Section 8. Amino Acid Metabolism Porphyrins, heme, bile pigments 11/22/04

Heme In addition to the heme in hemogloblin and myoglobin, molecules with the porphyrin ring structure include cytochromes, and in plants, the chlorophylls. Heme is synthesized in most cells. Reticulocytes make ~ 4 x hemes per second. In all cases, the precursors are glycine and succinyl CoA. In addition to the heme in hemogloblin and myoglobin, molecules with the porphyrin ring structure include cytochromes, and in plants, the chlorophylls. Heme is synthesized in most cells. Reticulocytes make ~ 4 x hemes per second. In all cases, the precursors are glycine and succinyl CoA. 1

Heme Structure The core protoporphyrin ring structure is constant. The sidechains vary in other porphyrins. The protein structure determines the properties of the prosthetic group. The core protoporphyrin ring structure is constant. The sidechains vary in other porphyrins. The protein structure determines the properties of the prosthetic group. (p270) 2

Protoporphyrin IX Structure Fe(II) displaces the two protons in the center. Stryer 4th 3

B 12 Coenzyme Structure Not heme, but similiar. Compared to a protoporphyrin ring, the corrin ring has smaller space in center to fit Co(II), smaller than Fe(II). Vitamin B 12, cobalamin, may have Co(III) with 5’-deoxyadenosine substituted by CN or CH 3. Not heme, but similiar. Compared to a protoporphyrin ring, the corrin ring has smaller space in center to fit Co(II), smaller than Fe(II). Vitamin B 12, cobalamin, may have Co(III) with 5’-deoxyadenosine substituted by CN or CH 3. Stryer 4th 4

Succinyl CoA Biosynthesis One heme precursor, succinyl CoA. can be made from propionyl CoA, or from  -ketoglutarate in the Krebs cycle. Propionyl CoA can be made from amino acids, or from the oxidation of odd-numbered fatty acids. After propionyl CoA is carboxylated, the conversion of methylmalonyl CoA to succinyl CoA requires vitamin B 12. One heme precursor, succinyl CoA. can be made from propionyl CoA, or from  -ketoglutarate in the Krebs cycle. Propionyl CoA can be made from amino acids, or from the oxidation of odd-numbered fatty acids. After propionyl CoA is carboxylated, the conversion of methylmalonyl CoA to succinyl CoA requires vitamin B 12. 5

Heme Biosynthesis: Part 1  -ALA synthetase catalyzes the committed step. Reactions occur in the mitochondrial matrix. 8 succinyl CoA & 8 glycine.  -ALA synthetase catalyzes the committed step. Reactions occur in the mitochondrial matrix. 8 succinyl CoA & 8 glycine. 6

Heme Biosynthesis: Part 2 Notice that one of the pyrrole rings was “rotated” by the uroporphyrinogen cosynthetase making the cyclic uroporphyrinogen III asymmetric (look at the A-P distribution). The multiple arrows are several decarboxylations. Notice that one of the pyrrole rings was “rotated” by the uroporphyrinogen cosynthetase making the cyclic uroporphyrinogen III asymmetric (look at the A-P distribution). The multiple arrows are several decarboxylations. 7

Ferrochelatase Mechanism Ferrochelatase (FC) binds Fe(II), displacing 2 H + on the enzyme. Next it “domes” the protoporphyrin IX. Fe(II) is inserted, exchanging 2 H +. It appears that distortion of the protoporphyrin planar structure occurs after the metal binds to FC. From Blackwood etal, Biochemistry 37:779 (1998). Ferrochelatase (FC) binds Fe(II), displacing 2 H + on the enzyme. Next it “domes” the protoporphyrin IX. Fe(II) is inserted, exchanging 2 H +. It appears that distortion of the protoporphyrin planar structure occurs after the metal binds to FC. From Blackwood etal, Biochemistry 37:779 (1998). 8

Stoichiometry This ignores the conversion of some of the acetate and propionate groups to methyl and vinyl groups. 9

Control Mechanisms  -ALA synthetase catalyzes the committed step. Heme inhibits the activities of  -ALA synthetase,  -ALA dehydrase and ferrochelatase. Heme also binds a nuclear receptor, which reduces the transcription of the mRNA for  - ALA synthetase. Heme inhibits transport of  -ALA synthetase into the mitochondria.  -ALA synthetase catalyzes the committed step. Heme inhibits the activities of  -ALA synthetase,  -ALA dehydrase and ferrochelatase. Heme also binds a nuclear receptor, which reduces the transcription of the mRNA for  - ALA synthetase. Heme inhibits transport of  -ALA synthetase into the mitochondria. 10

