The Tetrapyrrolic Cofactors

By the same token, however, these processes are also complicated in reaction centers because they are heavily loaded with cofactors, which are likely to exert a considerable influence on the reaction center structure. The effects of the tetrapyrrolic cofactors will be even more pronounced in antenna complexes, where the pigments constitute an even larger proportion. A satisfactory modeling of the organisation principle of the reaction center is therefore expected to give valuable insight, too, into the construction principles of antennae. 

As we will see in subsequent chapters, many metalloproteins have their metal centres located in organic cofactors (Lippard and Berg, 1994), such as the tetrapyrrole porphyrins and corrins, or in metal clusters, such as the Fe-S clusters in Fe-S proteins or the FeMo-cofactor of nitrogenase. Here we discuss briefly how metals are incorporated into porphyrins and corrins to form haem and other metallated tetrapyrroles, how Fe-S clusters are synthesized and how copper is inserted into superoxide dismutase. 

A view of the core of the reaction center of Rh. viridis69 is shown in Figure 2.36. It consists of three tetrapyrrolic cofactors the so-called special pair (SP), which is a dimer of bacteriochlorophylls, a monomeric bacteriochloro-phyll (BCh), and a bacteriopheophytin (BPh). As noted above, all these chro-mophores are arranged within the protein structure with oblique orientations to one another. In this bacterial triad, SP functions as the electron donor in... 

The tetrapyrroles are a group of natural products which include the haems (e.g. haem hl), the chlorophylls (e.g. chlorophyll a 2) and the corrinoids (e.g. coenzyme Bi2 4), see Scheme 1 [1 -6]. In addition to these well-known and widespread enzymic cofactors, other tetrapyrroles are used in more restricted cases. 

Fig. 10 presents the values of distances between the cofactors in the reaction centers of Rb. sphaeroides and Rp. viridis obtained in a structural determination at 2.65 A resolution as reported by Ermler, Fritzsch, Buchanan and Michel Some of these results will now be described. The pyrrole rings I of and Db are 3.2 A apart, but the ring centers are 1 A apart and thus the tetrapyrrole rings are inclined to each other by -15°. The Mg atoms of and Db are each ligated to L-His 173 and M-His200, respectively, as marked by upward arrows in Fig. 3 (A). The acetyl group on ring I of Da forms a hydrogen bond with...

For example, incorporation of nickel into carbon monoxide dehydrogenase of Rhodospirillum rubrum requires the prior reduction of an Fe-S cluster. Structural studies of this protein reveal that the added Ni completes a unique [lNi-4Fe-4S] center that is required for activity.Another example of a reductive activation step occurs during NiFe-hydrogenase biosynthesis, perhaps involving participation of the Fe-S cluster in HypD. Yet a third example from the Ni-enzyme literature involves the synthesis of methyl-X-coenzyme M reductase, a methanogen enzyme that contains the Ni-tetrapyrrole cofactor F43q. Formation of active enzyme requires both the reduction of Ni + to NF+ and reduction of a C=N bond in the organic macrocycle. 

Cobalamins are essential enzymatic cofactors in human biochemistry. Coba-lamins chemical structure is based on the tetrapyrrole ring, while the chemical properties of the Co bond located in the centre of their moiety have been the focus of extensive research. Cyanocobalamin, the most known form of cobalamins, is rarely found in nature. Methylcobalamin and adenosylcobalamin are the two active forms of cobalamins in vivo in humans. Weakening of the Co C bond and its homolytic or heterolytic cleavage have been uncovered as an essential mechanism in the biochemical role of cobalamins as cofactors in humans. Recent studies using modem computational methods and application of quantum chemistry models have widened our knowledge of cobalamins biochemistry and are expected to contribute to our further understanding of cobalamin-dependent enzymes. 

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