Heme Synthesis 5q5r6f

HEME SYNTHESIS G HARIPRASATH

RESEARCH SCHOLAR

DEPT OF BIOCHEMISTRY RMMC

ANNAMALAI UNIVERSITY

HEME SYNTHESIS – Metallo HC

Heme porphyrin.  Porphyrins – Cyclic compounds formed by 4 pyrrole rings linked through methyne or methenyl bridges (=CH-)  Porphyrins usually forms complexes with metal ions. 

HC

N H

CH CH

Pyrrole ring



Heme is the prosthetic group of hemoglobin, myoglobin, and the cytochromes.  



It is  asymmetric.

Porphyrin of heme – Protoporphyrin.

One ferrous (Fe2+) atom in the center of the tetrapyrrole ring. 

So heme is ferro protoporphyrin.

Acetate, Propionyl, Methyl and Vinyl groups are often attached as side chains and depending on their positions they form different isomers.  Important porphyrins: Uroporphyrins, Protoporphyrins and Coproporphyrins.  Protoporphyrins has Methyl, Propionate and Vinyl side chains.  Uroporphyrins – has acetate and propionate chains.  Coproporphyrins – has Methyl and propionate chains.  Type III porphyrins are normally important for humans. 

 Heme

is also the Prosthetic group for Myoglobin The Cytochromes Catalase and Tryptophan pyrrolase Nitric Oxide Synthase  Magnesium porphyrin – chlorophyll  Porphyrins - used in cancer phototherapy. Upon istration taken up more by cancer tissues. Then the patient is exposed to argon laser – which excites porphyrins producing cytotoxic effects. Photodynamic therapy.

BIOSYNTHESIS OF HEME Site – All tissues in the body except mature erythrocytes (lacks Mitochondria).  85% in erythroid precursor cells in bone marrow. Under the control of erythropoietin.  Remaining mainly in hepatocytes.  Organelle – Partly cytoplasmic & Partly mitochondrial. 

STEPS Synthesis of Porphobilinogen (Monopyrrole)  Synthesis of Uroporphyrinogen (Tetrapyrrole)  Conversion of Uroporphyrinogen to Protoporphyrin.  Formation of heme 

SYNTHESIS OF PORPHOBILINOGEN

Heme synthesis begins with condensation of glycine & succinylCoA, with decarboxylation, to form d-aminolevulinic acid (ALA).  1. ALA synthesis – Mitochondria.  Rate limiting step.  ALA synthase – requires pyridoxal phosphate & Mg++ for its activity.  d-Aminolevulinate Synthase (ALA Synthase) is the rate limiting step of the heme synthesis pathway.  Heme – feed back inhibitor.  Hemin – produced by oxidation of heme. Not taken up by protein. Inhibits ALA synthesis.  2. Phorphobilinogen synthesis – Cytosol.  PBG Synthase (Porphobilinogen Synthase), also called ALA Dehydratase, catalyzes condensation of two molecules of daminolevulinic acid (ALA) to form porphobilinogen (PBG).   ALA dehydratase – contains zinc.  Sensitive to inhibition by lead – lead poisoning – increased ALA & anemia. 

FORMATION OF UROPORPHYRINOGEN III BY CONDENSATION OF FOUR PORPHOBILINOGENS

Four PBG molecules condense  Catalysed by PBG deaminase or HMB synthase.  Four molecule of PBG condense in a head to tail manner to form Hydroxy methylbilane (HMB).  Uroporphyrinogen III synthase: Converts HMB to Uroporphyrinogen III.  Normal condition – Uroporphyrinogen III gets formed.  Under certain porphyrias – HMB cyclises spontaneously to form Uroporphyrinogen I. 

CONVERSION OF UROPORPHYRINOGEN TO PROTOPORPHYRIN

Enzyme – Uroporphyrinogen decarboxylase.  Four Acetate groups of Uroporphyrinogen III were decarboxylated to Methyl groups to form coproporphyrinogen III.  Site – cytosol.  Conversion of Coproporphyrinogen III to Protoporphyrinogen IX:  Enzyme – Coproporphyrinogen oxidase.  Oxidative decarboxylation.  Protoporphyrinogen IX is converted to protoporphyrin IX by protoporphyrinogen oxidase  Oxidation occurs by molecular oxygen.  6H+ are liberated. 

HEME FORMATION

By incorporation of iron (Fe 2+).  Partly spontaneous.  Enhanced by Ferrochelatase (heme synthetase).  May be inhibited by lead.  Insertion of an atom of Fe++ into central position of protoporphyrin IX.  Heme produced can be then coupled to various proteins-conjugated proteins: hemoglobin, myoglobin, cytochrome C, catalases and peroxidases. 

