BIOCHEMISTRY TRANS 9b - Amino Acid Conversion to Special Products.pdf

BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products Brendo M. Jandoc, M.D. TOPIC OUTLINE ii. Coproporphyrin I. Synthesis and Degradation...

BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products Brendo M. Jandoc, M.D. TOPIC OUTLINE ii. Coproporphyrin I. Synthesis and Degradation of Porphyrins · methyl (-CH3) A. Porphyrin Structure · propionate B. Porphyrin Biosynthesis C. Porphyrias D. Heme Degtadation E. Jaundice (Icterus) II. Transport and Storage of Iron III. SUMMARY I. SYNTHESIS AND DEGRADATION OF PORPHYRINS Porphyrins  Porphyrins are cyclic compounds that readily bind metal ions— 3. Side Chain Distribution usually Fe2+ or Fe3+. - ordered around the tetrapyrrole nucleus  Heme - most prevalent metalloporphyrin in humans - 4 types designated in roman numerals I to IV - consists of a. Type III · Fe++ - contain an asymmetric distribution on ring D · tetrapyrrole ring of protoporphyrin IX - the only physiologically important in human - prosthetic group for b. Congenital Erythropoietic Porphyria · hemoglobin - type I (contain symmetric arrangement of substituents) · myoglobin porphyrins are synthesized in appreciable quantities · cytochromes · catalase 4. Porphyrinogens · tryptophan pyrrolase - chemically reduced form - 6-7 gm of hemoglobin synthesized per day to replace heme - porphyrin precursors lost through catabolism - colorless - concomitant with hemeprotein turnover - intermediates between porphobilinogen and · simultaneous synthesis and degradation of the protoporphyrin in heme synthesis associated porphyrins and recycling of the bound iron - ex: uroporphyrinogen ions 5. Heme · quantitative importance in the body’s nitrogen balance - porphyrin-ring structure constructed from 4 pyrrole rings A. Porphyrin Structure - final product of porphyrin synthetic pathway 1. Ring Structure - contains Fe++ coordinated in the center of the - cyclic tetrapyrrole ring of protoporphyrin IX - formed by the linkage of 4 pyrrole rings through methyl - quantitatively the most important porphyrin in humans bridges  Functions - prosthetic groups of several proteins and enzymes · hemoglobin · cytochrome c · catalase · certain peroxidases B. Protoporphyrin Biosynthesis a. Major Sites of Heme Biosynthesis i. Livers - synthesizes a number of heme proteins (particularly cytochrome P450) - fluctuating demands for heme proteins highly variable rate of heme synthesis responding to alterations in the cellular heme pool ii. Erythrocyte-Producing Cells of the Bone Marrow - active in hemoglobin synthesis 2. Side Chains - relatively constant heme synthesis matching the - vary in nature rate of globin synthesis - attached to each of the 4 pyrrole rings b. Biosynthetic Reactions - Examples i. Mitochondria i. Uroporphyrin - initial step · acetate (-CH2-COO-) - last 3 steps · propionate (-CH2-CH2-COO-) Trans FINALS ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 1 of 7  BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products ii. Cytosol uroporphyrinogen III synthase → isomerization reaction → - intermediate steps formation of asymmetric uroporphyrinogen III c. Mature RBCs - lacks mitochondria unable to synthesize heme 1. δ-Aminolevulinic Acid (ALA) Formation - rate controlling step of porphyrin synthesis a. Carbon and Nitrogen Atoms of Porphyrin molecules are provided by: i. Glycine · nonessential amino acid ii. Succinyl CoA · TCA cycle intermediate → condensation reaction → ALA formation b. Enzyme - ALA synthase c. Coenzyme - pyridoxal phosphate d. End Product Inhibition by Hemin - porphyrin production exceeds globin or other apoprotein availability → excess heme → oxidation of Fe++ to Fe+++ → hemin → elevated concentration → decreases activity of ALA synthase - Heme - inhibits ALA synthase activity · represses ALA synthase gene transcription e. Effects of Drugs on ALA Synthase Activity - large number of drugs (ex: Phenobarbital, Griseofulvin, hydantoins) administered (metabolized by microsomal cytochrome P450 system mono- oxygenase system (hepatic heme protein oxidase system) → increased cytochrome P450 synthesis → enhanced heme Conversion of Porphobilinogen to Uroporphyrinogens (component of cytochrome