number Done by Corrected by Doctor Diala

Transcription

1 number 36 Done by Baraa Ayed Corrected by Moath Darweesh Doctor Diala 1 P a g e

2 Today we are going to cover these concepts: Porphyrin structure Biosynthesis of Heme Regulation of heme synthesis A clinical application (porphyrias) Introduction to degradation of heme Porphyrin structure: They are cyclic compounds that resemble a large closed ring that contain small rings inside it. They bind to metal ions usually Ferrous (Fe+2) or Ferric ( Fe+3) One example of these metalloporphyirn compounds is Heme which is found in hemoglobin, myoglobin, the cytochromes, catalase, nitric oxide synthase, and peroxidise. Note: porphyrin compounds ( for example heme ) is an example of nitrogen containing compounds that are synthesized from amino acids pool Remember: we said previously that amino acids pool has sources( that supply it) and fates ( one of these fates is usage some of these amino acids in producing nitrogen containg compounds ( like porphyrin compounds ) Now, let's discuss the structure of porphyrin ( look to the picture below) They resemble large closed rings that contain 4 small rings (A, B, C and D in this picture). Each ring which is pyrrole ring is Five-sides ring that contain 1 doublebond and 1 nitrogen atomand is attached to two side chains( in the picture, they are pointed as two symbols( A and P ) A which is refer to acetate and P which is refer to propionate. Note that this is an example of these side chains. So,porphyrin compounds differ from each other depending on the difference in these side chains. 2 P a g e

3 The medical significance of porphyrins is related to the following structural features of these molecules: 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 ) Coproporphyrin contains methyl ( CH3) and (propionate group) Protoporphyrin IX (and heme) contains vinyl ( CH=CH2), methyl, and propionate groups. ( later on you will notice that all these structures are intermediate structures in Heme biosynthesis) So, you can now know the name of the porphyrin compound in the previous picture which is Uroporphyrin. Distribution of side chains: note that in the previous pictures, at the right side; the side chains are arranged symmetrically ( if we start from A ring and in the cycle we will have this arrangement ( A,P,A,P,A,P,A,P) and if we look at the left side; the side chains arrangement is (A,P,A,P,,A,P,P,A)So, we notice that there is a reverse in the direction of side chains at D ring and this arrangement is asymmetric. The first compound is called Uroporphyrin I(symmetricarrangement) and the second is called Uroporphyrin III(asymmetric substitution on D ring) and this compound is a physiologically important in humans. SO, porphyrin compound are different from each other depending in the nature of side chains and also the orientation (the distribution) of these chains. Prophyrinogens: these are the precursors (inactive form) for porphyrin compounds which are intermediates in heme biosynthesis to give the most functional compound which is the heme. Biosynthesis of heme: The major sites of heme biosynthesis are: 1. Liver which is the metabolism site in our body which contains cytochrome P450 and as we said that cytochrome contains heme group so, the liver should synthesis heme. The rate of heme synthesis is variable rate depending on demands for heme proteins (CYP) 2.Erythrocyte-producing cells of the bone marrow: which are active inhemoglobin synthesis and hemoglobin contains heme so these cells should synthesis heme. This site is the major site in hemesynthesis about 85% of all heme synthesis 3 P a g e

4 So, the major site of producing the heme which is found in hemoglobin is Erythrocyte-producing cells of the bone marrow and the major site of producing heme that found in other molecules like cytochrome is liver Before we start the steps of heme biosynthesis, you should know: The initial ( first step) and last steps in porphyrins formation occur in mitochondria and The intermediate steps occur in the cytosol The steps of Heme biosynthesis: 1)Formation of δ-aminolevulinic acid (ALA): All the C and N atoms of the porphyrin are provided by Glycine (a nonessential amino acid) and succinyl coenzyme A (intermediate of Krebs cycle) that condense to form ALA and catalyzed by ALA synthase (ALAS)and the Coenzyme is pyridoxal phosphate(plp). Note that this isthe rate-limiting step in porphyrin synthesis. Note that there are two isoforms of ALA synthase which are ALAS1 and ALAS2. Both form ALA but they are differing in location; ALAS1 found in liver whereas ALAS2 found in Erythrocyte-producing cells. * This reaction should be repeated two times because we need two molecules of ALA in the next step * This step occur in the mitochondria (the initial step) *Note: heme which is a last product of this pathway act as inhibitor of ALAS enzyme. Also, high concentration of iron may lead to inhibition of this enzyme 2) Formation of porphobilinogen: The condensation of two molecules of ALA to form porphobilinogen by Zn-containing ALA dehydratase(porphobilinogen synthase) and this interaction between the two ALA molecules lead to release two water molecules and forming a first ring. * This enzyme is cytosoal enzymeand this step occur in the cytosol (remember we said that the initial step and the last 3 step occur in the mitochondria whereas the intermediate steps occur in the cytosol) *Lead act as inhibitor to this enzyme and that lead to causinganemia and also: less synthesis of heme> not enough amount of hemoglobin that carry oxygen to different to tissue> less amount of oxygen that reach the tissue like the brain and that cause mental retardation specially in child because they are in development stage and that affect their growth. 4 P a g e

