BIOC 312 Lecture 8-4 Heme Biosynthesis and Degradation PDF

Conversion of amino acids to specialized products Nucleic acid metabolism  Porphyrins Porphyrins: Are cyclic compounds that readily bind metal ions: ferrous or ferric ion. Four pyrrole rings The formation of complexes with metal ions bound to the nitrogen...

Conversion of amino acids to specialized products Nucleic acid metabolism  Porphyrins Porphyrins: Are cyclic compounds that readily bind metal ions: ferrous or ferric ion. Four pyrrole rings The formation of complexes with metal ions bound to the nitrogen atom of the pyrrole ring. Porphyrins  Structure A) Side chains: May vary in the nature of the said chains attached to each of the four pyrrole rings. Uroporphyrin I has Acetate and Propionate side chains Coproporphyrin has Methyl and Propionate side chains Protoporphyrin IX has Methyl, Vinyl and Propionate  Structure B) Side chains distribution: Only Type III porphyrins are normally important for humans. asymmetric Symmetric  Porphyrins Example of some important human and animal hemeproteins : Rapidly synthesized and degraded Structure of Heme Ferrous iron (Fe2+) V Protoporphyrin IX: contains tetrapyrrole rings linked M together by methenyl bridges I M M IV II P V III The side chains are: methyl = M M vinyl = V propionyl = P P Where does heme biosynthesis take place? 1- Hepatocytes Synthesizes 15% of heme proteins particularly Cytochrome P450 Rate of heme 2- synthesis is variable (alteration of heme pool). Erythrocyte-producing cells –bone marrow Active in HB synthesis Rate of heme synthesis is constant About 85% of heme occurs in Erythroid tissue. Note: mature RBCs can NOT make heme??????  Where exactly in the cells? The pathway takes place in the mitochondria and the cytosol The pathway starts and ends at the mitochondria  Heme synthesis steps process 1 and 2 Formation of a pyrrole molecule: a compound containing four carbon atoms and one nitrogen atom joined in a ring structure 3 Linkage of four pyrrole molecules to form a linear tetramer 4 Formation of the porphyrin (tetrapyrrole) ring 5-7 Decarboxylation and oxidation 8 Incorporation of the iron atom  First step in heme pathway is a committed step It’s involve a condensation reaction between Glycine (a nonessential amino acid) and succinyl CoA (an intermediate from TCA cycle). This reaction is catalyzed by the mitochondrial enzyme Aminolevulinic acid synthase (ALAS) which requires Coenzyme PLP Rate limiting step in the heme synthesis pathway. There are two isoforms of this enzyme: ALA-S1 is located in all tissues ALA-S2 is produced in erythroid tissue.  Succinyl Glycine + CoA _ end product 1 Pyrrole formation Aminolevulinic acid synthase (ALAS1) 2 Aminolevulinic acid Pyrrole formation (ALA) 2 H2O ALA dehydratase Or PBG synthase 4 Porphobilinogen (PBG) 3 Linear tetrpyrrole Hydroxymethylbilane synthase Hydroxymethylbilane 4 Tetrapyrrole ring Uroporphyrinogen III synthase Uroporphyrinogen III CO2 Uroporphyrinogen decarboxylase 5 Decarboxylation Coproporphyrinogen III Hemin 6 Decarboxylation/ oxidation CO2 Coproporphyrinogen oxidase protoporphyrinogen IX oxidation protoporphyrinogen oxidase 7 Ferrochelatase protoporphyrin IX Heme Fe+2 8 Heme formation  Effect of alcohol and drugs on heme synthesis ALAS1 CP450 Alcohol/drugs Detoxification of drugs heme Low level of heme in liver cells -> increase the synthesis of ALAS1- > ALA synthesis  Succinyl Lead poisoning Glycine + CoA Aminolevulinic acid synthase 2 ALA ALA dehydratase Lead 4 Porphobilinogen 1- factory workers 2- kid eats paint Hydroxymethylbilane synthase containing lead Hydroxymethylbilane Uroporphyrinogen III synthase Uroporphyrinogen III Uroporphyrinogen decarboxylase Coproporphyrinogen III Coproporphyrinogen oxidase protoporphyrinogen IX Lead protoporphyrinogen oxidase Ferrochelatase protoporphyrin IX Heme Fe+2 Porphyria: Vampire