Prof. of Medical Biochemistry Heme Metabolism By Dr. Maha Sallam Prof. of Medical Biochemistry

Extended Modular Program

Extended Modular Program Identify the structure of heme 2. Have an overview on heme synthesis 3.Study regulation of heme synthesis. Extended Modular Program

Heme Heme is the colored prosthetic group of hemoglobin and a number of proteins called hemoproteins. ( Can you mention some examples ?)

Examples of some important hemoproteins: Hemoglobin: Transport of oxygen in blood. Myoglobin: Storage of oxygen in muscle Cytochrome C: Involvement in electron transport chain ( respiratory chain ) Cytochrome P450: metabolism of xenobiotics Catalase: Degradation of hydrogen peroxide Tryptophan Pyrrolase: Oxidation of tryptophan. Cytoplasmic guanyl cyclase enzyme: formation of cyclic GMP © 5

Hemoproteins are rapidly synthesized and degraded New heme is synthesized each day to replace heme lost through the normal turnover of RBCs. So we have 2 important subjects : Synthesis of Heme Degradation of Heme To Bilirubin

Structure of Heme Heme is composed of iron in the ferrous state attached to the center of porphyrin ring. Porphyrins are cyclic compounds formed by the linkage of four pyrrole rings through 4 -HC= methenyl bridges. Fe+2 porphyrin ring © 7

porphyrin ring Fe+2

Porphyrins can form complexes with metal ions bound to the 4 nitrogen atoms of the pyrrole rings. Examples : 1-Iron porphyrins as heme of hemoglobin 2- Magnesium-containing porphyrins as chlorophyll.

Biosynthesis of Heme Heme synthesis occurs in most tissues except mature RBCs (have no mitochondria). The major sites are: Erythrocytic: Erythrocyte producing cells of bone marrow, which are active in hemoglobin synthesis. Non-Erythrocytic: especially Liver, which synthesizes a number of heme proteins (particularly cytochrome P450). 10

Overview on heme synthesis Heme biosynthesis begins and ends in mitochondria, but 3 intermediate reactions occur in cytoplasm. The reactions are irreversible. Mitochondria Cytoplasm

Steps of Heme Synthesis Extended Modular Program

1-Synthesis of Delta Aminolevulinic acid Kreb´s cycle ALA Synthase Leaves the mitochondria to the cytosol Amino Acids © 13

delta-aminolevulinic acid (ALA). 1- Synthesis of Delta Aminolevulinic acid: It starts in mitochondria by condensation reaction between succinyl-CoA and glycine to produce delta-aminolevulinic acid (ALA). It is catalyzed by ALA synthase in the presence of Pyridoxal phosphate. ( coenzyme of Vit B6) It is the rate-controlling enzyme of Heme synthesis. © 14

2-Formation of Porphobilinogen PBG in cytoplasm PBG synthase © 15

2- Formation of Porphobilinogen ALA leaves the mitochondria to cytoplasm where two molecules of ALA are condensed by the enzyme ALA dehydratase (zinc-containing enzyme) to form porphobilinogen (PBG) and removes two molecules of water. This enzyme is sensitive to inhibition by heavy metal ions as lead that replace zinc. PBG © 16

3-Condensation of 4 PBG to form the first Porphyrinogens

4-modification of the side chains 5- oxidation of the rings 6-Insertion of iron by enzyme Ferrochelatase in mitochondria Fe2+

So heme synthesis begins and ends in mitochondria PBG 2 So heme synthesis begins and ends in mitochondria

Regulation of Heme synthesis ALA synthase the rate limiting enzyme There are 2 ALA synthase isomers, each produced by different genes and controlled by different mechanisms. ALAS1 is found in all tissues, whereas ALAS2 is erythroid-specific. Loss of function mutation in ALAS2 results in sideroblastic anemia and iron overload.

