Protoporphyrin III

Fig. 4. Final Stages in the synthesis of protoporphyrin III (IX) In liver mitochondria, and its conversion into heme. The bracketed numbers and Greek letters correspond to the Fischer system. The unbracketed numbers represent the 1-24 porphyrin numbering system.

Heme and its immediate precursor, protoporphyrin IX (Figure 32-4), are both type III porphyrins (ie, the methyl groups are asymmetrically distributed, as in type III coproporphyrin). However, they are sometimes identified as belonging to series IX, because they were designated ninth in a series of isomers posmlated by Hans Fischer, the pioneer worker in the field of porphyrin chemistry. 

Maldotti (96) studied the kinetics of the formation of the pyrazine-bridged Fe(II) porphyrin shish-kebab polymer by means of flash kinetic experiments. Upon irradiation of a deaerated alkaline water/ethanol solution of Fe(III) protoporphyrin IX and pyrazine with a short intense flash of light, the 2 1 Fe(II) porphyrin (pyrazine)2 complex is formed, but it immediately polymerizes with second-order kinetics. This can be monitored in the UV-Vis absorption spectrum, with the disappearance of a band at 550 nm together with the emergence of a new band due to the polymer at 800 nm. The process is accelerated by the addition of LiCl, which augments hydrophobic interactions, and is diminished by the presence of a surfactant. A shish-kebab polymer is also formed upon photoreduction of Fe(III) porphyrins in presence of piperazine or 4,4 -bipyridine ligands (97). 

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). 

HRP C contains two different types of metal center (i.e., iron(III) protoporphyrin IX-heme group and two calcium atoms) that are fundamental for the integrity of the enzyme. The heme group is attached to the enzyme at His 170 by a coordinate bond between the histidine side-chain NE2 atom and the heme iron atom. The second axial coordination site is unoccupied in the resting state of the enzyme but available to hydrogen peroxide during enzyme turnover. Small molecules such as carbon monoxide, cyanide, fluoride, and azide bind to the heme iron atom at this distal site, giving six-coordinated PX complexes. 

This type of double-seal gave very good protection against O2. Fe(III) protoporphyrin IX was prepared after the method of Falk (10). The Fe(III) complex was converted to Fe(II) using the method oF Brault and Rougee (11) in the N2-H2 atmosphere glove bag. 

Killday etal. (1988) also provided evidence for internal autoreduction of ferric nitrosyl heme complexes, as previously proposed by Giddings (1977). Heating of chlorohemin( iron-III) dimethyl ester in dimethyl sulfoxide solution with imidazole and NO produced a product with an infrared spectra identical to that of nitrosyl iron(ll) protoporphyrin dimethyl ester prepared by dithionite reduction. Both spectra clearly showed the characteristic nitrosyl stretch at 1663 and 1665 cm. They thus proposed a mechanism for formation of cured meat pigment which includes internal autoreduction of NOMMb via globin imidazole residues. A second mole of nitrite is proposed to bind to the heat-denatured protein, possibly at a charged histidine residue generated in the previous autoreduction step. 

III) protoporphyrin(IX) moiety, and in the resting state a water molecule is bound as the second ligand. Understandably, there have been many experimental and theoretical studies on Fe(III) P-H20 systems. However, only two computational articles were published on Fe(II)-H20. 

The mimetic biosensor for ELD described in Section III.B.4.b, incorporating polymerized Fe-protoporphyrin IX (75c) and Os(II) ions, can be used for determination of diacyl peroxides in organic solution. The electrode was tested for determination of BzOOBz extracted from a pharmaceutical gel381. 

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