Heme Synthesis and Hemoglobin PDF

Heme Synthesis By: Dr. Safinaz Hamdy El Khoulany Lecturer of Medical Biochemistry and Molecular Biology Heterocyclic compounds: Cyclic compounds where at least one of the atoms in the ring is not carbon Macrocycle compounds: Large cycle compounds with : -ring size at least 9 atoms...

Heme Synthesis By: Dr. Safinaz Hamdy El Khoulany Lecturer of Medical Biochemistry and Molecular Biology Heterocyclic compounds: Cyclic compounds where at least one of the atoms in the ring is not carbon Macrocycle compounds: Large cycle compounds with : -ring size at least 9 atoms (porphyrins >9) -at least 3 donor atomsi.e. can donate free electrons (porphyrins contain 4 donor sites which are nitrogen) They have high affinity for metal ions methenyl bridge Porphyra  Hemoproteins Heme+ protein (Protoporphyrin IX + Iron (ferrous) )+ protein linkage of four pyrrole rings through methenyl bridges (CH) + Iron (ferrous) + protein The iron is held in the center of the heme molecule by bonds to the four nitrogens of the porphyrin ring.  1 2 8 3 4 7 6 5 Chromophores )‫(المواد الحاملة للون‬ Include conjugated system of double bonds (i.e. alternating double bond and single bond) i.e conjugation Porphyrins -They are from these comounds - They have high affinity to absorb light in the visible region - Thus, they are deeply colored Hemoproteins N.B: Protoporphyrinogen IX: methylene bridges (CH 2) Heme+ prtein (Protoporphyrin IX + Iron (ferrous) )+ protein linkage of four pyrrole rings through methenyl bridges (CH) + Iron (ferrous) + protein rin methenyl bridges (CH) rinogen methylene bridges (CH2) H OIL oxidation is loss of H+ Reduction RIG reduction is gain of H+ (CH) (CH2) Oxidized form Oxidation H Reduced form Colored molecule Colorless molecule (Double bonds in (no double bonds) conjugated system)  Porphin ring (Unsubstituted) 8 1 There is no 7 IV I 2 substitution Porphyrin ring There is substitution 6 III II 3 by substituent or side chain of the 5 4 H at the numbers in the figure C H by substituent or side chain Different porphyrins vary in the nature of the side chains that are attached to each of the four pyrrole rings: Uroporphyrin contains acetate (–CH2–COO–) and propionate (–CH2–CH2–COO– ) side chains Coproporphyrin contains methyl (–CH3) and propionate groups Protoporphyrin IX (or III) (and heme) contains vinyl (–CH=CH2), methyl, and propionate groups.  Types of porphyrins There are many types in nature The most important in human is Uroporphyrin , Coproporphyrin, Protoporphyrin IX These 3 types are subclassified into: Type I porphyrins: Contain symmetric substitution or side chains in all rings Type III porphyrins: Contain asymmetric substitution or side chains in ring IV at least  Types of porphyrins There are many types in nature The most important in human is Uroporphyrin , Coproporphyrin, Protoporphyrin IX These 3 types are subclassified into: Type I porphyrins: Contain symmetric substitution or side chains in all rings Type III porphyrins: Contain asymmetric substitution or side chains in ring IV at least  In hemoglobin and myoglobin, the heme group serves to reversibly bind oxygen Rxdjhxdgm the heme group of a cytochrome functions as an electron carrier that is alternately oxidized and reduced nc mh Hemeproteins are a group of specialized proteins that contain heme group  M V M M P V M P  Metalloporphyrins They are complexes of porphyrins with metals This is by the nitrogen atoms which all form bonds with the metal The metal is kept in a pocket in the centre of the porphyrin ring by interaction with these nitrogen atoms Metal may be: 1- magnesium in plant chlorophyll (green colour of the plant) 2- Ferrous Iron (Fe 2+) in Hemoglobin of blood 3- Cobalt in vitamin B 12 Porphyrin metabolism  Heme Synthesis Site: In all cells containing mitochondria (Mature RBCs can’t synthesize Heme) Major sites of synthesis are: Liver Erythrocyte producing cells in BM Synthesize hemoproteins Synthesize hemoglobin esp.Cytochromes Rate of synthesis is variable Rate of synthesis is constant The first step in synthesis is catalzed by ALA synthase enzyme encoded by different genes ALA S-1 ALA S-2 In liver Erythroid tissue Site of reactions: 1st and last 3 Remaining In mitochondria In Cytosol I) Formation of δ aminolevulinic acid:  All carbons from glycine and succinyl CoA  This step is catalyzed by ALA synthase (Mitochondrial enzyme)  This reaction requires pyridoxal phosphate (vit B6) as coenzyme This is rate-limiting step Rate