Metabolism of tetrapyrrols

Pavla Balínová

Tetrapyrrols• circular compounds

binding a metal ion (most frequently Fe2+ and Fe3+)

• consist of 4 pyrrol rings interconnected via methine bridges

Examples:• heme (Fe2+)• chlorophyll (Mg2+)• vitamin B12 (Co2+)

Figure was assumed from http://en.wikipedia.org/wiki/Porphyrin

Where can we find a heme??

Hemoproteins• Hemoglobin (Hb)• Myoglobin (Mb)• Cytochrome c • Catalases

(decomposition of H2O2 to H2O and O2)

Figure was assumed from http://en.wikipedia.org/wiki/Heme

Heme structure

pyrrol

methine bridge

Figure was assumed from a book T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed., Wiley‑Liss, Inc., New York, 1997.

Biosynthesis of heme• Organ location: bone marrow 85% and liver • Subcellular location: mitochondria and cytosol

• Substrates: succinyl-CoA and glycine

• Important intermediates: δ-aminolevulinic acid (ALA), porphobilinogen, uroporphyrinogen III, protoporphyrin IX

• Key regulatory enzyme: ALA synthase

Heme biosynthesis

Figure was assumed from http://www.porphyrin.net/mediporph/_netbiochem/synthesis/_synthmain.html

Delta-aminolevulinic acid (ALA)• synthesis of heme starts in mitochondria• succinyl-CoA and glycine (Gly) undergo

condensation → δ-aminolevulinic acid (ALA)• reaction is catalyzed by enzyme ALA synthase

-OOC-CH2-CH2-CO-S-CoA + NH3+-CH2-COO-

CO2 -OOC-CH2-CH2-CO-CH2-NH3

+

Porphobilinogen (PBG)• ALA leaves the mitochondria → cytoplasm• 2 molecules of ALA condense to form porphobilinogen• reaction is catalyzed by enzyme porphobilinogen

synthase

Figure was assumed from http://en.wikipedia.org/wiki/Porphobilinogen

pyrrol ring

Uroporphyrinogen III

Figure was assumed from book T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed., Wiley‑Liss, Inc., New York, 1997.

Uroporphyrinogen → coproporphyrinogen III

• enzyme hydroxymethylbilane synthase catalyzes the linkage of 4 PBG molecules and cleavage of 4 NH4

+ to yield uroporphyrinogen III• 4 acetate residues are decarboxylated into

methyl groups → coproporphyrinogen III returns to the mitochondria again

Protoporphyrinogen IX

Figure was assumed from book T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed., Wiley‑Liss, Inc., New York, 1997.

Protoporphyrinogen IX → protoporphyrin IX

• oxidation of protoporphyrinogen IX produces the conjugated π-electron system of protoporphyrin IX

Figure was assumed from book T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed., Wiley‑Liss, Inc., New York, 1997.

Heme

• Fe2+ is incorporated into protoporhyrin IX• reaction is catalyzed by enzyme ferrochelatase

Figure was assumed from http://www.porphyrin.net/mediporph/_netbiochem/synthesis/ferrochelatase.html

Regulation of heme biosynthesisALA synthase is a key regulatory enzyme• it is an allosteric enzyme that is inhibited by heme = feedback inhibition• requires pyridoxal phosphate• certain drugs and steroid hormones can increase heme synthesis

Porphobilinogen synthase is inhibited by lead ions Pb2+ in case of lead poisoning.

Ferrochelatase can be also inhibited by Pb2+. Its activity is influenced by availability of Fe2+ and ascorbic acid.

Porphyrias• are hereditary or acquired disturbances of heme synthesis

• in all cases there is an identifiable abnormality of the enzymes which synthesize heme

• this leads to accumulation of intermediates of the pathway and a deficiency of heme → excretion of heme precursors in feces or urine, giving them a dark red color

● accumulation of porphyrinogens in the skin can lead to photosensitivity • the neurological symptoms

Heme degradation• around 100 – 200 million aged erythrocytes per hour are broken down in the human organism• Organ location: RES (reticuloendothelial cells) in the spleen, liver and bone marrowHb is degraded to:● globin → AAs → metabolism● heme → bilirubin● Fe2+ → transport with transferrin and used in the next heme biosynthesis

Not only Hb but other hemoproteins also contain heme groups which are degraded by the same pathway.

Conversion of heme to bilirubin

Figure was assumed from http://web.indstate.edu/thcme/mwking/heme-porphyrin.html

BilirubinBilirubin (Bil) is released from RES into the blood. BUT! Bil is only poorly soluble in plasma, and therefore during transport it is bound to albumin. ↓ LIVERIn the hepatocytes, Bil is conjugated by 2 molecules of glucuronic acid → bilirubin diglucuronide (soluble in water, „conjugated Bil“). Conjugation is catalyzed by UDP-glucuronosyltransferase.

Bilirubin diglucuronide

Figure was assumed from http://web.indstate.edu/thcme/mwking/heme-porphyrin.html

Bile pigments bilirubin diglucuronide ↓ BILE ↓ INTESTINEBil is reduced to urobilinogen and stercobilinogen by bacteria → oxidation to urobilin and stercobilinBile pigments are mostly excreted in feces, but a small proportion is resorbed (enterohepatic circulation).Small amount of urobilinogen is excreted with urine.

Determination of bilirubin in serum

Blood tests • Bil reacts directly when dyes are added to the

blood sample → conjugated bilirubin = direct

• free Bil does not react to the reagents until alcohol (methanol) or caffeine is added to the solution. Therefore, the measurement of this type of bilirubin is indirect → unconjugated bilirubin = indirect

• Total bilirubin measures both unconjugated and conjugated Bil (normal value up to 20 µmol/L).

Hyperbilirubinemias• Hyperbilirubinemia = an elevated bilirubin level

(> 10 mg/L) → Bil can diffuses from the blood into peripheral tissues and gives it a yellow color (jaundice = icterus)

Jaundice can have various causes: • Increased erythrocyte degradation – hemolytic

jaundice• Impaired conjugation of bilirubin in the liver –

hepatocellular jaundice• Disturbance of bile drainage (gallstones) –

obstructive jaundice

In the urine, only conjugated bilirubin can be present.

IcterusIcterus is the yellow coloration of skin and mucus membranes of jaundice (hyperbilirubinemia with various ethiology) • Hemolytic icterus: elevated level of unconjugated Bil in blood

• Neonatal jaundice usually appears after a few days after birth (elevated hemolysis, decreased activity of UDP-glucuronosyltransferase → ↑ unconjugated Bil)In severe cases, unconjugated Bil can cross the blood-brain barrier and lead to brain damage (kernicterus).