Conversion of Amino Acids to Specialized Products

1. Conversion of Amino Acids to Specialized Products 1 Dr. Nikhat Siddiqi

2. In addition to serving as building blocks for proteins, amino acids are precursors of many nitrogen-containing compounds that have important physiologic functions. These molecules include porphyrins, neurotransmitters, hormones, purines, and pyrimidines. 2 Dr. Nikhat Siddiqi

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4. Porphyrin Metabolism 4 Dr. Nikhat Siddiqi

5. Porphyrins are cyclic compounds that readily bind metal ions�usually Fe2+ or Fe3+. The most prevalent metalloporphyrin in humans is heme, which consists of one ferrous (Fe2+) iron ion coordinated in the center of the tetrapyrrole ring of protoporphyrin IX. Heme is the prosthetic group for hemoglobin, myoglobin, the cytochromes, catalase, and tryptophan pyrrolase. 5 Dr. Nikhat Siddiqi

6. Structure of porphyrins Porphyrins are cyclic molecules formed by the linkage of four pyrrole rings through methenyl bridges. Three structural features of these molecules are relevant to understanding their medical significance. 6 Dr. Nikhat Siddiqi

7. Structures of uroporphyrin I and uroporphyrin III 7 Dr. Nikhat Siddiqi

8. Side chains Different porphyrins vary in the nature of the side chains that are attached to each of the four pyrrole rings. Uroporphyrin contains acetate (�CH2�COO-) and propionate (�CH2�CH2�COO-) side chains, coproporphyrin contains methyl (�CH3) and propionate groups, and protoporphyrin IX (and heme) contains vinyl (�CH=CH2), methyl, and propionate groups. 8 Dr. Nikhat Siddiqi

9. Distribution of side chains The side chains of porphyrins can be ordered around the tetrapyrrole nucleus in four different ways, designated by Roman numerals I to IV. Only Type III porphyrins, which contain an asymmetric substitution on ring D, are physiologically important in humans. 9 Dr. Nikhat Siddiqi

10. Porphyrinogens These porphyrin precursors (for example, uroporphyrinogen) exist in a chemically reduced, colorless form, and serve as intermediates between porphobilinogen and the oxidized, colored protoporphyrins in heme biosynthesis. 10 Dr. Nikhat Siddiqi

11. Biosynthesis of heme The major sites of heme biosynthesis are the liver, which synthesizes a number of heme proteins (particularly cytochrome P450), and the erythrocyte-producing cells of the bone marrow, which are active in hemoglobin synthesis. In the liver, the rate of heme synthesis is highly variable, responding to alterations in the cellular heme pool caused by fluctuating demands for heme proteins. In contrast, heme synthesis in erythroid cells is relatively constant, and is matched to the rate of globin synthesis. 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. 11 Dr. Nikhat Siddiqi

12. Summary of heme synthesis 12 Dr. Nikhat Siddiqi

13. Formation of d-aminolevulinic acid (ALA) All the carbon and nitrogen atoms of the porphyrin molecule are provided by two simple building blocks: glycine (a nonessential amino acid) and succinyl coenzyme A CoA (an intermediate in the citric acid cycle). Glycine and succinyl CoA condense to form ALA in a reaction catalyzed by ALA synthase. This reaction requires pyridoxal phosphate as a coenzyme, and is the committed and rate-controlling step in hepatic porphyrin biosynthesis. 13 Dr. Nikhat Siddiqi

14. End-product inhibition by hemin When porphyrin production exceeds the availability of globin (or other apoproteins), heme accumulates and is converted to hemin by the oxidation of Fe2+ to Fe3+. Hemin decreases the activity of hepatic ALA synthase by causing decreased synthesis of the enzyme, through inhibition of mRNA synthesis and use (heme decreases stability of the mRNA). In erythroid cells, heme synthesis is under the control of erythropoietin and the availability of intracellular iron. 14 Dr. Nikhat Siddiqi