P450) consumption → decreased Uroporphyrinogen Synthase I liver (intracellular) heme concentration → increased ALA - also called porphobilinogen (PBG) deaminase or synthase synthesis (derepression) → increase ALA synthesis hydroxymethylbilane (HMB) synthase f. Lead - other inhibitor b. Disorders 2. Porphobilinogen Formation i. Acute Intermittent Porphyria  Enzyme  Symptoms - δ-ALA dehydratase (porphobilinogen synthase) → dehydration · abdominal pain of 2 ALA molecules → porphobilinogen (contains pyrrole ring) · neurologic abnormalities - extremely sensitive to inhibition by heavy metal ions ii. Congenital Erythropoietic Porphyria - partly responsible for ALA elevation and anemia in lead  Symptoms poisoning · photosensitivity · severe skin lesions 3. Uroporphyrinogen Formation · hemolytic anemia a. Enzyme - uroporphyrinogen I synthase (hydroxymethylbilane 4. Protoporphyrin IX synthase) → symmetric condensation of 4 molecules of - formed from uroporphyrinogen III porphobilinogen molecules → open-chain tetrapyrrole → cyclization → uroporphyrinogen I (tetrapyrrole ring) → Trans FINALS ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 2 of 8  BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products - removes 6 hydrogen atoms → protoporphyrin III 5. Protoheme IX (Heme) · formed from protoporphyrin IX by the insertion of iron (Fe++) by ferrochetalase a. Enzyme Deficiency  protoporphyria b. Symptoms · dermatologic problems Steps in the Biosynthesis of the Porphyrin Derivatives from a. Uroporphyrinogen Decarboxylase - decarboxylation of 4 of the side chains of uroporphobilinogen III → coproporphyrinogen III i. Enzyme Deficiency  porphyria cutanea tarda ii. Symptoms · photosensitivity · dermatologic problems Porphobilinogen * Uroporphyrinogen I Synthase - also called porphobilinogen deaminase or hydroxymethylbilane synthase C. Porphyrias o rare, inherited (or occasionally acquired) defects in heme synthesis, resulting in the accumulation and increased excretion of porphyrins or porphyrin precursors. a. Classification Decarboxylation of Uroporphyrinogens to Coproporphyrinogens in - depending on whether the enzyme deficiency occurs in Cytosol the RBCs or liver b. Coproporphyrinogen Oxidase i. Erythropoietic - decarboxylates 2 of the side chains of ii. Hepatic coproporphyrinogen III → protoporphyrinogen III · acute i. Enzyme Deficiency · chronic  coproporphyria b. Autosomal Dominant Disorders ii. Symptoms - except congenital erythropoietic porphyria (genetically · abdominal pain recessive) · neurologic abnormalities c. Mutations · photosensitivity in some cases - heterogenous (not all are at the same DNA locus) c. Protoporphyrinogen Oxidase Trans FINALS ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 3 of 8  BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products bi. Characteristics of Acute Attacks  gastrointestinal  neurologic / psychiatric  cardiovascular symptoms bii. Barbiturates, Ethanol  induce synthesis of heme-containing cytochrome P450 microsomal drug oxidation system →further decreases amount of available heme → promotion of increased ALA synthase synthesis → precipitation of symptoms i. Acute Intermittent Porphyria - lead to accumulation of ALA and porphobilinogen · abdominal pain · neuropsychiatric disturbances ii. Hereditary Coproporphyria iii. Variegate Porphyria c. Erythropoietic Porphyrias ci. Characteristics · skin rashes · blisters · early in childhood cii. Complicated By Biochemical Causes of the Major Signs and Symptoms of the Porphyrias · cholestatic liver cirrhosis · progressive hepatic failure 1. Clinical Manifestations i. Congenital Erythropoietic Porphyria o Porphyrias Leading to Tetrapyrrole Intermediate ii. Erythropoietic Protoporphyria Accumulation 2. Increased ALA Synthase Activity - thought to be due to porphyrin-mediated formation - porphyrias → decreased heme (ALA synthase repressor) of superoxide radicals from oxygen → oxidatively synthesis → increased ALA synthase synthesis damage membranes → release of lysosomal (derepression) → increased synthesis of intermediates that enzymes → destruction of cellular components occur prior to the genetic block →photosensitivity and pruritus (skin itches and burns 3. Treatment when exposed