PORPHYRIN METABOLISM

BY JYOTI ARORA

STRUCTURE• Porphyrins are cyclic compounds that bind to metallic

ions (usually Fe2+ or Fe3+)• Porphyrin + metal = metalloporphyrin• The most prevalent metallopophyrin in human is heme,

which consists of one Fe2+ coordinated in the center of tetrapyrrol ring through methyl bridges. The porphyrin in heme, with its particular arrangement of four methyl, two propionate, and the two vinyl substituents, is known as protoporphyrin IX.

• Heme is the prosthetic group for hemoglobin, myoglobin, the cytochromes, catalase and tryptophan pyrrolase.

Protoporphyrin III

prefix or suffix ring substituents between rings uro- acetate, propionate -- copro- methyl, propionate -- proto- methyl, propionate, vinyl -porphyrinogen -- methylene -porphyrin -- methene

Uroporphyrinogen I Coproporphyrinogen I

Overview of Heme Synthesis

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.

Succinyl CoA + Glycine

-aminolevulinic acid

-aminolevulinic acid

Porphobilinogen Uroporphyrinogen III Coproporphyrinogen III

Coproporphyrinogen III

Protoporphyrinogen IX

Protoporphyrin IX

Heme

ALA synthase

cytoplasm

mitochondrial matrix

SYNTHESIS• The porphyrins are constructed from four molecules of the monopyrolle

derivative porphobilinogen, which itself is derived from two molecules of δ-aminolevulinate.

• There are two major pathways to δ-aminolevulinate: in higher eukaryotes, glycine reacts with succinyl-CoA in the first step to yield

α-amino-β-ketoadipate, which is then decarboxylated to δ-aminolevulinate. In plants, algae, and most bacteria, δ-aminolevulinate is formed from

glutamate.The glutamate is first esterified to glutamyl-tRNAGlu ; reduction by NADPH converts the glutamate to glutamate 1-semialdehyde, which is cleaved from tRNA. An aminotransferase converts the glutamate 1-semialdehyde to δ-aminolevulinate.

Synthesis contd.

• In all organisms, two molecules of δ-aminolevulinate condense to form porphobilinogen and then the condensation of four molecules of porphobilinogen results in the formation of uroporphybilinogen lll.

• uroporphybilinogen lll is converted to heme by a series of decarboxylation and oxidation reactions

• The introducing of Fe+2 into protoporphyrin spontaneously, but the rate is enhanced by ferrochelatase ( an enzyme inhibited by lead.

REGULATION OF HEME BIOSYNTHESIS• The two major sites of heme biosynthesis are erythroid cells, which synthesize ~85%

of the body’s heme groups, and the liver, which synthesizes ~ 80% of the remainder. • An important function of heme in liver is as the prosthetic groups of the cytochromes

P450, a family of oxidative enzymes involved in detoxification, whose members are required throughout a liver cell’s lifetime in amounts that vary with conditions.

• In contrast, erythroid cells, in which heme is, of course, a hemoglobin component, engage in heme synthesis only on differentiation, when they synthesize hemoglobin in vast quantities.This is a onetime synthesis; the heme must last the erythrocyte’s lifetime (normally 120 days) since heme and hemoglobin synthesis stop on red cell maturation (protein synthesis stops on the loss of nuclei and ribosomes).

• The different ways in which heme biosynthesis is regulated in liver and in erythroid cells reflect these different demands: In liver, heme biosynthesis must really be “controlled,” whereas in erythroid cells, the process is more like breaking a dam.

Regulation contd.

IN LIVER :• The main control target in heme biosynthesis is ALA synthase, the

enzyme catalyzing the pathway’s first committed step. Heme, or its Fe(III) oxidation product hemin, controls this enzyme’s activity through three mechanisms:

(1) feedback inhibition, (2) inhibition of the transport of ALA synthase (ALAS) from its site of synthesis in the cytosol to its reaction site in the mitochondrion, and (3) repression of ALAS synthesis.IN ERYTHROCYTES• In erythroid cells, heme exerts quite a different effect on its

biosynthesis. Heme induces, rather than represses, protein synthesis in reticulocytes (immature erythrocytes).

Regulation contd.

• Moreover, the rate-determining step of heme biosynthesis in erythroid cells may not be the ALA synthase reaction.

• Experiments on various systems of differentiating erythroid cells implicate ferrochelatase and porphobilinogen deaminase in the control of heme biosynthesis in these cells. There are also indications that cellular uptake of iron may be rate limiting.

• Iron is transported in the plasma complexed with the iron transport protein transferrin. The rate at which the iron–transferrin complex enters most cells, including those of liver, is controlled by receptor-mediated endocytosis. However, lipid-soluble iron complexes that diffuse directly into reticulocytes stimulate in vitro heme biosynthesis.

CATABOLISM OF HEME• When hemoglobin is destroyed in the body, globin is degraded to its constituent

amino acids, which are reused, and the iron of heme enters the iron pool, also for reuse. The iron-free porphyrin portion of heme is also degraded, mainly in the reticuloendothelial cells of the liver, spleen, and bone marrow.

• The catabolism of heme from all of the heme proteins appears to be carried out in the microsomal fractions of cells by a complex enzyme system called heme oxygenase. By the time the heme derived from heme proteins reaches the oxygenase system, the iron has usually been oxidized to the ferric form, constituting hemin.