Diseases Related to Heme Synthesis Congenital erythropoietic porphyria is due to deficient uroporphyrinogen III cosynthetase activity in red blood cell precursors. The symmetric product uroporphyrinogen is not degraded. Short rbc lifetime, photosensitive skin, red enamel (UV light). Deficient ferrochelatase causes erythropoietic protoporphyria, which has similar but milder symptoms. Congenital erythropoietic porphyria is due to deficient uroporphyrinogen III cosynthetase activity in red blood cell precursors. The symmetric product uroporphyrinogen is not degraded. Short rbc lifetime, photosensitive skin, red enamel (UV light). Deficient ferrochelatase causes erythropoietic protoporphyria, which has similar but milder symptoms. 11

Acute Intermittent Porphyria Due to deficient uroporphyrinogen synthetase activity in the liver, coupled to elevated  -ALA synthetase activity. The result is high [  -ALA] and [porphobilinogen]. Symptoms are abdominal pain, dark urine, neurological aberrations. King George III ? Vincent Van Gogh ? Due to deficient uroporphyrinogen synthetase activity in the liver, coupled to elevated  -ALA synthetase activity. The result is high [  -ALA] and [porphobilinogen]. Symptoms are abdominal pain, dark urine, neurological aberrations. King George III ? Vincent Van Gogh ? 12

Heme Degradation Heme is degraded in the liver. The final product, bilirubin diglucuronide, is transported in bile from the liver to the gall bladder and excreted. It, and several related structures, are bile pigments. Notice energy is used to reduce biliverdin to a waste product. Heme is degraded in the liver. The final product, bilirubin diglucuronide, is transported in bile from the liver to the gall bladder and excreted. It, and several related structures, are bile pigments. Notice energy is used to reduce biliverdin to a waste product. 13

Hb Heme Life Cycle Synthesized in red blood cell precursors. Spends about 120 days in the bloodstream. Identified as “old” by the spleen, which disrupts the rbc membrane, freeing the Hb. Transported to the liver as a haptoglobin-hemoglobin complex. Degraded in the liver to amino acids, Fe(III) and bilirubin diglucuronide. The iron is transported to other cells, bound to transferrin, for reuse. Bilirubin diglucuronide excreted by gall bladder. Synthesized in red blood cell precursors. Spends about 120 days in the bloodstream. Identified as “old” by the spleen, which disrupts the rbc membrane, freeing the Hb. Transported to the liver as a haptoglobin-hemoglobin complex. Degraded in the liver to amino acids, Fe(III) and bilirubin diglucuronide. The iron is transported to other cells, bound to transferrin, for reuse. Bilirubin diglucuronide excreted by gall bladder. 14

Bile About 500 ml per day of bile is made by the liver, which excretes it and transfers it to the gall bladder. The gall bladder concentrates bile and excretes it into the intestinal lumen. The three main dissolved constituents are glycocholate (~80%), phospholipids (~15%) and cholesterol (~5%). Glycocholate, a fat-solubilizing detergent, is reabsorbed by the intestinal epithelium. The other constituents, such bile pigments, are minor. About 500 ml per day of bile is made by the liver, which excretes it and transfers it to the gall bladder. The gall bladder concentrates bile and excretes it into the intestinal lumen. The three main dissolved constituents are glycocholate (~80%), phospholipids (~15%) and cholesterol (~5%). Glycocholate, a fat-solubilizing detergent, is reabsorbed by the intestinal epithelium. The other constituents, such bile pigments, are minor. 15

Gallstone Formation At typical total concentrations in bile, the solubility of the constituents is limited (Percent is percent of total solid). The hatched area includes the combinations of cholesterol, phospholipid and bile salt that are soluble. Precipitation can lead to gallstones, which are painful if they block the bile duct. At typical total concentrations in bile, the solubility of the constituents is limited (Percent is percent of total solid). The hatched area includes the combinations of cholesterol, phospholipid and bile salt that are soluble. Precipitation can lead to gallstones, which are painful if they block the bile duct. * 80% bile salt 15% phospholipid 5% cholesterol 16

Neonatal Jaundice The immature liver cannot add glucuronate to bilirubin. Bilirubin circulates in the blood instead of being transported to the gall bladder. Bilirubin can cross the immature blood brain barrier and causes brain damage. Two therapies are successful : –Visible light irradiation. –Sn-substituted protoporphyrin. Sn-protoporphyrin inhibits heme oxygenase. The immature liver cannot add glucuronate to bilirubin. Bilirubin circulates in the blood instead of being transported to the gall bladder. Bilirubin can cross the immature blood brain barrier and causes brain damage. Two therapies are successful : –Visible light irradiation. –Sn-substituted protoporphyrin. Sn-protoporphyrin inhibits heme oxygenase. 17

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