disease Porphyrias are rare metabolic diseases resulting from a defect (Deficiency of an enzyme) in heme biosynthesis. It’s result in the accumulation of porphyrins or porphyrins precursors. It classified as: 1) hepatic porphyria (chronic or acute) 2) erythropoietic porphyria  Succinyl Chronic hepatic Glycine + CoA porphyria or porphyria cutanea tarda Aminolevulinic acid (PCT) synthase Chronic Liver disease 2 Aminolevulinic acid Most common porphyria Patient are photosensitive Aminolevulinate dehydratase 4 Porphobilinogen Hydroxymethylbilane synthase Kidney Hydroxymethylbilane Uroporphyrinogen III synthase Photoactive Built up Uroporphyrinogen III molecule that Uroporphyrinogen is able to PCT decarboxylase absorb UV Coproporphyrinogen III light from the Urine red to brown Coproporphyrinogen oxidase sun causing (sun burn) protoporphyrinogen IX protoporphyrinogen oxidase Ferrochelatase protoporphyrin IX Heme Fe+2  Succinyl Acute hepatic Glycine + CoA porphyria Or hepatic acute intermittent Aminolevulinic acid porphyria (AIP) synthase drugs 2 ALA Built up ALA dehydratase 4 PBG Hydroxymethylbilane synthase In AIP Kidney Hydroxymethylbilane Uroporphyrinogen III synthase Uroporphyrinogen III Uroporphyrinogen decarboxylase Urine turns dark Red Coproporphyrinogen III Coproporphyrinogen oxidase protoporphyrinogen IX protoporphyrinogen oxidase Ferrochelatase protoporphyrin IX Heme Fe+2  Degradation of Heme Hb in RBCs life span is around 120 days About 80-85% of heme comes from RBC myoglobin Heme Cytochromes  Location of heme degradation Macrophage system (reticuloendothelial system) of RBC mainly occurs in liver and spleen.  Degradation of Heme Hemoglobin Heme Globin Fe2+ Return to Bilirubin Amino acids body iron stores Excreted Degradation of Heme Microsomal heme oxygenase in macrophage catalyzes three oxygenations results in: a) Open porphyrin ring -> biliverdin b) Release Fe2+ as Fe3+ c) Release CO Biliverdin is reduced to bilirubin by biliverdin reductase enzyme. 1 Formation of bilirubin Degradation of Heme Bilirubin is transferred to the liver as a (bilirubin-albumin complex). 2 Uptake of Bilirubin dissociate from bilirubin by the albumin and enter hepatocytes liver and binds to ligandin. The bilirubin is made more soluble by conjugation with two glucuronic acid from ( donor 3 UDP-glucuronic acid) in a Formation of reaction catalyzed by bilirubin CB UDP glucuronyl transferase (UGT) -> Bilirubin diglucuronide or conjugated bilirubin (CB) (conjugated bilirubin) which is secreted into bile. 4 Secretion of Unconjugated bilirubin is not bilirubin into bile secreted into bile  Summary of the bilirubin metabolism Urobilin excreted in urine Formation of colorless urobilinogen by bacteria in the intestine Stercobilin excreted in stool 2-5% 20%  Disorder linked to heme degradation pathway: Jaundice Definition Jaundice: is the clinical syndrome refers to yellow color of skin, nails and sclerae caused by bilirubin deposition ( elevated bilirubin levels in blood above the normal level < 1 mg/dl | hyperbilirubinemia). Types of jaundice Jaundice prehepatic hepatic Post-hepatic Prehepatic jaundice: Hemolytic jaundice Liver can conjugate and excrete > 3000 mg bilirubin/day - normal is 300 mg/day. Massive hemolysis causes more than can be processed may be due to sickle cell anemia (PK or G6PDH deficiency) Ø It produces faster bilirubin that cannot be conjugated -> increased unconjugated bilirubin (UCB) in blood. Ø Increased CB excreted into bile Ø Urobilinogen is increased in blood, urine  Hepatocellular jaundice Damage of liver cells (cirrhosis or hepatitis) causes unconjugation hyperbilirubinemia as a result of decreased conjugation. CB is made but not efficiently secreted from liver to bile (leak into the blood so CB is elevated in blood) Urobilinogen is increased in urine because hepatic damage decreases the enterohepatic circulation