Heme is the most important porphyrin containing com- pound. It is primarily synthesized in the liver and the erythrocyte-producing cells of bone marrow (erythroid cells). However, mature erythrocytes lacking mitochon- dria are a notable exception.
Structure of Heme
1. Heme is a derivative of the porphyrin. Porphyrins are cyclic compounds formed by fusion of four pyrrole rings linked by methenyl (=CH—) bridges.
2. Since an atom of iron is present, heme is a ferropro- toporphyrin. The pyrrole rings are named as I, II, III, IV and the bridges as alpha (a), beta (b), gamma (g) and delta (d). The possible areas of substitution are denoted as 1–8 (Fig. 8.1).
Fig. 8.1: Structure of heme
3. Type III is the most predominant in biological sys- tems. It is also called series 9.
Biosynthesis of Heme
Heme can be synthesized by almost all the tissues in the body. Heme is synthesized in the normoblasts, but not in the matured erythrocytes. The pathway is partly cytoplas- mic and partly mitochondrial.
Step 1: Formation of d-aminolevulinate Acid
Glycine, a non-essential amino acid and succinyl-CoA, an intermediate in the citric acid cycle are the start- ing materials for porphyrin synthesis. Glycine com- bines with succinyl-CoA to form delta-aminolevulinate (ALA). This reaction catalyzed by a pyridoxal phos- phate-dependent d-aminolevulinate synthase occurs in the mitochondria. It is a rate-controlling step in porphy- rin synthesis.
Step 2: Synthesis of Porphobilinogen
Two molecules of d-aminolevulinate condense to form porphobilinogen (PBG) in the cytosol. This reaction is cat- alyzed by a Zn-containing enzyme, ALA dehydrogenase. It is sensitive to inhibition by heavy metals such as lead.
Step 3: Formation of Uroprophyrinogen
Condensation of four molecules of the PBG results in the formation of the first porphyrin of the pathway, namely uroporphyrinogen (UPG). The enzyme for this reaction is PBG deaminase [otherwise called uroporphyrin I synthase or hydroxymethylbilane (HMB) synthase]. HMB molecule will cyclize spontaneously to form uroporphyrinogen I. It is
Chapter 8: Tissue Biochemistry 91
converted to uroporphyrinogen III by the enzyme uropor- phyrinogen III synthase.
Step 4: Synthesis of Coproporphyrinogen
The UPG-III is next converted to coproporphyrinogen (CPG-III) by decarboxylation. Four molecules of CO2 are eliminated by uroporphyrinogen decarboxylase.
Step 5: Synthesis of Protoporphyrinogen
Further metabolism takes place in the mitochondria. CPG is oxidized to protoporphyrinogen (PPG-III) by copropor- phyrinogen oxidase. This enzyme specifically acts only on type III series.
Step 6: Generation of Protoporphyrin
The protoporphyrinogen-III is oxidized by the enzyme protoporphyrinogen oxidase to protoporphyrin-III (PP- III) in the mitochondria. The oxidation requires molecular oxygen.
Step 7: Generation of Heme
The incorporation of ferrous ion (Fe2+) into protopor-
phyrin-IX is catalyzed by the enzyme heme synthetase (ferrochelatase). This enzyme can be inhibited by lead (Fig. 8.2).
Regulation of Heme Synthesis
1. The ALA synthase is regulated by repression mecha- nism. Heme inhibits the synthesis of ALA synthase by acting as a co-repressor.
2. The ALA synthase is also allosterically inhibited by he- matin. When there is excess of free heme, the Fe2+ is
oxidized to Fe3+ (ferric), thus forming hematin.
3. The compartmentalization of the enzymes of heme synthesis makes the regulation easier for the regula- tion. The rate-limiting enzyme is in the mitochondria. Some steps take place inside mitochondria, while rest occurs in cytoplasm.
4. Drugs like barbiturates induce heme synthesis. Barbi- turates require the heme-containing cytochrome p450
for their metabolism.
5. The steps catalyzed by ferrochelatase and ALA dehy- dratase are inhibited by lead.
6. Isonicotinic acid hydrazide (INH) that decreases the availability of pyridoxal phosphate may also affect heme synthesis.
7. High cellular concentration of glucose prevents in- duction of ALA synthase.
Fig. 8.2: Biosynthesis of heme
Disorders of Heme Synthesis
Porphyrias
Porphyrias are the metabolic disorders of heme synthesis characterized by the increased excretion of porphyrins or porphyrin precursors. Porphyrias are either inherited or acquired. They are broadly classified into two categories (Table 8.1):
• Erythropoietic: Enzyme deficiency occurs in the erythrocytes
• Hepatic: Enzyme defect lies in the liver. Acute intermittent porphyria
Acute intermittent porphyria is characterized by increased excretion of porphobilinogen and 8-aminolevulinate. The urine gets darkened on exposure to air due to the conver- sion of porphobilinogen to porphobilin and porphyria. The other characteristic features of acute intermittent por- phyria are as follows:
1. The symptoms include abdominal pain, vomiting and cardiovascular abnormalities. The neuropsychiatric
disturbances observed in these patients are be- lieved to be due to reduced activity of tryptophan pyrrolase, resulting in accumulation of tryptophan and serotonin.
2. The symptoms are more severe after administration of drugs (e.g. barbiturates) that induce the synthesis of cytochrome P450. This is due to the increased activity of
ALA synthase causing accumulation of PBG and ALA. 3. These patients are not photosensitive since the en- zyme defect occurs prior to the formation of uropor- phyrinogen.
4. Acute intermittent porphyria is treated by administra- tion of hematin, which inhibits the enzyme ALA syn- thase and the accumulation of porphobilinogen.
Acute intermittent porphyria symptoms (5 P’s):
• Pain in abdomen • Polyneuropathy
• Psychological abnormalities • Pink urine
• Precipitated by drugs (e.g. barbiturates, oral contraceptives
and sulfa drugs)
Congenital erythropoietic porphyria
1. Congenital erythropoietic porphyria is a rare congeni- tal disorder caused by autosomal recessive mode of inheritance, mostly confined to erythropoietic tissues. 2. The individuals excrete uroporphyrinogen I and cop- roporphyrinogen I, which oxidize respectively to uro- porphyrin I and coproporphyrin I (red pigments). 3. The patients are photosensitive (itching and burning
of skin when exposed to visible light) due to the ab- normal prophyrins that accumulate.
4. Increased hemolysis is also observed in the individu- als affected by this disorder.
Porphyria cutanea tarda
Porphyria cutanea tarda is also known as cutaneous he- patic porphyria and is the most common porphyria, usually associated with liver damage caused by alcohol
overconsumption or iron overload. Cutaneous photo- sensitivity is the most important clinical manifestation of these patients.