Lecture 2 - Heme Synthesis and Porphyria PDF

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: BLOOD and body fluids (blf) 103 heme synthesis and porphyria By: Dr. Marwa Ali Assistant Professor of Medical Biochemistry and Molecular Biology Ain Shams university Me...

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: BLOOD and body fluids (blf) 103 heme synthesis and porphyria By: Dr. Marwa Ali Assistant Professor of Medical Biochemistry and Molecular Biology Ain Shams university Medical Biochemistry and Molecular Department: Biology 27/12/2024 2  Objectives By the end of this lecture the student will be able to: 1. Outline the structure of porphyrins 2. Illustrate the steps of heme synthesis 3. Discuss regulatory steps of heme synthesis 4. Outline different types of Porphyria 5. Correlate biochemical basis of porphyria with its clinical manifestations 3  What is Heme? Heme is the colored prosthetic group of hemoglobin and a number of proteins called hemeproteins  Some important heme-proteins Hemoglobin Myoglobin Electron transport chain cytochromes (cyt aa3, cyt.c) Cytochrome P450 Catalse and Peroxidase (degradation of H2O2) Tryptophan pyrrolase Cytoplasmic guanyl cyclase (activated by NO)  Structure Of Heme Heme = Iron in the ferrous state + porphyrin Porphyrins = cyclic compounds formed of four pyrrole rings Fe+2 © Heme group  Porphyrins They are cyclic compounds formed of 4 pyrrole rings linked by methenyl bridges. They form complexes with metal ions that bind to Nitrogen of pyrrole rings: Iron porphyrins (Heme), Magnesium porphyrins (Chlorophyll)  Porphyrinogens They are reduced forms of porphyrins that have no conjugated double bonds and colorless No double bonds Structure Of Porphyrins They constitute a system of conjugated double bonds which absorb light Double bonds  MCQ 1- Porphyrin ring present in all of the following except: A. peroxidase B. Xanthine oxidase C. Tryptophan pyrrolase D. catalase E. guanyl cyclase  Biosynthesis of Heme Mainly in the bone marrow (hemoglobin synthesis) and the liver (cytochrome P450 synthesis ). Heme biosynthetic pathway is partly mitochondrial and partly cytosolic. The reactions are Irreversible. Steps: The initial porphobilinogen synthase reaction and the last (A pyrrol) three steps in the formation of (tetrapyrrol) porphyrins occur in mitochondria, but the intermediate steps occur in the cytosol Steps: porphobilinogen synthase (A pyrrol) (tetrapyrrol)  Glycine and succinyl CoA condense to form ALA by mitochondrial ALA synthase (ALAS). It is the committed step in porphyrin TCA cycle biosynthesis. PLP COASH  Condensation of two molecules of ALA by Zn- containing cytosolic ALA dehydratase (porphobilinogen synthase). It is inhibited by heavy metal ions e.g. lead that replace the zinc. (cytosolic enzyme) porphobilinogen synthase  Condensation of four porphobilinogens produces a linear tetrapyrrole, which is cyclized forming uroporphyrinogen III by uroporphyrinogen III Synthase enzyme. uroporphyrinogen III Uroporphyrinogen III undergoes decarboxylation forming coproporphyrinogen III. This reaction occur in the cytosol. Coproporphyrinogen III enters the mitochondrion, followed by oxidation reactions. Iron (as Fe2+) is introduced into protoporphyrin by ferrochelatase enzyme.  Overall reactions of heme biosynthesis uroporphyrinogen III Synthase  MCQ 2- An amino acid required for porphyrin synthesis is: A. proline B. serine C. glycine D. histidine E. alanine  Regulation of Heme Synthesis Glucose - ALA synthase is the rate limiting regulatory enzyme  1- Effect of heme (hemin): When porphyrin production exceeds the availability of the apoproteins that require it, heme accumulates and is converted to hemin by the oxidation of Fe2+ to Fe3+. Hemin decreases the amount and the activity of ALAS enzyme. Feed Back inhibition  2- Effect of drugs (in liver only): Barbiturates , alcohol and carcinogens  3- Effect of Lead: Lead has an inhibitory effect on ALA dehydratase & Ferrochelatase  4- Effect of Glucose: Glucose has inhibitory effect on ALA synthase  MCQ 3- -Aminolevulinic