to visible light) a. Intravenous Hemin Injection a. Chronic Porphyria: Porphyria Cutanea Tarda - decreases ALA synthase synthesis - most common b. Sunlight Avoidance - chronic disease of the c. β-Carotene · liver - free radical scavenger · erythroid tissues i. Enzyme Deficiency in Uroporphyrinogen Decarboxylase D. Heme Degradation  clinical expression of enzyme deficiency is influenced by:  Red Cell Destruction - hepatic iron overload a. RBCs – After approximately 120 days in the circulation RBCs - sunlight exposure are taken up and degraded by the reticuloendothelial - infections system (RES) particularly the liver and spleen · hepatitis B o Degraded Heme · hepatitis C - 85% from RBCs · HIV - 15% from: ii. Clinical Onset · turnover of immature RBCs  4th or 5th decade of life · cytochrome from extrathyroid tissues iii. Porphyrin Accumulation b. Red Cell Destruction in Other Sites (Other Than the Spleen and Liver)  cutaneous symptoms i. 2 Carrier Proteins - colored urine - bind hemoglobin · red to brown in natural a. Haptoglobin sunlight · bind methemoglobin dimers · pink to red in fluorescent b. Hemopexin light · bind free heme ii. Iron – ferric state (methemoglobin) b. Acute Hepatic Porphyrias – free heme 1. Bilirubin Formation Trans FINALS ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 4 of 8  BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products → The first step in the degradation of heme is catalyzed by the - also bind to albumin (more weakly than bilirubin) microsomal heme oxygenase system of the c. Anionic Drugs reticuloendothelial cells. - displace bilirubin from albumin → bilirubin to enter CNS a. Microsomal Heme Oxygenase System of Reticuloendothelial → potential neural damage in infants System + NADPH + O2 - Examples - adds hydroxyl group to the methenyl bridge between · sulfonamides 2 pyrrole rings oxidation of Fe++ → Fe+++ · salicylates → 2nd oxidation (same enzyme) results in the cleavage of the 3. Bilirubin Diglucuronide Formation porphyrin ring (rupture of methylidyne group of heme - bilirubin + 2 molecules of glucuronic acid → bilirubin between pyrrole rings carrying vinyl groups) → release of diglucuronide (more water soluble) Fe+++ and carbon monoxide (original bridge carbon) → In the hepatocyte, the solubility of bilirubin is increased by the addition green pigment biliverdin is produced as ferric iron and CO of two molecules of glucuronic acid. are released → reduction by NADPH → bilirubin (poorly a. Enzyme soluble, red-orange) - bilirubin glucuronyl transferase (uridine diphosphate b. Bile Pigments (UDP) - glucuronyl transferase) - bilirubin b. UDP-Glucuronic Acid - bilirubin derivatives - glucuronate donor c. Changing Colors of a Bruise - reflects the varying pattern of intermediates occurring during heme degradation 2. Hepatic Uptake of Bilirubin Conjugation of Bilirubin with Glucuronic Acid UDP - Glucuronic Acid - glucuronate donor - formed from UDP-glucose UDP - Glucuronosyltransferase - also called bilirubin-UGT Bilirubin Diglucuronide (Conjugated, "Direct-Reacting" Bilirubin) Glucuronic Acid - attached via ester linkage to the two propionic acid groups of bilirubin to form an acylglucuronide c. Bilirubin Conjugates - also bind to albumin but much more weakly than does a. Bilirubin unconjugated bilirubin - slightly soluble in the plasma → transported to the liver d. Deficiencies by binding noncovalently to albumin → dissociates from i. Crigler-Najjar Syndrome the carrier albumin molecule → enters a hepatocyte via · UDP- glucuronyl transferase deficiency facilitated diffusion → binding with ligandin (intracellular · results in severe jaundice protein) ii. Neonatal Jaundice b. Bilirubin Conjugates Trans FINALS ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 5 of 8  BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products 4. Bilirubin Excretion into Bile b. Posthepatic (Obstructive) Jaundice - bilirubin diglucuronide transported - jaundice is not caused by overproduction of bilirubin or against concentration gradient into the decreased conjugation, but instead results from obstruction of bile canaliculi and then into the bile the bile duct - energy-dependent, rate-limiting step i. Hepatic Tumors, Bile Stones - susceptible