Appreciate that the some of the enzymes of heme biosynthesis are mitochondrial and others are cytosolic. Indicate which steps in the conversion of heme to bilirubin are cytosolic and which are mitochondrial. Understand the causes and general clinical pictures of the various porphyrias.
Explain the biochemical nature of jaundice, name some of its causes, and suggest how to approach determining its biochemical underpinnings. Heme is synthesized from porphyrins and iron, and the products of degradation of heme are the bile pigments and iron.
The biochemistry of the porphyrins and of heme is basic to understanding the varied functions of hemoproteins, and the porphyrias, a group of diseases caused by abnormalities in the pathway of porphyrin biosynthesis. This enzyme is also called hydroxymethylbilane synthase or uroporphyrinogen I synthase.
PBG deaminase isoform 1 is composed of amino acids, PBG deaminase isoform 2 is composed of amino acids, PBG deaminase isoform 3 is composed of amino acids, and PBG deaminase isoform 4 is composed of amino acids. PBG deaminase isoform 2 is an erythroid cell-specific form of the enzyme. Once produced, hydroxymethylbilane has two main fates, one is due to enzymatic action, the other is non-enzymatic.
Non-enzymatic alteration in hydroxymethylbilane is a cyclization to the compound called uroporphyrinogen I. The latter fate of hydroxymethylbilane is of significance only in patients with defects in enzymes downstream of PBG deaminase. Of significance to patients harboring a defective heme biosynthetic enzyme is the fact that defects prior to hydroxymethylbilane synthesis ARE NOT associated with photosensitivity, whereas, defects from this point on ARE associated with photosensitivity.
Defects in the PBG deaminase gene result in the autosomal dominant hepatic porphyria called acute intermittent porphyria , AIP. The most important fate of hydroxymethylbilane is the regulated, enzymatic conversion to uroporphyrinogen III, the next intermediate on the pathway to heme synthesis.
This step is mediated by the enzyme, uroporphyrinogen-III synthase. Defects in the UROS gene are associated with the autosomal recessive erythroid porphyria called congenital erythropoietic porphyria , CEP In the Figure below you can place your mouse over intermediate names to see their structures.
Clicking on enzyme names will take you to a descriptive page of the porphyria resulting from deficiency in that enzyme. Pathway of Heme Biosynthesis.
Heme biosynthesis begins in the mitochondria from glycine and succinyl-CoA, continues in the cytosol, and ultimately is completed within the mitochondria. The heme that it produced by this biosynthetic pathway is identified as heme b. The resultant products have methyl groups in place of acetate and are known as coproporphyrinogens, with coproporphyrinogen III being the important normal intermediate in heme synthesis.
The UROD gene is located on chromosome 1p Following its synthesis, coproporphyrinogen III is transported to the interior of the mitochondrion, where two propionate residues are decarboxylated, yielding vinyl substituents on the two pyrrole rings. The colorless product is protoporphyrinogen IX.
The CPOX gene is located on chromosome 3q Mutations in the CPOX gene result in the autosomal dominant acute hepatic porphyria called hereditary coproporphyria , HCP In the mitochondrion, protoporphyrinogen IX is converted to protoporphyrin IX structure shown below by protoporphyrinogen IX oxidase.
These six mRNAs encode four distinct protein isoforms. The oxidase reaction requires molecular oxygen and results in the loss of six protons and six electrons, yielding a completely conjugated ring system, which is responsible for the characteristic red color of the hemes. The enzyme catalyzing this reaction is known as ferrochelatase. The ferrochelatase gene symbol: FECH is located on chromosome 18q Ferrochelatase isoform a is composed of amino acids and isoform b is composed of amino acids.
Defects in the ferrochelatase gene are associated with the autosomal dominant erythroid porphyria called erythropoietic protoporphyria , EPP. Protoporphyrin IX Heavy Metal Inhibition of Heme Metabolism The enzymes ferrochelatase, ALA synthase and ALA dehydratase a sulfhydryl containing enzyme are sensitive to inhibition by heavy metal poisoning, with inhibition of ferrochelatase being the most sensitive and most significant to the clinical manifestations of heavy metal poisoning.
A characteristic of lead poisoning is an increase in ALA in the circulation in the absence of an increase in heme. Indeed, due to the inhibition of ferrochelatase and the associated loss of heme synthesis, the feed-back inhibition of ALA synthase is no longer active leading to increased ALA synthesis even in the presence of a heavy metal such as lead. The consequences of heavy metal inhibition of ferrochelatase are an acquired porphyria as opposed to an inherited disease referred to as plumbism so-called because the symbol for lead is Pb.