more goes to urine the urine is darker and stool is pale, clay-colored AST and ALT levels are elevated due to hepatic damage Post-hepatic: Obstructive jaundice Obstruction of the bile duct – Tumor or bile stones – Prevents passage of CB into intestine – GI pain and pale, clay-colored stools – Increased CB in blood excreted in urine (darker)  Jaundice in newborns Most newborn infants shows a rise in UCB in the first prenatal week because the enzyme bilirubin UGT is low. Maximum expression (adult level) at ~ 4 weeks Excess UCB can cause toxic encephalopathy Elevated bilirubin is treated with blue fluorescent light converts bilirubin to more polar compound (water soluble isomers) can be excreted in bile without conjugation Catecholamines and melanin  Catecholamines Biologically-active water-soluble amines derived from tyrosine that serve as: – neurotransmitters in the CNS – hormones in circulation in response to psychological stress (fight or flight response) or hypoglycemia: Dopamine and Norepinephrine are synthesized in the brain Epinephrine (adrenaline) are synthesize in the adrenal medulla Function: 1. Catecholamines act as regulators of carbohydrate and lipid metabolism 2. They are released from storage vesicles in the adrenal medulla in response to fright, exercise, cold and low level blood glucose 3. They increase the degradation of glycogen and triacylglycerol, blood pressure and the output of the heart Melanin Is a pigment occurs in several tissues eye, hair and skin. It is synthesized from tyrosine in the epidermis by pigment forming cells called melanocytes. It is function to protect cells from harmful effects of sunlight. Albinism results from a defect in melanin production due to defects in copper-containing tyrosinase.  Synthesis of melanin Melanocytes Melanin Albinism Cu2+ containing tyrosinase Synthesis of Catecholamines Rate limiting step hydroxylation decarboxylation hydroxylation methylation  Degradation of Catecholamines Once released, catecholamines have a half-life of 1 min due to subsequent inactivation. It is degraded by: 1) Monoamine oxidase (MAO) inactivates catecholamines by oxidative deamination to yield the aldehyde which oxidized to corresponding acids urine 2) Catechol-O-methyltransferase (COMT) inactivates catecholamines by methylation using S-adenosylmethionine (SAM) as the one-carbon donor MAO inhibitors block catecholamine degradation, allowing their accumulation in the presynaptic neuron and subsequent leakage into circulation, providing an antidepressant action. urine Histamine Synthesis Histamine is a chemical messenger mediating a range of cellular responses including: allergic, inflammatory reactions and gastric acid secretion. Powerful vasodilator decarboxylation PLP Synthesized by histidine decarboxylase in a reaction requiring PLP It is secreted by mast cells as a result of allergic reactions or trauma. Serotonin Synthesis §Serotonin also called 5’hydroxytryptamine is synthesized predominately in the GI tract hydroxylation §Synthesized from tryptophan §Inactivated by MAO §It has multiple physiologic roles including decarboxylation § Pain perception § Regulation of sleep, appetite, temperature, blood pressure, cognitive function and mood  Amidino group Creatine Synthesis §Amidinotransferase is a mitochondrial enzyme catalyzing the rate-limiting step of the pathway Kidney §Methyltransferase uses S- Adenosylmethionine SAM as a one- carbon donor to yield creatine SAM §Creatine kinase catalyzing the Liver phosphorylation of creatine using ATP as the phosphate donor § Degradation: Creatine and Creatine phosphate spontaneous cyclizes to creatinine, which is secreted in the urine phosphorylation §Serum creatinine level is routinely Brain, Heart measured on admission as a test of And Skeletal muscle kidney function. Kidney impairment results in elevated serum creatinine urine