Biosynthesis of Porphyrin

K.H.TimotiusJune 2014

Outline

1. Structure of porphyrin2. Biosynthesis of heme3. Porphyria

1. Chemistry of porphyrin

• porphyrin, any of a class of water-soluble, nitrogenous biological pigments (biochromes), derivatives of which include the hemoproteins (porphyrins combined with metals and protein).

• Examples of hemoproteins are a. the green, photosynthetic chlorophylls of higher plants; b. the hemoglobins in the blood of many animals; c. the cytochromes, enzymes that occur in minute quantities

in most cells and are involved in oxidative processes; and d. catalase, also a widely distributed enzyme that accelerates

the breakdown of hydrogen peroxide.

1. Chemistry of porphyrin

• Porphyrins have complex cyclic structures. • All porphyrin compounds absorb light intensely at

or close to 410 nanometres. • Structurally, porphyrin consists of four pyrrole rings

(five-membered closed structures containing one nitrogen and four carbon atoms) linked to each other by methine groups (−CH=).

• The iron atom is kept in the centre of the porphyrin ring by interaction with the four nitrogen atoms.

Spectrometry UV-VIS

2. Synthesis of Porphobilinogen and heme

• The first reaction in heme biosynthesis takes place in the mitochondrion and involves the condensation of one glycine and one succinylCoA by the pyridoxal phosphate-containing enzyme, δ-aminolevulinic acid synthase (ALAS).

• Delta-aminolevulinic acid (ALA) is also called 5-aminolevulinic acid.

• This reaction is both the rate-limiting reaction of heme biosynthesis, and the most highly regulated reaction

hemoprotein

• Hemoglobin: transport oksigen dalam darah• Mioglobin: penyimpanan oksigen di otot• Sitokrom c: terlibat dalam rantai transpor

elektron• Sitokrom P450: hidroksilasi xenobiotik• Katalase: penguraian hidrogen peroksida• Triptofan pirolase: oksidasi triptofan• Vitamin B12: ?

2. Synthesis of Porphobilinogen and heme

• Two forms of ALAS: ALAS1 and ALAS2.• ALAS1 is considered a house-keeping gene and is expressed in all

cells. • ALAS2 is an erythroid-specific form of the enzyme and is expressed

only in fetal liver and adult bone marrow. • The ALAS1 gene is located on chromosome 3, whereas the ALAS2

gene is located on the X chromosome. • Deficiencies in ALAS2 result a disorder called X-linked sideroblastic

anemia, XLSA. Sideroblasts are erythroblasts with non-heme iron-containing organelles, called siderosomes. XLSA has also been called congenital sideroblastic anemia, hereditary sideroblastic anemia, hereditary iron-loading anemia, X-linked hypochromic anemia, hereditary hypochromic anemia, and hereditary anemia.

2. Synthesis of Porphobilinogen and heme

• Following synthesis mitochondrial ALA is transported to the cytosol, where ALA dehydratase (also called porphobilinogen synthase) dimerizes two molecules of ALA to produce the pyrrole ring compound porphobilinogen.

• The next step in the pathway involves the head-to-tail condensation of four molecules of porphobilinogen to produce the linear tetrapyrrole intermediate, hydroxymethylbilane.

2. Synthesis of Porphobilinogen and heme

• The enzyme for this condensation is porphobilinogen deaminase (PBG deaminase).

• This enzyme is also called hydroxymethylbilane synthase or uroporphyrinogen I synthase. Hydroxymethylbilane has two main fates.

• The most important is regulated, enzymatic conversion to uroporphyrinogen III, the next intermediate on the path to heme.

• This step is mediated by a holoenzyme comprised of uroporphyrinogen synthase plus a protein known as uroporphyrinogen III cosynthase. Hydroxymethylbilane can also non-enzymatically cyclize forming uroporphyrinogen I.

3. Regulation of Heme Biosynthesis

• Although heme is synthesized in virtually all tissues, the principal sites of synthesis are erythroid cells (≈85%) and hepatocytes (accounting for nearly all the rest of heme synthesis).

• The differences in these two tissues and their needs for heme result in quite different mechanisms for regulation of heme biosynthesis.

3. Regulation of Heme Biosynthesis

• In hepatocytes, heme is required for incorporation into the cytochromes, in particular, the P450 class of cytochromes that are important for detoxification. In addition numerous cytochromes of the oxidative-phosphorylation pathway contain heme.

• The rate-limiting step in hepatic heme biosynthesis occurs at the ALA synthase catalyzed step, which is the committed step in heme synthesis. Heme itself functions as a co-repressor in the inhibition of ALA synthase gene expression.

• The Fe3+ oxidation product of heme is termed hemin. Hemin acts as a feed-back inhibitor on ALA synthase. Hemin also inhibits transport of ALA synthase from the cytosol (its' site of synthesis) into the mitochondria (its' site of action).

3. Regulation of Heme Biosynthesis

• In erythroid cells all of the heme is synthesized for incorporation into hemoglobin and occurs only upon differentiation when synthesis of hemoglobin proceeds.

• When red cells mature both heme and hemoglobin synthesis ceases. The heme (and hemoglobin) must, therefore, survive for the life of the erythrocyte (normally this is 120 days). In reticulocytes (immature erythrocytes) heme stimulates protein synthesis. The mechanism of this mode of heme-mediate regulation of protein synthesis is described in the Protein Synthesis section. Additionally, control of heme biosynthesis in erythrocytes occurs at numerous sites other than at the level of ALA synthase. Control has been shown to be exerted on ferrochelatase, the enzyme responsible for iron insertion into protoporphyrin IX, and on porphobilinogen deaminase.

5. Clinical Aspect of Heme Metabolism

two types: Porphyrias and Bilirubinemias. 1. Disorders that arise from defects in the

enzymes of heme biosynthesis are termed the porphyrias and cause elevations in the serum and urine content of intermediates in heme synthesis.

2. Inherited disorders in bilirubin metabolism lead to hyperbilirubinemia.

Porphyrias• are both inherited and acquired disorders in heme synthesis. • These disorders are classified as either erythroid or hepatic, depending upon

the principal site of expression of the enzyme defect. • Eight different porphyrias have been classified encompassing defects in each of

the enzymes of heme synthesis. • Defects in hepatic uroporhyrinogen decarboxylase (UROD) results in type I

porphyria cutanea tarda (PCT I), whereas deficiencies in the non-hepatic forms of UROD result in type II PCT (PCT II). PCT is the most commonly occurring type of porphyria.

• It should be noted that no porphyria has been identified resulting from defects in the house-keeping form of ALAS (ALAS1).

• The most commonly occurring hepatic porphyria is acute intermittent porphyria, AIP which is caused by a defect in porphobilinogen deaminase, (PBG deaminase). This enzyme is also called hydroxymethylbilane synthase or rarely uroporphyrinogen I synthase.

Porphyrias

• All of the porphyrias lead to excretion of heme biosynthetic byproducts that turn the urine red and when deposited in the teeth turn them reddish brown.

• Accumulation of these byproducts in the skin renders it extremely sensitive to sunlight causing ulceration and disfiguring scars.

• Increased hair growth (hypertrichosis) is also a symptom of the porphryias leading to appearance of fine hairs over the entire face and on the extremities.

• This latter symptom lends to the description of "werewolf syndrome" in many porphyria patients.

Pustaka

• Murray, R.K., D.K.Granner, and V.W.Rodwell (Eds.), 2009. Biokimia Harper. Edisi 27. Bab 31. Porfirin dan Pigmen Empedu (Robert K. Murray) Halaman 288- 303. Penerbit Buku Kedokteran, Jakarta