• the hemin is reduced to heme with NADPH, and, with the aid of more NADPH, oxygen is added to the α-methyne bridge between pyrroles I and II of the porphyrin. The ferrous iron is again oxidized to the ferric form. With the further addition of oxygen, ferric ion is released, carbon monoxide is produced, and an equimolar quantity of biliverdin results from the splitting of the tetrapyrrole ring.

Catabolism contd.

• In birds and amphibia, the green biliverdin IX is excreted; in mammals, a soluble enzyme called biliverdin reductase reduces the methylene bridge between pyrrole III and pyrrole IV to a methylene group to produce bilirubin, a yellow pigment

• Bilirubin formed in peripheral tissues is transported to the liver by plasma albumin. The further metabolism of bilirubin occurs primarily in the liver. It can be divided into three processes:

(1) uptake of bilirubin by liver parenchymal cells, (2) conjugation of bilirubin with glucuronate in the endoplasmic reticulum, and (3) secretion of conjugated bilirubin into the bile. Each of these processes will be considered separately.

THE LIVER TAKES UP BILIRUBIN• Bilirubin is only sparingly soluble in water, but its

solubility in plasma is increased by noncovalent binding to albumin. Bilirubin in excess of this quantity can be bound only loosely and thus can easily be detached and diffuse into tissues.

• In the liver, the bilirubin is removed from albumin and taken up at the sinusoidal surface of the hepatocytes by a carrier-mediated saturable system. This facilitated transport system has a very large capacity, so that even under pathologic conditions the system does not appear to be rate-limiting in the metabolism of bilirubin.

Catabolism contd.

Conjugation of Bilirubin with Glucuronic Acid Occurs in the Liver• Bilirubin is nonpolar and would persist in cells (eg, bound to lipids) if

not rendered water-soluble. Hepatocytes convert bilirubin to a polar form, which is readily excreted in the bile, by adding glucuronic acid molecules to it. This process is called conjugation and can employ polar molecules other than glucuronic acid (eg, sulfate).

• The conjugation of bilirubin is catalyzed by a specific glucuronosyltransferase. The enzyme is mainly located in the endoplasmic reticulum, uses UDP-glucuronic acid as the glucuronosyl donor, and is referred to as bilirubin-UGT. Bilirubin monoglucuronide is an intermediate and is subsequently converted to the diglucuronide

Catabolism contd.

Bilirubin Is Secreted into Bile• Secretion of conjugated bilirubin into the bile occurs by an active transport

mechanism, which is probably rate-limiting for the entire process of hepatic bilirubin metabolism.

• The protein involved is MRP-2 (multidrug-resistance-like protein 2), also called multispecific organic anion transporter (MOAT). It is located in the plasma membrane of the bile canalicular membrane and handles a number of organic anions.

Conjugated Bilirubin Is Reduced to Urobilinogen by Intestinal Bacteria• As the conjugated bilirubin reaches the terminal ileum and the large intestine, the

glucuronides are removed by specific bacterial enzymes (β-glucuronidases), and the pigment is subsequently reduced by the fecal flora to a group of colorless tetrapyrrolic compounds called urobilinogens. In the terminal ileum and large intestine, a small fraction of the urobilinogens is reabsorbed and reexcreted through the liver to constitute the enterohepatic urobilinogen cycle.

Catabolism contd.

DISORDERS IN HEME METABOLISMPORPHYRIA Porphyria are 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. Most porphyrias show a prevalent autosomal dominant pattern, except congenital eythropoietic porphyria, which is recessive.

The most common form of porphyria is acute intermittent porphyria.

Disorders: porphyria contd.

• One of the rarer porphyrias results in an accumulation of uroporphyrinogen I, an abnormal isomer of a protoporphyrin precursor. This compound stains the urine red, causes the teeth to fluoresce strongly in ultraviolet light, and makes the skin abnormally sensitive to sunlight. Many individuals with this porphyria are anemic because insufficient heme is synthesized. This genetic condition may have given rise to the vampire myths of folk legend.

JAUNDICE• When bilirubin in the blood exceeds 1 mg/dL (17.1 μmol/L),

hyperbilirubinemia exists. Hyperbilirubinemia may be due to the production of more bilirubin than the normal liver can excrete, or it may result from the failure of a damaged liver to excrete bilirubin produced in normal amounts. In the absence of hepatic damage, obstruction of the excretory ducts of the liver—by preventing the excretion of bilirubin—will also cause hyperbilirubinemia. In all these situations, bilirubin accumulates in the blood, and when it reaches a certain concentration (approximately 2–2.5 mg/dL), it diffuses into the tissues, which then become yellow. That condition is called jaundice or icterus.

Disorders contd.

Elevated Amounts of Unconjugated Bilirubin in Blood Occur in a Number of Conditions• Hemolytic anemias • Neonatal “Physiologic Jaundice” • Type I Crigler–Najjar syndrome• Crigler–Najjar Syndrome, Type II.• Gilbert Syndrome • Toxic Hyperbilirubinemia

Disorders: jaundice contd.

Obstruction in the Biliary Tree Is the Most Common Cause of Conjugated Hyperbilirubinemia• Obstruction of the Biliary Tree• Dubin–Johnson Syndrome• Rotor Syndrome

Some Conjugated Bilirubin Can Bind Covalently to Albumin

Disorders: jaundice contd.

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