Biosynthetic pathway of heme

Biosynthetic pathway of heme. The pathway consists of eight irreversible reactions, four each in the mitochondrion and the cytosol. The primary site of regulation is the ALA synthase step. 

Aminolevulinic acid (ALA) is a common intermediate in the biosynthetic pathways of heme, chlorophylls, phycobilins, and cobalamins. Relationships between the various pathways are shown in Figure 21.27. 

These enzymes are homodimeric periplasmic proteins (encoded by nirB) that contain two different heme groups per monomer. One is a c-type heme bound via a Cys-X-X-Cys-His motif and has a role in electron transfer the other, a noncovalently linked d heme, is an unusual ferric-dioxoisobacteriochlorin, restricted to this class of enzyme and forms the active site. The biosynthetic pathway of heme di is unique among the tetrapyrroles in having 0x0, methyl, and acrylate side chains, involving some seven nir genes. The Em values of heme d model complexes are 200mV lower than iron porphyrins. 

Figure 32-2 Biosynthetic pathway of porphyrins and heme. Q, —CHzCHjCOOH C , —CHjCOOH Me,—CHajVn, —CH = GHj...

The basic concept upon which ALA-PDT is based was conceived as we studied the biochemical basis for the group of metabolic diseases known as the porphyrias [43]. Some of the porphyrias are associated with a generalized photosensitization which is caused by the accumulation of specific types of porphyrin in the blood and/or tissues, where each type of porphyrin accumulates as the result of a specific abnormality in the biosynthetic pathway for heme. Although at that time it was generally believed that the expression of such abnormalities was restricted to the liver and the hemopoietic system (those tissues which synthesize large amounts of heme), it was obvious that all nucleated cells must have at least some capacity to synthesize heme because they all use heme-containing enzymes for the tricarboxylic acid cycle. 

In principle, it should be possible to induce a transient uroporphyria, coproporphyria, or protoporphyria in normally non-porphyric cells and tissues by administering a drug which reversibly suppresses the synthesis and/or the activity of the appropriate enzyme(s) in the biosynthetic pathway for heme. However, this approach seems in general unwise, since it would suppress heme production in every cell in the body, and heme-containing enzymes are essential for aerobic energy production. An alternate approach is to administer large doses of ALA, the first... 

The open-chain tetrapyrrole chromophores of phycobiliproteins (Fig. 2) share a common biosynthetic pathway with heme and chlorophyll from 5-aminole-vulinate to protoporphyrin IX (a pathway also shared by phytochrome of higher plants). Each phycobilipro-tein consists of a- and 3-chains. The M, of these polypeptides varies among species, but the a-chain (M,... 

HeyerNJ, Bittner Jr AC, Echeverria D, Woods JS. 2006. A cascade analysis of the interaction of mercury and coproporphyrinogen oxidase (CPOX) polymorphism on the heme biosynthetic pathway and porphyrin production. Toxicol Lett 161 159-166. 

Simmonds PL, Luckhurst CL, Woods JS. 1995. Quantitative evaluation of heme biosynthetic pathway parameters as biomarkers of low-level lead exposure in rats. J Toxicol Environ Health 44 351-367. 

Alterations in the heme biosynthetic pathway noted after 7 days. Erythrocyte ALAD activity was depressed and did not recover until 5 weeks after termination of lead treatments No effect on gastric contractions or egestion of pellets of undigested materials No effect on frequency or timing of pellet egestion... 

Although not specific for kerosene, aminolevulinic acid (ALA) could potentially be used as an adjunct or supplemental biomarker for kerosene exposure. Kerosene may affect heme metabolism by decreasing the activities of enzymes in the heme biosynthetic pathway (hepatic -ALA dehydratase and -ALA synthetase) (Rao and Pandya 1980). Therefore, it may be possible that this effect would generate increased ALA in the urine of exposed individuals. Additional studies of acute, intermediate, and chronic exposure are needed to identify biomarkers of effects for specific target organs following exposure to fuel oils. 

Scheme 4. The heme biosynthetic pathway. In vivo administration of d-aminolevulinic acid induces accumulation of fluorescent protoporphyrin IX (PpIX) preferably in malignant tissues...