5 However, in adult specially who work in lead factories; they will be more sensitive to lead poisoning and they will suffer from fewer amounts of oxygen that reach the tissue but not mental retardation which happens in child. The parents are responsible to avoid the toys that contain high amount of lead from their children to avoid lead poisoning. *This reaction repeated 4 times to form the four rings (A, B, C and D) Summary for the first two reactions 3) Formation of Uroporphyrinogen III: The condensation of four porphobilinogens rings produces Uroporphyrinogen III (the first porphyrin) by the enzyme Uroporphyrinogen III synthase. 4) Formation of coproporphyrinogen III:then the cyclic Uroporphyrinogen III will undergo decarboxylation of its 4 acetate side chain(note each ring of these 4 rings have two side chains (acetate and propionate) so we have 4 acetate side chains that undergo decarboxylation to form 4 methyl groups by the enzyme Uroporphyrinogen III decarboxylase and that lead to release 4 CO2 5 P a g e

6 NOTE: the purpose of this pathway is synthesis of heme and as we said previously "heme side chains are methyl, vinyl and propionate". So, now we have the methyl groups and propionate groups but still we want to get the vinyl (by two propionate groups in the next reaction). EXTRA NOTE:uro prefix means excreted in urine,andcopro means excreted in stool. 5) Formation of Heme:Coproporphyrinogen III enters the mitochondrion and two propionate side chainsundergodecarboxylation then dehydrogenationto form vinyl groups, generating protoporphyrinogen IX(the number is 9) by the enzyme coproporphyrinogen III Oxidase then Protoporphyrinogen IX is oxidized to protoporphyrin IX by the enzyme protoporphyrinogen III oxidase(it removes hydrogens from protoporphrinogen).then, the introduction of iron (as Fe2+) into protoporphyrin IX can occurspontaneously orby the enzyme Ferrochelatase(the rate of adding Fe++ is enhanced by this enzyme).this enzyme is inhibited by lead So, lead poisoning affect heme synthesis in early stage of hemesynthesis (by inhibition the enzyme ALA dehydrase) and in late stage of heme synthesis( by inhibition of Ferrochelatase) Remember: heme side chains are{ 4 methyl, two propionate, two vinyl } 6 P a g e

7 Regulation of heme biosynthesis: In the liver; End-product inhibition of ALAS1 by hemin: actually not heme itself cause the inhibition, however when heme accumulate, the iron (Fe+2) is oxidized to become (Fe+3) and now is called Hemin.Hemin decreases the activity of hepatic ALAS1 by reducing its synthesis (mrna synthesis) and mitochondrial import. In erythroid cells, ALAS2 is controlled by the availability of intracellular iron. High level of iron inhibit the enzyme Effect of drugs on ALA synthase activity: Many drugs increase hepatic ALAS1 activity.how? These drugs are metabolized by cytochrome P450 monooxygenase system a -heme protein oxidase system found in the liver-. In response to these drugs, the synthesis of cytochrome P450 proteins increases, leading to an enhanced consumption of heme. Low heme concentration increases ALAS1 synthesis Porphyrias: They are group of diseases that related to pigmentation and colours. Porphyria refers to the purple colour caused by pigment-like porphyrins in the urine of patients and in the skin. This disease is rare, inherited, it may be acquired but mostly it is inherited. It caused by defect in heme synthesis pathway like deficiency in certain enzyme that results in accumulation of certain intermediate and reduction of another intermediate. ( Any defect in the pathway of heme synthesis) Some symptoms like the change of urine colour into red and skin eruptions There are many types of porphyrias: Acute hepatic porphyria 7 P a g e

8 Chronic hepatic porphyria Erythropoietic porphyria It is not required to know the details of these diseases JUST THEIR NAMES The biochemical changes: Decrease in the production of the final product (heme) due to any defect in any enzyme and that lead to hemeanemiaand Increased ALA synthase activity (specially ALAS1 ) (due to less amount of heme) Less heme results in an increased synthesis of ALA synthase1. That will be without any benefit because there is defect in certain enzyme in the pathway that will result indecreased synthesis of heme. Note that all compound that was before the enzyme that has deficiency, will accumulate. But the final product of the pathway will decrease. Treatment: Treatment for pain and vomiting during acute attacks which is IV injection of hemin and glucose, which decreases the synthesis of ALAS1, toreduce symptoms that result from accumulation of compounds that was before the defect enzyme.( we reduce the symptoms but not cure the disease) Heme Degradation: RBCs are degraded by the reticuloendothelial system (liver and spleen) but the main site for heme degradation is spleen. Heme degradation lead to Bilirubin formation: In any macrophage of reticuloendothelial system, heme degradation occur and by the action of heme oxygenase enzyme,the ring open and that lead to releasing Fe++ and formation of Biliverdin by adding OH to the heme. The next step is reduction of Biliverdin by Biliverdin Reductase that added hydrogen atoms to Biliverdin forming Bilirubin and this reaction as accomplished by oxidation of NADPH to NADP Notethat purpose of forming Bilirubin is to use it in pigmentation of watery solution like Urine and bile. However, we notice that bilirubin is highly hydrophobic compound because of releasing Fe++ and adding hydrogen atoms. So, how can Bilirubin be used in pigmentation of these 8 P a g e

9 solutions? By going to liver.when bilirubin go to liver, liver will solubilise it so it can go to gall bladder and excreted with urine (giving it its yellow colour). ** The doctor will discuss this concept in detail in the next lecture because she gives introduction to it in this lecture. *Don't forget to refer to slides 9 P a g e

10 10 P a g e