2 Structure and Properties of Iron Protoporphyrin IX propionatemethylpyrrole ringvinylDerived from protoporphyrin IXPattern of side chains defines isomerBinds metals: Heme- Fe2+ (ferrous)Hemin- Fe3+ (ferric)Zinc protoporphyrin (ZnPP)- Zn2+Extended conjugation across ring systemPhotoreactive generation of Reactive Oxygen Species (ROS)
3 Amino Acid Pools are in Steady State Majority of amino acids used for de novo protein synthesis (80%) derives from the degradation of existing proteinsOnly 30 g (6%) used for synthesis of specialized productsOnly 70 g (14%) of total amino acid utilization is used for energy or stored as glycogen/fatty acids in the well-fed state (nitrogen balance)
4 Overview of Heme Synthesis Protoporphyrin IXSuccinyl CoA + GlycineALA synthaseProtoporphyrinogen IX-aminolevulinic acidCoproporphyrinogen IIImitochondrial matrixcytoplasm-aminolevulinic acidPorphobilinogenUroporphyrinogen IIICoproporphyrinogen IIIUroporphyrinogen ICoproporphyrinogen IHeme 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.
6 -Aminolevulinate (ALA) Dehydratase Asymmetry of the reaction results in acetate and propionate side chainsThe enzyme active site contains a required cysteine, making the enzyme sensitive to inactivation by lead (Pb2+) and other heavy metalsIncreased urinary excretion of -aminolevulinate is a leading indicator of heavy metal poisoning
7 Formation of the Final Ring Requires a Bi-functional Enzyme This step occurs by elimination of the primary amines as the methylene adds across the double bond of the pyrrole ring.Note: Uroporphyrinogen I synthase is alternate name for Hydroxymethylbilane synthase
8 Uroporphyrinogen Decarboxylase Remodels the Acetate Side Chains Spontaneously oxidizes to the biologically inactive Coproporphyrin I and III which are subsequently excreted in the urine
9 Coproporphyrinogen III Oxidase Catalyzes the Oxidative Decarboxylation of Specific Propionate Side Chains
10 Protoporphyrinogen IX Oxidase This reaction oxidizes the methylene bridge carbons between the pyrrole rings to methenyl bridge carbons, allowing extended conjugation through the entire tetrapyrrole ring system for the first time.
11 Ferrochelatase Inserts Fe2+ into Protoporphyrin IX to yield heme The reaction also requires ascorbic acid and cysteine as reducing agentsLead (Pb2+) acts as a competitive inhibitor of Fe2+ but does not insert into protoporphyrin IXIron deficiency leads to insertion of Zn2+ to yield zinc protoporphyrin (ZnPP), an important clinical indicator of iron deficiency
12 Porphyrias Caused by hereditary or acquired defects in heme synthesis - Accumulation and increased excretion of metabolicprecursors (each unique)- Most porphyrias show a prevalent autosomal dominant pattern, except congenital eythropoietic porphyria, which is recessiveCan be hepatic or erythropoietic, reflecting the two major locations of heme synthesis- hepatic can be acute or chronicThose with tetrapyrrole intermediates show photosensitivity due to extended conjugated double bonds- Formation of superoxide radicals- Skin blisters, itches (pruritis)- Skin may darken, grow hair (hypertrichosis)
14 Acquired PorphyriasLead poisoning- inhibition of ferrochelatase and ALA dehydratase- displaces Zn+2 at enzyme active siteChildren- developmental defects- drop in IQ- hyperactivity- insomnia- many other health problemsAdults- severe abdominal pain- mental confusion- many other symptoms
15 Most heme from RBCs (85%) - rest from turnover of cytochromes, p450s, immature erythrocytes. RBCs last 120 days, degraded by reticuloendothelial (RE) system [liver and spleen].Microsomal heme oxygenase hydroxylates methenyl bridge carbon and oxidizes Fe2+ to Fe3+. Second reaction open ring and release methenyl carbon as CO.The resulting biliverdin is poorly soluble due to ring stacking and aggregation.Serum albumin carries bilirubin in circulation, ligandin in hepatocytes.
17 Types of Jaundice Hemolytic jaundice - Liver can handle 3000 mg bilirubin/day - normal is 300- Massive hemolysis causes more than can be processed- cannot be conjugated- increased bilirubin excreted into bile, urobilinogenis increased in blood, urine- unconjugated bilirubin in blood increases = jaundiceObstructive jaundice- Obstruction of the bile duct- tumor or bile stones- gastrointestinal pain - nausea- pale, clay-colored stools- can lead to liver damage and increased unconjugated bilirubin
18 Hepatocellular jaundice - Liver damage (cirrhosis or hepatitis) cause increasedbilirubin levels in blood due to decreased conjugation- Conjugated bilirubin not efficiently exported to bileso diffuses into blood- Decreased urobilinogen in enterohepatic circulationso urine is darker and stool is pale, clay-colored- AST and ALT levels are elevated due to hepatic damage- Nausea and anorexia
19 Jaundice in NewbornsPremature babies often accumulate bilirubin due tolate onset of expression of bilirubin glucuronyltransferase- Maximum expression (adult level) at ~ 4 weeks- Excess bilirubin can cause toxic encephalopathy(kernicterus)- Treated with blue fluorescent light- converts bilirubin to more polar compound- can be excreted in bile without conjugation- Crigler-Najjar syndrome is deficiency in bilirubinglucuronyltransferase