Porphyrin ring system biosynthesis

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

Branching of pathways is relevant in several cases. Thus, intermediates of the porphyrin biosynthetic pathway serve as precursors for chlorophyll (17, Fig. 2) and for the corrinoid ring systems of vitamin B12 (20, Fig. 2) (17). 1-Deoxy-D-xylulose 5-phosphate (43) serves as an intermediate for the biosynthesis of pyridoxal 5 -phosphate (39, Fig. 5), for the terpenoid precursor IPP (86) via the nonmevalonate pathway (Fig. 11), and for the thiazole moiety of thiamine pyrophosphate (46, Fig. 4). 7,8-Dihydroneopterin triphosphate (29, Fig. 3) serves as intermediate in the biosynthetic pathways of tetrahydrofolate (33) and tetrahydrobiopterin (31). The closely related compound 7,8-dihydroneopterin 2, 3 -cyclic phosphate is the precursor of the archaeal cofactor, tetrahydromethanopterin (34) (58). A common pyrimidine-type intermediate (23) serves as precursor for flavin and deazaflavin coenzymes. Various sulfur-containing coenzymes (thiamine (9), lipoic acid (7), biotin (6), Fig. 1) use a pyrosulfide protein precursor that is also used for the biosynthesis of inorganic sulfide as a precursor for iron/sulfur clusters (12). 

Biosynthesis of Porphyrins, the Prosthetic Groups of Cytochromes. The cytochromes are all proteins conjugated with porphyrins, which means that they are derivatives of the heterocyclic ring system, porphin. 

The porphyrin system is a complicated structure consisting of four pyrrole rings connected by methine groups (=CH—). It is the basis of the red blood pigment, the green leaf pigment, and of the cytochromes (redox enzymes). Efforts to determine its chemical structure occupied many decades (KQster, H. Fischer), and finally were crowned by the synthesis of the porphyrin molecule (H. Fischer). We will not follow the tortuous route of the research chemist here, but instead will construct the ring system from simple components, just as the cell does. Thus the biosynthesis will be discussed at the same time. 

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