Limiting step / control step “Where the entire pathway is regulated”’  Product Substrate II) Porphobilinogen Synthase (ALA Dehydratase) - It catalyzes condensation of two molecules of δ-aminolevulinate (ALA) to form the pyrrole ring of porphobilinogen (PBG). - It is cytosolic enzyme  Product Substrate II) Porphobilinogen Synthase (ALA Dehydratase) - It is zinc- containing enzyme - It is extremely sensitive to inhibition by heavy metals e.g. lead that replaces zinc The Zn++ binding sites in mammalian Porphobilinogen Synthase, which include cysteine S ligands, can be occupied by lead (Pb++) Zn++ SH CYS Pb++  Product Substrate II) Porphobilinogen Synthase (ALA Dehydratase) - It is zinc- containing enzyme - It is extremely sensitive to inhibition by heavy metals e.g. lead that replaces zinc The Zn++ binding sites in mammalian Porphobilinogen Synthase, which include cysteine S ligands, can be occupied by lead (Pb++) Zn++ SH CYS Pb++ II) Porphobilinogen Synthase (ALA Dehydratase) The Zn++ binding sites in mammalian Porphobilinogen Synthase include cysteine S ligands, which, and they can can be occupied by lead (Pb++) Inhibition of Porphobilinogen Synthase by Pb++ Elevated blood ALA Lead poisoning Hydroxymethylbilane (Linear tetrapyrrol) Ring closure Oxidation of bridge Decarboxylation (-4CO2) (CH2 to CH) Oxidative Decarboxylation (-2CO2 and -4H) III) Porphobilinogen Deaminase (hydroxymethylbilane synthase):  Its deficiency causes Acute Intermittent Porphyria  has a dipyrromethane prosthetic group (dipyrrol) , linked at the active site via a cysteine S. The enzyme itself catalyzes formation of this prosthetic group. P A A P  Substrate III) Porphobilinogen Deaminase (hydroxymethylbilane synthase) Product reaction mechanism: 1- Elimination of ammonia from Porphobilinogen , generating an intermediate 2- This intermediate is then attacked by the enzyme porphobilinogen deaminase P A 3- This intermediate is then open to further reaction with porphobilinogen. PBG units are added to the enzyme. A P 4- Hydrolysis of the link to the enzyme releasing the tetrapyrrole hydroxymethylbilane A P A P Linear tetrapyrrol  P IV) Formation of uroporphyrinogen III A A P A P P A Uroporphyrinogen III Synthase catalyzes ring closure & flipping over of one pyrrole to yield an asymmetric tetrapyrrole. (Asymmetric)  P IV) Formation of uroporphyrinogen III A A P A P P A Uroporphyrinogen III Synthase catalyzes ring closure & flipping over of one pyrrole to yield an asymmetric tetrapyrrole. (Asymmetric) Hydroxymethylbilane (Linear tetrapyrrol) Ring closure Oxidation of bridge Decarboxylation (-4CO2) (CH2 to CH) Oxidative Decarboxylation (-2CO2 and -4H) (CH2)  V) Formation of coproporphyrinogen III Uroporphyrinogen III (cyclic tetrapyrrole) undergoes decarboxylation of its acetate groups into methyl groups, generating coproporphyrinogen III. This is catalyzed by Uroporphyrinogen decarboxylase These reactions occur in the cytosol.  V) Formation of coproporphyrinogen III Uroporphyrinogen III (cyclic tetrapyrrole) undergoes decarboxylation of its acetate groups into methyl groups, generating coproporphyrinogen III. This is catalyzed by Uroporphyrinogen decarboxylase These reactions occur in the cytosol. Decarboxylation (-4CO2)  VI) Formation of Protoporphyrinogen III  Coproporphyrinogen III is transported into mitochondria  In mitochondria: Oxidative decarboxylation converts 2 of 4 propionyl side chains to vinyl groups This is catalyzed by Coproporphyrinogen Oxidase (deficiency causes Hereditary Coproporphyria) Oxidative Decarboxylation (-2CO2 and -4H)  VI) Formation of Protoporphyrinogen III  Coproporphyrinogen III is transported into mitochondria  In mitochondria: Oxidative decarboxylation converts 2 of 4 propionyl side chains to vinyl groups This is catalyzed by Coproporphyrinogen Oxidase (deficiency causes Hereditary Coproporphyria) Oxidative Decarboxylation (-2CO2 and -4H)  VII) Formation of protoporphyrin IX Protoporphyrinogen IX is oxidized to protoporphyrin IX  Oxidation adds double bonds This is catalyzed by (Protoporphyrinogen Oxidase). We have to convert the single Oxidation of bridge bonds to double bonds in or der to (CH2 to CH) have alternating single & double bonds which means the rings becomes aromatic and aromaticity is essential for the functions of heme Protoporphyrinogen Oxidase VIII) Formation of Heme Hydroxymethylbilane (Linear tetrapyrrol) Ring closure Oxidation of bridge Decarboxylation (-4CO2) (CH2 to CH) Oxidative Decarboxylation (-2CO2 and -4H)