Dioxoisobacteriochlorin

More prevalent is cytochrome cd2 nitrite reductase.340 343 346 The water-soluble periplasmic enzyme is a homodimer of 60-kDa subunits, each containing a c-type heme in a small N-terminal domain and heme dv a ferric dioxoisobacteriochlorin (Fig. 16-6). The... 

Reaction of oxochlorin 54 with methyllithium gave alcohol 62, which was dehydrogenated to the exomethylenechlorin 63 (69LA(725)167). Oxohydro-porphyrins have found multiple interest. A knowledge of the properties of 56 may help to elucidate the structure of heme dy, which most probably is a dioxoisobacteriochlorin and not a chlorin (86JA1352). 

Heme dx isolated from Pseudomonas aeruqinosa and Paracocens denitrifi-cant was proposed not to be a chlorin but a 3,8-dioxoisobacteriochlorin similar to compound 55. This was confirmed by a comparison of the chromophore with that of a model compound (86JA1352). 

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. 

The synthesis performed by Montforts <95A(3(E)784> <89AG(E)480> <92TL765> <95TA867> <96HCA1572> leads to the enantiomer of porphyrin di ewMOl via the dioxoisobacteriochlorin intermediate enf-98. The key step in this synthesis is a stereoselective... 

Three different synthetic routes leading to heme di (107) were developed in the laboratories of Battersby (75), Chang (72, 75), and Montforts (74). Chang s partial synthesis of heme di is of less practical use due to its low selectivity and should therefore not be reviewed here. The partial synthesis performed by Montforts leads to the enantiomer of porphyrin di, the metal free porphyrin ligand of ent-101 via the dioxoisobacteriochlorin intermediate (127). 

The cytochrome cdi nitrite reductases (heme cdi NiR s) are typically dimers of 60 kDa subunits, each of which contain a single heme c and a heme d chromophore. The heme c has typical spectroscopic properties, and appears to act as an electron donor to the heme di, which appears to constitute the active site of the enzyme. The heme d has anomalous spectroscopic properties and a novel structure, a dioxoisobacteriochlorin (6) (Fig. 6). The mechanism of action of the heme cd NiR s is reasonably well established, and is shown in Figure 7. 

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