Proteins that function as electron transferases

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Proteins that function as electron transferases typically place their prosthetic groups in a hydrophobic environment and may provide hydrogen bonds (in ad- dition to ligands) to assist in stabilizing both the oxidized and the reduced forms of the cofactor. Metal-ligand bonds remain intact upon electron transfer to min- imize inner-sphere reorganization 4 (discussed in Section III). Many of the com- plex multisite metalloenzymes (e.g., cytochrome c oxidase, xanthine oxidase, the nitrogenase FeMo protein) contain redox centers that function as intramole- cular electron transferases, shuttling electrons to/from other metal centers that bind exogenous ligands during enzymatic turnover. There are four classes 3 ,5 of electron transferases, each of which contains many members that exhibit important structural differences: flavodoxins, blue copper proteins, iron-sulfur proteins, and cytochromes. The flavodoxins 6 are atypical in that they contain an organic redox cofactor, flavin mononucleotide (FMN; see Figure 6.3). These proteins have molecular Figure 6.3 Reduction of FMN, weights in the 8-13 kDa range, and are found in many species of bacteria and algae. The FMN cofactor is found at one end of the protein, near the molecular surface, but only the dimethylbenzene portion of FMN is significantly exposed to the solvent (Figure 6.4). FMN can act as either a 1- or a 2-electron redox center. In solution, the semiquinone form of free FMN is unstable, and dispro- portionates to the quinone (oxidized) and hydroquinone (reduced) forms. Hence, free FMN functions in effect as a 2-electron reagent. FMN in flavodoxins, on the other hand, can function as a single-electron carrier. This is easily discerned
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318 Figure 6.4 Stereo view of the structure of a clostridial f1avodoxin. See R. D. Andersen et al., Proc. Natl. Acad. Sci. USA 69 (1972),3189-3191. Figure kindly provided by M. L. Ludwig. by comparing reduction potentials for free and protein-bound FMN (Table 6.1). Clearly, the protein medium is responsible for this drastic alteration in oxida- tion-state stability. From an NMR study 7 of the M. elsdenii flavodoxin quinone/ semiquinone and semiquinone/hydroquinone electron self-exchange rates, it was concluded that the latter is approximately 300 times faster than the former, in keeping with the view that the physiologically relevant redox couple is semiqui- none/hydroquinone. The blue copper proteins are characterized by intense S(Cys) ....... ,. Cu charge- transfer absorption near 600 nm, an axial EPR spectrum displaying an unusually small hyperfine coupling constant, and a relatively high reduction potentia1. 4 ,8-10 With few exceptions (e.g., photosynthetic organisms), their precise roles in bac- terial and plant physiology remain obscure. X-ray structures of several blue copper proteins indicate that the geometry of the copper site is approximately trigonal planar, as illustrated by the Alcaligenes denitrijicans azurin structure (Figure 6.5).11,12 In all these proteins, three ligands (one Cys, two His) bind tightly to the copper in a trigonal arrangement. Differences in interactions be-
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