15. Effect of drugs on ALA synthase activity Administration of any of a large number of drugs, such as griseofulvin (an antifungal agent), hydantoins and phenobarbital (anticonvulsants used to treat epilepsy), results in a significant increase in hepatic ALA synthase activity. These drugs are metabolized by the microsomal cytochrome P450 monooxygenase system�a hemeprotein oxidase system found in the liver. In response to these drugs, the synthesis of cytochrome P450 proteins increases, leading to an enhanced consumption of heme�a component of cytochrome P450 proteins. This, in turn, causes a decrease in the concentration of heme in liver cells. The lower intracellular heme concentration leads to an increase in the synthesis of ALA synthase (derepression), and prompts a corresponding increase in ALA synthesis. 15 Dr. Nikhat Siddiqi

16. Formation of porphobilinogen The condensation of two molecules of ALA to form porphobilinogen by ALA dehydratase is extremely sensitive to inhibition by heavy metal ions. This inhibition is, in part, responsible for the elevation in ALA and the anemia seen in lead poisoning. 16 Dr. Nikhat Siddiqi

17. Formation of uroporphyrinogen The condensation of four porphobilinogens produces the linear tetrapyrrole, hydroxymethylbilane, which is isomerized and cyclized by uroporphyrinogen III synthase to produce the asymmetric uroporphyrinogen III. This cyclic tetrapyrrole undergoes decarboxylation of its acetate groups, generating coproporphyrinogen III. These reactions occur in the cytosol 17 Dr. Nikhat Siddiqi

18. Formation of heme Coproporphyrinogen III enters the mitochondrion, and two propionate side chains are decarboxylated to vinyl groups generating protoporphyrinogen IX, which is oxidized to protoporphyrin IX. The introduction of iron (as Fe+2) into protoporphyrin IX occurs spontaneously, but the rate is enhanced by ferrochelatase, an enzyme that, like ALA dehydratase, is inhibited by lead. 18 Dr. Nikhat Siddiqi

19. Porphyrias Porphyrias are rare, inherited (or occasionally acquired) defects in heme synthesis, resulting in the accumulation and increased excretion of porphyrins or porphyrin precursors. 19 Dr. Nikhat Siddiqi

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21. Clinical manifestations The porphyrias are classified as erythropoietic or hepatic, depending on whether the enzyme deficiency occurs in the erythropoietic cells of the bone marrow or in the liver. Hepatic porphyrias can be further classified as acute or chronic. Individuals with an enzyme defect leading to the accumulation of tetrapyrrole intermediates show photosensitivity�that is, their skin itches and burns (pruritis) when exposed to visible light. 21 Dr. Nikhat Siddiqi

22. Chronic porphyria Porphyria cutanea tarda, the most common porphyria, is a chronic disease of the liver and erythroid tissues. The disease is associated with a deficiency in uroporphyrinogen decarboxylase. Porphyrin accumulation leads to cutaneous symptoms, and urine that is red to brown in natural light, and pink to red in fluorescent light. 22 Dr. Nikhat Siddiqi

23. Acute hepatic porphyrias Acute hepatic porphyrias (acute intermittent porphyria, hereditary coproporphyria, and variegate porphyria) are characterized by acute attacks of gastrointestinal, neurologic/psychiatric, and cardiovascular symptoms. Porphyrias leading to accumulation of ALA and porphobilinogen, such as acute intermittent porphyria, cause abdominal pain and neuropsychiatric disturbances. Symptoms of the acute hepatic porphyrias are often precipitated by administration of drugs such as barbiturates and ethanol, which induce the synthesis of the heme-containing cytochrome P450 microsomal drug oxidation system. This further decreases the amount of available heme, which, in turn, promotes the increased synthesis of ALA synthase. 23 Dr. Nikhat Siddiqi

24. Erythropoietic porphyrias The erythropoietic porphyrias (congenital erythropoietic porphyria and erythropoietic protoporphyria) are characterized by skin rashes and blisters that appear in early childhood. 24 Dr. Nikhat Siddiqi

25. Increased ALA synthase activity One common feature of the porphyrias is a decreased synthesis of heme. In the liver, heme normally functions as a repressor of ALA synthase. Therefore, the absence of this end product results in an increase in the synthesis of ALA synthase (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. 25 Dr. Nikhat Siddiqi

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