REGULATION OF HEME SYNTHESIS 

 









Regulator enzyme – ALA synthase & Hydroxy methyl bilane synthase. Heme – negative regulator. Absence of heme: Synthesis of ALA synthase & HMB synthase increases greatly. Enzyme synthesis, as well as its transport to the mitochondria, is also inhibited by elevated levels hemin (oxidised product of heme). Many xenobiotics on istration increases ALA synthase activity .Eg., barbiturates-phenobarbitol and griseofulvin These drugs are usually metabolized by the microsomal cytochrome P450 mono-oxygenase system. Which diminishes intracellular heme conc. ALA synthase Requires pyridoxal phosphate (Vitamin B6) as a coenzyme. Bio availability of iron too plays an important role.

PORPHYRIAS Defect in heme biosynthesis – accumulation of porphyrins or its precursors – porphyrias.  Can be hereditary or acquired (common).  Classification:  Hepatic  Erythropoietic.  Hepatic forms – neurological symptoms  Erythropoietic forms – cutaneous symptoms.  Salient features:  Occurrence: Mutation of any gene encoding enzymes of heme synthesis.  Charectiresed by non synthesis of metabolic product beyond the point of enzyme defect. Accumulation of intermediates prior to block.  All Porphyrias are autosomal dominant. Exception – Congenital erythropoietic porphyria (autosomal recessive). 

1.

2.

3. 4. 5. 6.

Enzyme defect earlier in the pathway – ALA, PBG and HMB accumulates. Causes Abdominal pain and neuropsychiatric symptoms. Enzyme defect later in the pathway – porphyrinogens are accumulated. Upon oxidation they produces corresponding porphyrins which cause photo sensitivity resulting in skin injury. (generation of free radicals which damage cellular component). Consumption of antioxidants – may benefit. Treatment: gene replacement therapy (experimental). istration of heme or hematin. Avoiding drugs that induce cyt P450.

TYPES OF PORPHYRIAS ALA synthase deficiency: mutation in gene encoding ALA synthase.  No accumulation of porphyrins.  Sideroblastic anemia.  May respond to supplementation of pyridoxine.  Plumboporphyria: Deficiency of PBG synthase or ALA dehydratase. Occurrence (rare) – Lead poisoning. 

PORPHYRIAS ARE RESPONSIBLE FOR THE ENZYME DEFECTS

Acute interittent porphyria:  Enzyme defect: HMB synthase.  Products accumulated: Porphobilinogen & ALA.  Urine – colourless but darkens on exposure to air due to photooxidation.  Abdominal pain and neuropsychiatric symptoms.  Psychiatric symptoms – reduced activity of tryptophan pyrrolase.  istration of intravenous hematin may ameliorate the symptoms.  istration of Barbiturates, ethanol, steroids can precipiate the manifestations.  No photosensitivity. 

Congenital erythropoietic porphyria:  Gunther’s disease.  Autosomal recessive.  Enzyme defect: uroporphyrinogen III synthase (low activity).  Diversion of pathway. Type I isomers gets formed (Uroporphyin I and Coproporphyrin I).  Urine – dark red in colour.  High photosensitivity is observed.  Teeth exhibit fluoresence when exposed to UV light (erythrodontia). 

Porphyria Cutanea tarda: Most common.  Enzyme defect: partial deficiency of uroporphyrinogen decarboxylase.  Precipitated by excess alcohol consumption and iron overload.  Uroporphyrins I and III are excreted in urine.  Photosensitivity is observed.  Hereditary coproporphyria:  Enzyme defect: Coproporphyrinogen oxidase.  Coproporphyrinogen III and other metabolites of heme synthesis prior to enzyme block accumulates.  Photosensitivity and neurological attacks. 

Variegate Porphyria:  Enzyme defect – protoporphyrin oxidase.  Products accumulate – protoporphyrin, coproporphyrin, uroporphyrin, PBG.  Neurological attacks and photosensitivity are observed.  Erythropoietic Protoporphyria:  Enzyme defect – ferrochelatase.  Elevated conc of protoporphyrin in blood, bile and feces.  Protoporphyrin – strongly hydrophobic.  Mild photosensitivity.  No neurologic symptoms.  May exhibit cholelithiasis (gall stone disease) and portal hypertension. 

ACQUIRED PORPHYRIAS Secondary to CKD, few malignancies, liver disease.  Exposure to heavy metals – Pb, Hg, As.  Toxic porphyrias.  Associated with anemia. 