Due to the lack of enzymatic incorporation of iron into protoporphyrin IX, by heavy metal inhibited ferrochelatase, erythroblasts will acquire siderosomes. Siderosomes are histologically observable structures that result from iron deposition on mitochondria. Zn-protoporphyrin imparts a fluorescence capability that can be visualized by observing blood under appropriate wavelength light which causes the erythroid cells to "glow".
This same phenomenon is observable in patients with iron deficient anemia. Indeed, the measurement of ZPP is used as a screening tool for both heavy metal lead poisoning and iron deficiency. The Various Heme Molecules In addition to the heme b found in hemoglobin, there are two additional forms of heme found in cytochromes such as those involved in the process of oxidative phosphorylation. Cytochromes of the c type contain a modified iron protoporphyrin IX known as heme c.
Only cytochromes of the c type contain covalently bound heme. Heme a is also a modified iron protoporphyrin IX. Heme a is found in cytochromes of the a type such as those of complex IV of the oxidative phosphorylation pathway. The differences in these two tissues and their needs for heme result in quite different mechanisms for regulation of heme biosynthesis.
In hepatocytes, heme is required for incorporation into the cytochromes, in particular, the P class of cytochromes CYP that are important for xenobiotic 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.
Heme itself, and hemin acts as a feed-back inhibitors on ALA synthase. Hemin also inhibits transport of ALA synthase from the cytosol its site of synthesis into the mitochondria its site of action. Because certain pharmaceutical drugs are metabolized by the hepatic CYP system, which requires heme, increased utilization of heme occurs upon administration of these drugs. Of particular significance are the barbiturates.
Use of barbiturates should NEVER be prescribed for the pain associated with certain types of porphyrias. This is because the administration of barbiturates leads to their degradation by CYP enzymes in the liver, resulting in a reduction in overall heme levels as the heme needs to be incorporated into the CYP for their function.
This results in de-repression of ALA synthase with the result being an exacerbation of the symptoms of the porphyria due to increased ALA synthesis and subsequent heme biosynthesis products upstream of the defective enzyme. 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 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 page.
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. First, the porphyrin ring is hydrophobic and must be solubilized to be excreted. Second, iron must be conserved for new heme synthesis. Normally, senescent red blood cells and heme from other sources are engulfed by cells of the reticuloendothelial system phagocytic macrophages primarily of the spleen but also of the liver, lymph, and bone marrow.
The globin is recycled or converted into amino acids, which in turn are recycled or catabolized as required. Heme is oxidized, with the heme ring being opened by the endoplasmic reticulum enzyme, heme oxygenase. This is the only reaction in the body that is known to produce CO. Most of the CO is excreted through the lungs, with the result that the CO content of expired air is a direct measure of the activity of heme oxygenase in an individual. The heme oxygenase enzyme encoded by the HMOX1 gene is the rate-limiting enzyme of heme catabolism.
The HMOX1 gene is located on chromosome 22q The HMOX2 gene is located on chromosome 16p In the next reaction a second bridging methylene between rings III and IV is reduced by biliverdin reductase, producing bilirubin. The BLVRA gene is located on chromosome 7p13 and is composed of 11 exons generate two alternatively spliced mRNAs, both of which encode the same amino acid precursor protein.
Bilirubin is significantly less extensively conjugated than biliverdin causing a change in the color of the molecule from blue-green biliverdin to yellow-red bilirubin. The latter catabolic changes in the structure of tetrapyrroles are responsible for the progressive changes in color of a hematoma, or bruise, in which the damaged tissue changes its color from an initial dark blue to a red-yellow and finally to a yellow color before all the pigment is transported out of the affected tissue.
Peripherally arising bilirubin is transported to the liver in association with albumin, where the remaining catabolic reactions take place.
Pathway for the degradation of heme to bilirubin. The product of the heme oxygenase reaction is biluverdin. Biliverdin is converted to bilirubin via the action of biliverdin reductase. The various substituents on the pentameric rings of biliverdin and bilirubins are M: methyl, P: propyl, V: vinyl.
In hepatocytes, bilirubin-UDP-glucuronosyltransferase bilirubin-UGT: a member of the large UDP glucuronosyltransferase family of enzymes adds two equivalents of glucuronic acid to bilirubin to produce the more water soluble, bilirubin diglucuronide derivative. The increased water solubility of the tetrapyrrole facilitates its excretion with the remainder of the bile as the bile pigments.This mechanism is of therapeutic importance: infusion of heme arginate or hematin and glucose can abort attacks of acute intermittent porphyria in patients with an inborn error of metabolism of this process, by reducing transcription of ALA synthase. Peripherally arising bilirubin is transported to the liver in association with albumin, where the remaining catabolic reactions take place. Pathway of Heme Biosynthesis. Biliverdin is converted to bilirubin via the action of biliverdin reductase.