1-Effect of Heme Repression ( at gene level) When porphyrin production exceeds the availability of the proteins that require it, heme accumulates. The excess heme is converted to hemin (hematin) by oxidation of Fe2+ to Fe3+. Hemin inhibits synthesis of the enzyme ALAS1 by repressing transcription of its gene, decreasing synthesis and stability of mRNA of the enzyme and decreasing its import into mitochondria (in bone marrow ALAS2 is controlled by the availability of intracellular iron). . © 21

2- Effects of drugs The activity of ALASI in liver ( not in Bone marrow) can be increased by certain drugs as barbiturates and steroids or other compounds as insecticides and carcinogens. © 22

These drugs are metabolized in liver by cytochrome P450 , increasing consumption of heme containing proteins (during metabolism of these drugs in liver) .So heme concentration decreases with removal of its inhibitory effect on ALA synthase gene i.e (derepression) with increase ALA synthase 1 activity to meet the needs of cells.

3-Effect of Hypoxia In the erythropoietic tissues Heme synthesis is affected by Hypoxia that increases ALA synthase activity by increasing Erythropoietin hormone

4- Effect of Lead Lead has an inhibitory effect on ALA dehydratase Ferrochelatase The body has enough iron available but cannot incorporate it into hemoglobin. Lead poisoning causes Sideroblastic anemia. Sideroblasts are atypical nucleated erythrocytes with granules of iron accumulated in perinuclear mitochondria © 25

Porphyrias “The Vampire Disease” These are rare group of genetic disorders of heme synthesisresults in the accumulation and increased excretion of porphyrins or porphyrin precursors that have unfairly branded many sufferers with the term "vampire". These poor souls are extremely sensitive to sunlight that can easily result in burns and abrasions, and so they prefer darkness. They suffer from acute attacks of abdominal pains and vomiting . Their urine may have a purplish-red color leading some to wrongly believe that it results from drinking blood. They may have increased hair growth, and with repeated damage, their skin tightens and shrinks. When this occurs around the mouth, the canine teeth appear to be more prominent, and suggestive of fangs. 

Summary Heme is the colored prosthetic group of hemoglobin and a number of proteins called hemoproteins Heme biosynthesis begins and ends in mitochondria, but intermediate reactions occur in cytoplasm. The reactions are irreversible. ALA synthase is the rate limiting enzyme Hemin inhibits synthesis of ALA synthase at gene level The activity of ALA synthase in liver ( not in Bone marrow) can be increased by certain drugs Hypoxia increases ALA synthase activity in bone marrow Lead has an inhibitory effect on ALA dehydratase & Ferrochelatase enzymes causes Sideroblastic anemia.

Delta-aminolevulinic (ALA) synthase activity Revision Questions Delta-aminolevulinic (ALA) synthase activity A- is decreased by drugs as barbiturates & steroids B-catalyzes a rate limiting reaction in heme synthesis C- requires biotin coenzyme D-is inhibited by heavy metal ions as lead E- occurs in cytosol

Lead poisoning can cause anemia due to inhibition of : a- Heme oxygenase b- ALA synthase c-Ferroxidase d- ferrochelatase

Regulation of heme synthesis occurs by: A-Repression- derepression B-Induction C-Allosteric regulation D-Covalent Modification

Mention the name of 2 hemoproteins …………..&………… Hemoglobin: Transport of oxygen in blood. Myoglobin: Storage of oxygen in muscle Cytochrome C: Involvement in electron transport chain ( respiratory chain ) Cytochrome P450: metabolism of xenobiotics Catalase: Degradation of hydrogen peroxide Tryptophan Pyrrolase: Oxidation of tryptophan. Cytoplasmic guanyl cyclase enzyme

Heme is synthesized by condensation of A- Succinyl-CoA and glycine B-Active acetate and glycine C-Active succinate and alanine D-Active acetate and alanine

The color of hemoglobin is due to the presence of: A- globin protein B- alpha chain C- beta chain D- heme