Porphyrin metabolism & porphyrias

1. Porphyrin metabolism & porphyrias HMIM224

3. What are porphyrins ? • Porphyrinsare cyclic compounds that bind metal ions (usually Fe2+ or Fe3+) • Porphyrin + Metal = Metaloporphyrin • Most prevalent metalloporphyrin in humans is heme(metal here is iron ion) Heme consists of: • One ferrous ion (Fe2+) in the centre • ProtoporphyrinIX (a tetrapyrrole ring) Heme is the prosthetic group of hemoglobin, myoglobin, cytochromes, catalase, tryptophan pyrrolase So, heme + globin protein = hemoglobin

4. Structure of porphyrins • Porphyrinsare cyclic molecules formed by 4 pyrrole (tetrapyrrole) rings linked by methenyl bridges. • Different porphyrins vary in the nature of side chains that are attached to each of the 4 pyrrole rings Protoporphyrin IX contains vinyl, methyl & propionate Distribution of side chains - Side chains can be ordered around tetrapyrrole nucleus in 4 different ways designated I, II, III & IV series. Only type III porphyrins are physiologically important in humans - Protoporphyrin IX is a member of type III series • Porphyrinogens are porphyrin precursors intermediate between porphobilinogen & protoporphyrin PorphobilinogenPorphyrinogensProtoporphyrins

5. Pyrrole ring Methenyl bridge Side Chains Structure of Porphyrin Determine the type of porphyrin

6. propionate methyl pyrrole ring vinyl Structure and Properties of Iron Protoporphyrin IX Derived from protoporphyrin IX Pattern of side chains defines isomer Binds metals: Heme- Fe2+ (ferrous) Hemin- Fe3+ (ferric) Zinc protoporphyrin (ZnPP)- Zn2+ Extended conjugation across ring system

7. Biosynthesis of Heme

8. Overview of Heme Synthesis Heme SuccinylCoA + Glycine Protoporphyrin IX ALA synthase Protoporphyrinogen IX -aminolevulinic acid Coproporphyrinogen III mitochondrial matrix cytoplasm -aminolevulinic acid Uroporphyrinogen III Coproporphyrinogen III Porphobilinogen Coproporphyrinogen I Uroporphyrinogen I Heme synthesis occurs in all cells due to the requirement for heme as a prosthetic group on enzymes and electron transport chain. By weight, the major locations of heme synthesis are the liver and the erythroid progenitor cells of the bone marrow.

9. Biosynthesis of heme Site of biosynthesis: Liver & Bone Marrow (erythryoid producing cells) Steps: Step 1: Formation of d-amino levulinic acid (ALA): in mitochondria (Rate Controlling Step) Glycine+ SuccinylCoA (nonessential amino acid) (intermediate of citric acid cycle) Enzyme: ALA Synthase Coenzyme: Pyridoxal Phosphate (PLP) d-Amino levulinic acid (ALA)

10. Biosynthesis of heme(cont.) Clinical importance of first step: When heme (end product) is produced in excessive amounts, heme is converted to hemin. Hemin decreases action of ALA synthase in liver. (end product inhibition). The reverse occurs when heme biosynthesis is reduced. Drugsas grisofulvin (antifungal), hydantoin & phenobarbital (anticonvulsant) increase ALA synthase activity: as these drugs are metabolized by cytochrome p450 in liver resulting in more consumption of heme (component of cytochrome). Accordingly, heme concentration is reduced resulting in stimulation of action of ALA synthase.

11. ALA Synthesis of ALAS1 Decreased heme Synthesis of CYP 450 is increased Large Amount of Drug intake Metabolised by CYP 450

12. Biosynthesis of heme(cont.) Cytochrome P450 MonooxygenaseSystem: • Cytochrome P450s (CYPs) are actually a superfamily of related, heme-containingmonooxygenase enzymes that participate in abroad variety of reactions. This system performs different functions in two separate locations in cells. • The over-all reaction catalyzed by a cytochrome P450 enzyme is: R-H + O2+ NADPH + H+→R-OH + H2O NADP+ where R may be a steroid, drug, or other chemical. • The name P450 reflects the absorbance at 450 nm by the protein. Role of cytochrome P450 in detoxification of drugs & toxic compounds: • It may itself activate or inactivate a drug • It can make a toxic compound more soluble, thus facilitating its excretion in the urine or feces.