Lactoperoxidase and eosinophil peroxidase are protective enzymes responsible for the destruction of invading bacteria and virus. This latter symptom lends to the description of "werewolf syndrome" in many porphyria patients. All heme intermediates and degradation products that end in -ogen e. Each first exon has its own promoter element. Porphyrins are complex chemical compounds that are large heterocyclic organic ring structures. The rationale for the use of these agents is that they act as analogs of heme and strongly inhibit the activity of ALAS resulting in reductions in the heme biosynthetic intermediates that precipitate the porphyria attack.
Bilirubin has been shown to inhibit DNA synthesis, uncouple oxidative phosphorylation, and inhibit ATPase activity in brain mitochondria. Of significance to patients harboring a defective heme biosynthetic enzyme is the fact that defects prior to hydroxymethylbilane synthesis ARE NOT associated with photosensitivity, whereas, defects from this point on ARE associated with photosensitivity. This process is claimed to create a meaty flavor in the resulting products. Of particular significance are the barbiturates. Conjugated bilirubin does not require addition of alcohol to promote the azotization reaction and thus, this is referred to as measurement of direct bilirubin.
Disorders that arise from defects in the enzymes of heme biosynthesis are termed the porphyrias see Table below and the Porphyrias page and cause elevations in the serum and urine content of intermediates in heme synthesis. This mechanism is of therapeutic importance: infusion of heme arginate or hematin and glucose can abort attacks of acute intermittent porphyria in patients with an inborn error of metabolism of this process, by reducing transcription of ALA synthase. The latter is the result of inherited deficiencies in the enzyme responsible for bilirubin conjugation to glucuronic acid, bilirubin UDP glucuronosyltransferase bilirubin-UGT. Several inherited disorders in bilirubin metabolism have been identified. Accumulation of these byproducts in the skin renders it extremely sensitive to sunlight causing ulceration and disfiguring scars.
In normal individuals, intestinal bilirubin is acted on by bacteria to produce the final porphyrin products, urobilinogens and stercobilins, that are found in the feces. The heme and hemoglobin must, therefore, survive for the life of the erythrocyte normally this is days. In addition, numerous cytochromes of the oxidative-phosphorylation pathway contain heme. Delta-aminolevulinic acid ALA is also called 5-aminolevulinic acid.
Following its synthesis, coproporphyrinogen III is transported to the interior of the mitochondrion, where two propionate residues are decarboxylated, yielding vinyl substituents on the two pyrrole rings. Name the key regulated enzyme of hepatic heme biosynthesis. Defects in the PBG deaminase gene result in the autosomal dominant hepatic porphyria called acute intermittent porphyria , AIP. Heme a is found in cytochromes of the a type such as those of complex IV of the oxidative phosphorylation pathway.
In addition to the heme b of hemoglobin and its role in oxygen transport, hemes are critical for the biological functions of several enzymes such as the cytochromes of oxidative phosphorylation and the xenobiotic metabolizing enzymes of the cytochrome P family CYP.
Delta-aminolevulinic acid ALA is also called 5-aminolevulinic acid. The reaction with unconjugated bilirubin requires the addition of alcohol and thus is referred to as the measurement of indirect bilirubin. Siderosomes are histologically observable structures that result from iron deposition on mitochondria. The N-terminal region encoded by each unique first exon determines acceptor substrate specificity, while the amino acid C-terminal region encoded by the 4 common exons specifies interactions with the common donor substrate, UDP-glucuronic acid. The BLVRA gene is located on chromosome 7p13 and is composed of 11 exons generate two alternatively spliced mRNAs, both of which encode the same amino acid precursor protein.
In humans, the most clinically significant porphyrins are the hemes; heme a, heme b, and heme c.
Harper's Illustrated Biochemistry, 29th ed. Heme is synthesized from porphyrins and iron, and the products of degradation of heme are the bile pigments and iron. Clicking on enzyme names will take you to a descriptive page of the porphyria resulting from deficiency in that enzyme. The 9 viable first exons are independently spliced to the common exons 2 through 5 to generate 9 UGT1A transcripts with unique 5' ends and identical 3' ends.
Explain the biochemical nature of jaundice, name some of its causes, and suggest how to approach determining its biochemical underpinnings. The major function of heme in humans is its role in the coordination of O2 molecules in hemoglobin.