acid synthase activity: A. Is frequently decreased in liver 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. Overall reactions of heme biosynthesis Overall reactions of heme biosynthesis  Clinical Disorders of Heme Synthesis  1. Lead Poisoning Due to high exposure to: Lead paints, batteries and water lead pipes. Lead inhibits 2 enzymes: ALA dehydratase & Ferrochelatase ALA and protoporphyrin accumulate in urine Elevation in ALA and anemia  2. Porphyrias Porphyrias are rare, inherited ( or occasionally acquired in lead poisoning) defects in heme synthesis, resulting in the accumulation and increased excretion of porphyrins or porphyrin precursors  2. Porphyrias Porphyria” refers to the red-blue color caused by pigment-like porphyrins in the urine of patients with defects in heme synthesis  The porphyrias are classified as: 1-Erythropoietic 2-Hepatic Depending on whether the enzyme deficiency occurs in the erythropoietic cells of the bone marrow or in the liver  Clinical manifestations: If the enzyme defect prior If the enzyme defects after to the formation of the formation porphyrinogens porphyrinogens (tetrapyrroles) Abdominal and Photosensitivity neuropsychiatric signs (skin itches, burns and pruritus on exposure to visible light) Note: Photosenstivity is a result of the oxidation of colorless porphyrinogens to colored porphyrins, that participate in the formation of superoxide radicals from oxygen. These reactive oxygen species can oxidatively damage membranes and cause the release of destructive enzymes from lysosomes.  Clinical manifestations: One common feature of the porphyrias is a decreased synthesis of heme Increase in the synthesis of ALAS1 (derepression) Increased synthesis and accumulation of toxic intermediates that occur prior to the genetic block  Porphyria cutanea tarda It is the most common porphyria It occurs due to deficiency in uroporphyrinogen decarboxylase enzyme  Porphyria cutanea tarda Clinical onset is during the fourth or fifth decade of life Porphyrin accumulation leads to cutaneous symptoms and red to brown urine in natural light Note: Symptoms of the acute hepatic porphyrias often precipitated by drugs that cause induction of cytochrome P450 e.g. steroids , alcohol, Phenobarbital. These drugs are contraindicated for porphyria patients. WHY? WHY? Intake of drugs as barbiturates or ethanol Induce the synthesis of the heme containing cytochrome P450 required for their metabolism. Decrease heme level in liver cells Increase synthesis of ALAS More increase of porphyrins & exacerbation of symptoms  Treatment No curable treatment, only medical support during acute attacks and symptomatic treatment for pain and vomiting  Treatment 1. Intravenous injection of hemin and glucose to decreases the synthesis of ALAS1. 2. Avoidance of precipitating drugs. 3. Protection from sunlight. 4. Anti-oxidants: vitamin A (β-carotene) & Vit E in cases of photosensitivity.  MCQ 1- Biochemical basis of precipitation of porphyria by barbiturates is: A. Repression of ALA synthase B. Derepression of ALA synthase C. Rerepression of ALA synthase D. MiRNA mediated  MCQ 2- Most common porphyria is due to deficiency of: A. PBG deaminase B. Uroporphyrinogen decarboxylase C. Ferrocheletase D. Coproporphyrinogen oxidase E. ALA synthase  Summary Heme = Iron in the ferrous state + porphyrin The initial reaction and the last three steps in the formation of porphyrins occur in mitochondria, but the intermediate steps occur in the cytosol ALA synthase is the rate limiting regulatory enzyme 52  Summary Lead poisoning is due to high exposure to: Lead paints, batteries and water lead pipes. Porphyria” refers to the red-blue color caused by pigment-like porphyrins in the urine of patients with defects in heme synthesis Porphyria cutanea tarda occurs due to deficiency in uroporphyrinogen decarboxylase enzyme 53  References 1- "Lippincott's Illustrated Reviews in Biochemistry" by P.C.Champe, R.A.Harvey and D.R.Ferrier 2- "Harper's Biochemistry" by R.K.Murray, D.K.Granner, P.A. Mayes and V.W.Rodwell. 54 Marwa Ali