Protoporphyrin biosynthesis

L. Eriksen, N. Eriksen (1976). Possible pathways in protoporphyrin biosynthesis. In M. Doss (Ed.), Porphyrins in human diseases (pp.105-110). Karger, Basel. 

Taken together, these data suggest that restricting protoporphyrin biosynthesis not only modulated Chi accumulation, but also altered the Chi microenvironment -perhaps by an altered complement of Chl-bmding proteins. This possibility prompted our examination of thylakoid composition by Western blot analysis. 

The porphyrinogens described above are colorless, containing six extra hydrogen atoms as compared with the corresponding colored porphyrins. These reduced porphyrins (the porphyrinogens) and not the corresponding porphyrins are the actual intermediates in the biosynthesis of protoporphyrin and of heme. 

FIGURE 2.1.4 Biosynthesis steps of porphyrins from ALA to protoporphyrin IX. 

FIGURE 2.1.5 Biosynthesis steps of porphyrins from protoporphyrin IX to chlorophyll a. 

Although oxygen was found to be the only oxidant for conversion of coproporphyrinogen III to protoporphyrin IX, anaerobic systems must obviously exist for the biosynthesis of the latter molecule (43). Porphine itself has not been found in nature but spectral lines identical to those of bis-pyridylmagnesiumtetrabcnzoporphine have been detected in interstellar space (53). 

In tracing the evolutionary development of iron ligands it is of interest to examine the machinery employed by organisms which carry out reactions on those substances believed to have been present on the primitive Earth. Specific substrates acted on by this group include, besides ferrous iron itself, hydrogen sulfide, hydrogen gas, methane and reduced nitrogen compounds. Species which perform photosynthesis may be presumed to have the capacity to synthesize protoporphyrin IX since this substance is an intermediate in chlorophyll biosynthesis (43). 

Figure 7.1 The overall pathway of haem biosynthesis. 5-AminolaevuIinate (ALA) is synthesized in the mitochondrion, and is transferred to the cytosol where it is converted to porphobilinogen, four molecules of which condense to form a porphyrin ring. The next three steps involve oxidation of the pyrrole ring substituents to give protoporphyrinogen fX, whose formation is accompanied by its transport back into the mitochondrion. After oxidation to protoporphyrin IX, ferrochelatase inserts Fe2+ to yield haem. A, P, M and V represent, respectively acetyl, propionyl, methyl and vinyl (—CH2=CH2) groups. From Voet and Voet, 1995. Reproduced by permission of John Wiley Sons, Inc.

IX is accompanied by its transport back into the mitochondria whence it came, to undergo oxidation of its methylene groups to protoporphyrin IX and insertion of iron to yield the end product, haem. The two major sites of haem biosynthesis are erythroid cells, which synthesize around 85 % of the body s haem groups, and the liver, which synthesizes most of the remainder. A major function of haem in liver is as the prosthetic group of cytochrome P450, the importance of which in detoxification has been discussed in Chapter 2. The liver cell must synthesize cytochrome P450 throughout its lifetime in quantities that vary with conditions. In contrast, the... 

Zinc protoporphyrin IX is a normal metabolite that is formed in trace amounts during haem biosynthesis. However, in iron deficiency or in impaired iron utilization, zinc becomes an alternative substrate for ferrochelatase and elevated levels of zinc protoporphyrin IX, which has a known low affinity for oxygen, are formed. This zinc-for-iron substitution is one of the first biochemical responses to iron depletion, and erythrocyte zinc protoporphyrin is therefore a very sensitive index of bone-marrow iron status (Labbe et ah, 1999). In addition, zinc protoporphyrin may regulate haem catabolism by acting as a competitive inhibitor of haem oxygenase, the key enzyme of the haem degradation pathway. However, it has been reported... 

The incorporation of iron(II) into the protoporphyrin IX ring, the final step in heme biosynthesis, is catalyzed by ferrochelatase. Kinetic parameters were reported both for this process and for the reaction of Fe " with ferrochelatase, whose kinetics were used to characterize the latter. ... 

Other photosensitisers in clinical or pre-clinical trials include zinc phthalocya-nine, aluminium sulphonated phthalocyanines, benzoporphyrins, benzochlorins and purpurin-lS-iV-alkylamides, all of which absorb strongly in the 675-700 nm region. An alternative approach to the photosensitisation in PDT involves the use of 5-aminolaevulinic acid (ALA). This compound itself is not a sensitiser but in human cells it is the key metabolic precursor in the biosynthesis of protoporphyrin IX, which can act as a photosensitiser. Normally the biosynthetic process would continue beyond protoporphyrin IX to the iron containing haem. However, by adding extra ALA and iron chelators, the ferrochelatase action is inhibited and the normal feedback mechanism by-passed resulting in a build up of protoporphyrin IX in the cell. The mechanism is illustrated in Figure 4.24. ... 

Porphyrinogens Porphyrin precursors exist in the chemically reduced form called porphyrinogens. In contrast to the porphyrins, which are colored, the porphyrinogens, such as uroporphyrinogen, are colorless. As described in the next section, porphyrinogens serve as intermediates between porphobilinogen and protoporphyrin in the biosynthesis of heme. 

The insertion of ferrous iron into the porphyrin ring in the biosynthesis of heme is catalyzed by the enzyme ferrochelatase. A deficiency in ferrochelatase activity results in an accumulation or the excretion of unchelated protoporphyrin in patients with erythrohepatic protoporphyria. Ferrochelatase catalyzes the synthesis of a range of metalloporphyrins,628 and, for example, produces zinc protoporphyrin in erythrocytes of patients with iron-deficiency anaemia. 

In 1992 the X-ray crystal structure of porphobilinogen deaminase was solved, an enzyme that is involved with the biosynthesis of a linear tetrapyrrole precursor to protoporphyrin IX, found in haemoglobin... 

In iron deficiency anemia, Zn2+ may be used instead of iron by ferro-chelatase in the biosynthesis of heme. Red blood cell lysates in such instances contain increased quantities of Zn-hemoglobin. In addition, red cells from iron-deficient patients also contain increased amounts of protoporphyrin IX. Zinc-hemoglobin and protoporphyrin IX determinations are thus used in the diagnosis of iron deficiency anemia. 

Prostaglandin (PG) H Synthase. The enzyme PGH synthase is a homodimeric protein consisting of subunits with an approximate molecular weight of 72 kDa and one Fe(III)-protoporphyrin IX (PPIXFe(III)) prosthetic group per subunit. This protein is responsible for the central reaction in the biosynthesis of prostaglandins and is selectively inhibited by antiinflammatory drugs such as aspirin and indo-... 

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