to impairment in liver disease - bile duct obstruction → bilirubin passage into the intestines - Unconjugated Bilirubin prevented · normally not excreted - liver “regurgitates” conjugated bilirubin into the blood → excreted into the urine 5. Formation of Urobilins in the Intestines - prolonged obstruction → liver damage → increased - bilirubin is hydrolyzed and reduced by unconjugated bilirubin bacteria in the gut → urobilinogen ii. Presentations (colorless) - GI pain · most of the urobilinogen is oxidized by - nausea intestinal bacteria to stercobilin, which - stools that are gives feces the characteristic brown color - pale · some of the urobilinogen is reabsorbed - clay colored from the gut and enters the portal blood c. Hepatic (Hepatocellular) Jaundice to the kidney, where it is converted to i. Hepatocyte Damage urobilin and excreted, giving urine its - damage to liver cells results to decreased conjugation characteristic yellow color increasing unconjugated bilirubin levels in the blood [IMAGE: CATABOLISM OF HEME, p.7] - bilirubin that is conjugated → not efficiently secreted into the bile → diffuses (“leaks”) into the blood decreased E. Jaundice (Icterus) enterohepatic circulation → increased urinary urobilinogen  caused by deposition of bilirubin, secondary ia. Cirrhosis to increased bilirubin levels in the blood ib. Hepatitis (hyperbilirubinemia) → yellow color of the: ii. Decreased · skin - bilirubin uptake · sclerae - conjugated bilirubin production · nail beds iii. Increased  not a disease itself but a - unconjugated bilirubin in the blood symptom of an underlying - urobilinogen in the urine disorder - AST (SGOT) 1. Types of Jaundice - ALT (SGPT) a. Prehepatic (Hemolytic) Jaundice iv. Urine - liver has the capacity to conjugate and excrete over 3000 - dark in color mg bilirubin/day v. Stools - normal bilirubin production: 300 mg/day - pale  Bilirubin Production > Hepatic Conjugation Capacity - clay color - massive RBC lysis vi. vi. Symptoms · sickle cell anemia - nausea · pyruvate kinase deficiency - anorexia · glucose 6-phosphate dehydrogenase deficiency 2. Jaundice in Newborns · malaria o temporary → increased blood unconjugated bilirubin levels o low hepatic bilirubin glucuronyl transferase production → more bilirubin excreted in the bile - reaches adult levels in 4 weeks · increased amount a. Kernicterus of urobilinogen - elevated bilirubin in excess of entering the binding capacity of albumin → enterohepatic diffuse into basal ganglia → circulation toxic encephalopathy · increased urinary (kernicterus) urobilinogen b. Blue Fluorescent Light (Phototherapy) (Unconjugated bilirubin levels in - converts bilirubin → more the blood become elevated, polar → water-soluble causing Jaundice) photoisomers → excreted into the bile without conjugation to glucuronic acid Trans FINALS ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 6 of 8  BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products 3. Determination of Bilirubin Concentration C. Transport a. van den Bergh Reaction o Transferrin - diazotized sulfanilic acid + bilirubin → red - synthesized by the liver azodipyrroles (measured colorimetrically) - very high affinity to Fe+++ → binds and transport i. In Aqueous Solution Fe+++ (2 atoms per molecule) - water-soluble conjugated bilirubin reacts rapidly with the reagent → “direct reaction” D. Storage - unconjugated bilirubin 1. Ferritin · reacts more slowly - protein in cells · “indirect-reacting” - 24 subunits bind very large number of Fe+++ atoms · total minus the direct - arranged in a sphere → channels formed Fe+++ enters ii. In Methanol the core → deposited as hydroxyphosphates - conjugated and unconjugated bilirubin react → 2. Hemosiderin total bilirubin - very high Fe+++ intakes → some Fe+++ found in granules b. Normal Plasma of hemosiderin - 4% of total bilirubin is conjugated - complex of: · Fe+++ II. TRANSPORT AND STORAGE OF IRON (FE+++) · protein · polysaccharide A. Intake - 3% Fe+++ by weight B. Absorption CATABOLISM OF HEME Trans FINALS ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 7 of 8  BIOCHEMISTRY AMINO ACID: Conversion to Specialized Products III. SUMMARY Trans FINALS ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 8 of 8