Flavins cofactors

To summarize, squalene epoxidase is a flavoprotein capable of catalyzing the insertion of oxygen into the 2,3-double bond of squalene to give 2,3-oxidosqualene, with the second oxygen atom from 02 being reduced to water. The reducing equivalents necessary for this transformation are relayed from NADPH through NADPH-cytochrome c reductase to the flavin cofactor of the epoxidase. 

All characterized BVMOs contain a flavin cofactor that is crucial for catalysis while NADH or NADPH is needed as electron donor. An interesting observation is the fact that most reported BVMOs are soluble proteins. This is in contrast to many other monooxygenase systems that often are found to be membrane-bound or membrane-associated. In 1997, Willetts concluded from careful inspection of... 

Alcohol dehydrogenases found in certain microorganisms utilize a pyrroloquino-line quinone (PQQ) or flavin cofactor to pass electrons released upon oxidation of alcohols to the heme electron-acceptor protein, cytochrome c. These membrane-associated alcohol dehydrogenases form part of a respiratory chain, and the energy from fuel oxidation therefore contributes to generation of a proton gradient across... 

Incubation Conditions. The incubation mixtures consisted of the cell preparation, with or without a flavin cofactor (FMN,... 

Degradation of DDT, dieldrin and lindane by the flavoprotein preparation was almost more efficient in the absence than in the presence of FMN (e.g. Figure 3). On the contrary, photodegradation of mexacarbate was greatly enhanced by FMN and other flavin cofactors. It is well known that flavin cofactors, such as FMN, are active photosensitizers. Hence it is possible that the mechanisms or pathways involved for the photodegradation of DDT, dieldrin and lindane and that for mexacarbate are different... 

The report of Basran et al. (entry 5 of Table 2) contains two studies involving hydride transfer with nicotinamide cofactors. In morphinone-reductase catalyzed reduction by NADH of the flavin cofactor FMN (schematic mechanism in Fig. 5), the primary isotope effects are modest (around 4 for H/D), but exhibit a small value of Ajj/Aq (0.13) and an exalted isotopic difference in energies of activation (8.2kJ/mol) that alone would have generated an isotope effect around 30. The enthalpies of activation are in the range of 35-45 kJ/mol. This is behavior typical of Bell tunneling as discussed above. It can also be reproduced by more complex models, as will be discussed in later parts of this review. 

The small subunit is composed of two domains. The N-terminal domain shows the characteristic architecture of flavodoxin with the phosphate moiety of the flavin cofactor occupying the binding pocket of the proximal [4Fe-4S] cluster. This N-terminal domain, including the proximal cluster, is found in all [NiFe] hydrogenases and is consequently an essential feature, both structural and functional, of these enzymes. By contrast, the C-terminal domain that binds the other [FeS] clusters is less organised and more variable in [FeS] cluster content and amino acid sequence. 

In a further demonstration of the scope of the four-helix bundle maquette further complexity was added by the addition of both the flavin cofactor and heme groups [75]. Photoreduction of the hemes was successfully demon-... 

Another approach to preparing enantiomerically pure carboxylic acids and related compounds is via enanhoselective reduction of conjugated double bonds using NAD(P)H-dependent enoate reductases (EREDs EC 1.3.1.X), members of the so-called Old Yellow Enzyme family [44]. EREDs are ubiquitous in nature and their catalytic mechanism is well documented [45]. They contain a catalytic flavin cofactor and a stoichiometric nicotinamide cofactor which must be regenerated (Scheme 6.23). 

Figure 7. Degradation pathway of mexacarbate through the flavin cofactor-fer-redoxin system...

Table 3. Degradation of C-mexacarbate by ferredoxin in the presence and absence of a flavin cofactor ...

From detailed X-ray analysis and ab initio molecular orbital calculations, it has been suggested that the planar electron-rich aryl-oxazolidinone moiety interacts reversibly via the Ti-system with the flavin cofactor of the enzyme, which is also planar and known to be an electron acceptor [91]. Additionally, the side chains of the inhibitor, including the trifluoromethyl group, are engaged in interactions with the peptide core of the enzyme [87]. 

Two classes of monooxygenases are known. Those requiring a cosubstrate (BH2 of Eq. 18-36) in addition to the substrate to be hydroxylated are known as external monooxygenases. In the other group, the internal monooxygenases, some portion of the substrate being hydroxylated also serves as the cosubstrate. Many internal monooxygenases contain flavin cofactors and are devoid of metal ions. 

Metalloenzymes or metal ion-activated enzymes catalyze an enormous variety of organic reactions that are not restricted to any particular reaction class, but appear as catalysts for all types of reactions. Thus neither the presence of the metal ion nor the reaction type seems to be restrictive as far as metal-assisted enzyme catalysis is concerned. In some cases the metal ion appears to function as an electron acceptor or donor, but flavin cofactors have substituted as redox centers during evolution in some enzymes. 

A model of a flavin-based redox enzyme was prepared.[15] Redox enzymes are often flavoproteins containing flavin cofactors flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). They mediate one- or two-electron redox processes at potentials which vary in a range of more than 500 mV. The redox properties of the flavin part must be therefore tuned by the apoenzyme to ensure the specific function of the enzyme. Influence by hydrogen bonding, aromatic stacking, dipole interactions and steric effects have been so far observed in biological systems, but coordination to metal site has never been found before. Nevertheless, the importance of such interactions for functions and structure of other biological molecules make this a conceivable scenario. 

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