compounds make them more water soluble thus facilitating their renal excretion. 2) Glucuronic acid is also a component of the glycosoaminoglycans & proteoglycans Importance
- 47. a. In mammals, except human and guinea pigs, it is converted to L- ascorbic acid (vitamin C). b. It is converted to L-Xylulose then to D-Xylulose which can enter HMP pathway to complete its metabolism. 3) Glucuronic acid is oxidized to L-Gulonic acid which has 2 fates:
- 48. 4- Excretion of conjugated bilirubin: The conjugated bilirubin secreted with the bile into intestine (the unconjugated bilirubin not secreted) where it is hydrolyzed and reduced by the gut bacteria to urobilinogen (colorless compound). Most of urobilinogen is oxidized by intestinal bacteria to stercobilin, which give feces the characteristic brown color. Some of urobilinogen is reabsorbed from the gut and enters the portal blood and resecreted to the kidney, where it is converted to urobilin that gives the urine the characteristic yellow color 90% By bacteria 10%
- 50. Lecture 4
- 51. Hyperbilirubinemia The normal plasma bilirubin level range from 0.3 – 1 mg/dl . If the serum bilirubin exceeds 1 mg/dl, the condition is called hyperbilirubinemia. If the bilirubin level exceeds 2 mg/dl. Jaundice will occur with yellowish discoloration of sclera, conjunctiva and skin. The sclera is particularly affected because it is rich in elastin, which has a high affinity for bilirubin.
- 52. Note that: • unconjugated bilirubin can cross the blood-brain barrier into the central nervous system so encephalopathy due to hyperbilirubinemia (kernicterus) thus occurs only with unconjugated bilirubin. • Alternatively, because of its water-solubility, only conjugated bilirubin can appear in urine. choluric jaundice (choluria is the presence of bile pigment in the urine) occurs only in regurgitation conjugated hyperbilirubinemia, acholuric jaundice occurs if only the presence of an excess of unconjugated bilirubin.
- 53. MCQ The substance deposited in skin and sclera in jaundice is: A. biliverdin. B. only unconjugated bilirubin. C. only direct bilirubin. D. Both direct and indirect bilirubin E. hematin.
- 55. Type of Bilirubin Unconjugated Neonatal “physiological jaundice” Hemolysis Gilbert syndrome Crigler-Najjar syndromes types I & II Hepatic damage Conjugated Obstruction of the biliary tree Dubin–Johnson syndrome Rotor syndrome Hepatic damage
- 56. Site of defect Prehepatic Hemolytic Neonatal jaundice” Hepatic Hepatitis Crigler-Najjar syndromes I & II Gilbert syndrome Dubin–Johnson syndrome Rotor syndrome Posthepatic Obstruction of the biliary tree
- 57. 1) Hemolytic Jaundice Due to: In neonates Rh incompatibility between maternal and fetal blood. In children and even in adult from enzyme deficiency of G-6-P dehydrogenase or pyruvate kinase or sickle cell anemia. Extensive hemolysis produce bilirubin faster than it can be conjugated UCB levels in the blood become elevated more CB is made and excreted into the bile, the amount of urobilinogen entering the enterohepatic circulation is increased, and urinary urobilin and stercobilin is increased causing jaundice, normal colour of urine and stool
- 58. 2) Neonatal “Physiological Jaundice” This is transient hyperbilirubinemia due to accelerated rate of destruction of RBCs and to the immature hepatic system of conjugation. Elevated UCB, in excess of the binding capacity of albumin (20–25 mg/dl) Causing diffuse into the basal ganglia, cause toxic encephalopathy (kernicterus) ttt Phototherapy (converts bilirubin to more polar water-soluble isomers) and barbiturates (promoter of bilirubin-metabolism)
- 59. 3) Hepatocellular Jaundice due to cirrhosis or hepatitis Results in: Damage to liver cells can cause UCB levels in the blood to increase as a result of decreased conjugation Inflammatory oedema of hepatocytes will compress the intracellular canaliculi “mild obstruction” causing increased CB Urobilinogen decreased if micro-obstruction is present Causing the urine consequently darkens, whereas stools may be a pale, clay color
- 60. 4) Crigler-Najjar syndrome Type I Crigler-Najjar: autosomal recessive disorder due to mutations in the gene encoding bilirubin-UGT (UDP- glucuronyl-transferase)activity in hepatic tissues. serum UCB usually exceeds 20 mg/dL It is characterized by severe congenital jaundice ttt Phototherapy reduces plasma bilirubin levels somewhat, but phenobarbital has no effect. Type II (UDP-glucuronyl-transferase), have some activity and the condition has a more benign course than type I. Serum UCB concentrations usually do not exceed 20 mg/dl,ttt: patients respond to treatment with large doses of phenobarbital
- 61. 5) Gilbert’s disease due to congenital defect in conjugation by hepatocytes (70– 80% reduction) and 30% of the bilirubin UDP-glucuronosyl transferase activity is retained in Gibert syndrome the condition is harmless.
- 62. 6) Dubin Johnson syndrome It is a benign autosomal recessive. caused by mutations in the gene encoding the protein involved in the secretion of conjugated bilirubin into bile. So CB increases
- 63. Extended Modular Program 64 7) Rotor syndrome Its cause has not been identified, but it is related to Dubin Johnson syndrome This is a rare benign condition characterized by chronic conjugated hyperbilirubinemia and normal liver histology.
- 64. 8) Obstructive Jaundice Due to obstruction of the common bile duct as in Biliary cirrhosis – hepatoma - Gall stones- Cancer head of pancreas “The most severe form of Jaundice, bilirubin > 30 mg/dl”. Preventing passage of CB into the intestine The liver “regurgitates” CB into the blood (hyperbilirubinemia). The CB is eventually excreted in the urine (which darkens upon standing), and is referred to as “urinary bilirubin.” Urinary urobilinogen is absent. Causing GI pain and nausea and produce stools that are a pale, clay color.
- 65. Post hepatic jaundice Obstruction of bile duct Hepatic jaundice Defect in conjugation and/or excretion of bilirubin in bile Prehepatic jaundice Increase bilirubin production biliary stones cancer head of pancreas 1.Gilbert syndrome 2.Crigler-najjar syndrome Dubin- johnson syndrome Rotor syndrome Hepatic damage e.g; Hepatitis 1.hemolysis of RBCs as in sickle cell anemia, G6PD deficiency and RH incompatibility 2.Neonatal jaundice Causes: direct (conjugated) Indirect direct Indirect + direct Indirect (unconjugated) Type of elevated bilirubin Present (choluric) Absent Present (choleric) Decrease Absent (acholuric) Normal color increase Urine 1.Presence of conjugated bilirubin (dark colored urine) 2.urobilinogen Pale clay colored and bulky(steatorrhea) absent Pale clay colored decrease Normal increase Stool 1.Color and consistency 2.stercobilin Normal Increased ………. ……………. Present (itching) Increased Increased Normal …… …….. ……. Normal Normal Normal Low Elevated …….. Normal Blood test: Serum ALT and AST Serum ALP Blood hemoglobin Reticulocyte count Serum bile salts Serum cholestero