Heme b is utilized for formation of hemoglobin, myoglobin, and many enzymes. It reacts with appropriate protein precursors to form the cytochromes c. Heme b is converted by prenylation to heme o405 and by prenylation and oxidation to heme fl.405a The porphyrin biosynthetic pathway also has a number of branches that lead to formation of corrins, chlorins, and chlorophylls as shown schematically in Fig. 24-22. 

The next reaction in the biosynthetic pathway, the dimerization of two molecules of 125, is thought to occur through radical bond formation to give rise to 127 (Fig. 24). This unusual reaction - dimerization of two unreactive carbon centers -is catalyzed by an equally unusual enzyme, StaD, a heme-containing enzyme with 1,100 amino acids [158], which has relatively few sequence relatives in sequence databases. Each of the currently known StaD sequence relatives are thought to play equivalent roles in related biosynthetic pathways [145-149, 155, 159], and all characterized homologs contain heme iron. Work on the related enzyme RebD... 

Antibodies were generated against a bent A-alky 1 mesoporpbyrin, which is a known inhibitor of the enzyme ferrochelatase. Ferrochelatase catalyzes the insertion of Fe(II) into protoporphyrin, as part of the heme biosynthetic pathway (Lavallee, 1988). Antibody 7G12 was found to catalyze the insertion of divalent metal ions into porphyrins, with a rate similar to thatfound for ferrochelatase (Cochran and Schultz, 1990). The structural data presented below supports the hypothesis that the transition state for porphyrin metallation involves a distortion of the macrocyclic ring system to facilitate metal insertion, and that this bent porphyrin acts as a good transition-state mimic in the development of catalytic antibody 7G12. 

A second major lead-induced toxicity involves interruption of heme synthesis. Lead interacts at several steps in the heme biosynthetic pathway (Figure 21.13). As mentioned above, Pb inhibits the enzyme 8-aminolevulinic acid dehydratase (ALA-D), which catalyzes the second step of heme synthesis involving the condensation of two molecules of aminolevulinic acid (ALA) to form porphobilinogen. The result of this inhibition is the accumulation of aminolevulinic acid in the serum and increased excretion of ALA in the urine. A second major disruption of the heme biosynthetic pathway is Pb inhibition of ferrochelatase. This enzyme is responsible for the incorporation of the ferrous ion (Fe2+) into protoporphrin IX to produce heme (Figure 21.2). Accumulated protoporphrin is incorporated into red blood cells and chelates zinc as the cells circulate. This zinc-protoporphrin complex is fluorescent and used to diagnose Pb poisoning. 

Aminolevulinic acid dehydrase (ALA dehydrase) is the second enzyme of the heme biosynthetic pathway. It catalyzes the condensations of two molecules of ALA to form porphyrobilinogen (PBG). 

An example of this approach is demonstrated in an antibody mimic of the enzyme ferrochetalase (39). Ferrochelatase catalyzes the insertion of Fe + into protoporphyrin IX (3) as the last step in the heme biosynthetic pathway (40). Interestingly, N-alkylporphyrins are known to be potent inhibitors of this enzyme, because alkylation at one pyrrole lutrogen distorts the planarity of the porphyrin macrocycle (41). This finding was used in the design of hapten 4 to catalyze the incorporation of metal ions into mesoporphyrin IX (5) by eliciting an antibody that binds the substrate in a ring-strained conformation. 

In addition to direct trafficking to target metalloproteins, some metals need to enter specialized biosynthetic pathways for metal cofactor assembly. For example, iron in metaUoenzymes is usually present as part of heme or Fe-S clusters, so it must be routed into the biosynthetic pathways for these cofactors. In a similar vein, molybdenum is not biologically active unless it is first incorporated with a pterin compound to form molybdenum... 

Antibodies that recognize transition states should function as catalysts, if our understanding of the importance of the transition state to catalysis is correct. The preparation of an antibody that catalyzes the insertion of a metal ion into a porphyrin nicely illustrates the validity of this approach. Ferrochelatase, the final enzyme in the biosynthetic pathway for the production of heme, catalyzes the insertion of Fe2+ into protoporphyrin IX. The nearly planar porphyrin must be bent for iron to enter. The recently determined crystal structure of the ferrochelatase bound to a substrate analog confirms that the enzyme does indeed bend one of the pyrole rings, distorting it 36 degrees to insert the iron. 

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