13. Biosynthesis of heme(cont.) Step 2: Formation of porphobilinogen: 2molecules of d-Amino levulinic acid (ALA) condense to form porphobilinogenby the enzyme ALA dehydratase. Clinical importance: ALA dehydratase enzyme is inhibited by heavy metals as leadthat results in anemia. (lead poisoning). In this case: ALA in blood is elevated (lab investigation)

14. First & second steps of hemesynthesis Biosynthesis of heme (cont.)

15. Biosynthesis of heme(cont.) Further steps: (in mitochondria) Formation of protoporphyrin IX Then, ferrous ions (Fe2+) are introduced into protoporphyrin IX, either: simultaneously or: enhanced by ferrochelatase Clinical importance: Ferrochelatase enzyme is inhibited by lead

16. Biosynthesis of heme (cont.) Further steps of heme synthesis

17. Summary of biosynthesis of heme Glyine+ Succinyl CoA Enzyme: ALA Synthase STEP 1 PLP d-Amino levulinic acid (ALA) Enzyme: ALA dehydratase. STEP 2 porphobilinogen FURTHER STEPS Protoporphyrin IX Ferrous ion (Fe2+ ) introduction of iron Enzyme: ferrochelatase heme

18. Overview of Heme Synthesis Heme SuccinylCoA + Glycine Protoporphyrin IX ALA synthase Protoporphyrinogen IX -aminolevulinic acid uroporphyrinogendecarboxylase Coproporphyrinogen III mitochondrial matrix cytoplasm -aminolevulinic acid Uroporphyrinogen III Coproporphyrinogen III Porphobilinogen Coproporphyrinogen I Uroporphyrinogen I Heme synthesis occurs in all cells due to the requirement for heme as a prosthetic group on enzymes and electron transport chain. By weight, the major locations of heme synthesis are the liver and the erythroid progenitor cells of the bone marrow.

19. Porphyrias Porphyriaare rare inherited defects in heme synthesis. An inherited defect in an enzyme of heme synthesis results in accumulation of one or more of porphyrin precursors depending on location of block of the heme synthesis pathway. These precursors increase in blood & appear in urine of patients. Porphyriameans purple colour caused by pigment-like porphyrins in urine of patients. (Diagnosed by lab investigation) Most porphyrias show a prevalent autosomal dominant pattern, except congenital eythropoieticporphyria, which is recessive

20. Porphyrias(cont.) Clinical manifestations of porphyrias: Two types of porphyrias: erythropoietic (bone marrow) & hepatic Hepatic porphyrias are: acute & chronic porrphyrias Generally, individuals with an enzyme defect priorto the synthesis of the tetrapyrroles manifest abdominal and neuropsychiatric signs Those with tetrapyrrole intermediatesshow photosensitivity with formation of reactive oxygen species (ROS) that damage membranes by oxidation resulting in the following effects: - Skin blisters, itches (pruritis) - Skin may darken, grow hair (hypertrichosis)

21. PorphyriaCutaneaTarda • Chronic hepatic porphyria • The most common type of porphyria • a deficiency in uroporphyrinogendecarboxylase • Clinical expression of the enzyme deficiency is influenced by various factors, such as exposure to sunlight, the presence of hepatitis B or C • Clinical onset is during the fourth or fifth decade of life. • Porphyrin accumulation leads to cutaneous symptomsand urinethat is red to brown in natural light and pink to red in fluorescent light

22. Porphyrias(cont.)

23. Acute Hepatic Porphyrias e.g. Acute Intermittent Porphyria • Porphyrias leading to accumulation of ALA and porphobilinogencause abdominal pain and neuropsychiatric disturbances, ranging from anxiety to delirium. • Symptoms of the acute hepatic porphyrias are often precipitated by administration of drugs such as barbiturates and ethanol.

24. Types of Porphyrias

25. Degradation of Heme

26. Degradation of Heme RBCs last 120 days then, degraded by Reticulo-endothelial System (RES) (in liver & spleen) Hemoglobin (released from RBCs) Heme + globin (reused) Biliverdin Bilirubin Liver Bile Intestine feces

27. Degradation of Heme