Flavins hydride complexes

Unsubstituted dihydroflavin-anions might, as has been stated previously, be considered as Flavin-hydride complexes , though one might infer that the term complex is abused if used in this way. In fact there are two ways to define the term complex . Historically, this term means that one does not know how to explain the structure of a given species in terms of chemical bonds. Nowadays, however, pure chemistry attempts to comprehend practically everything, including Amor and Psyche, in terms of chemical bonds, and the term complex is confined, in pure chemistry, to a species in which the coordination number of the central atom exceeds its oxidation number. In bio-... 

The chemistry of flavins is complex, a fact that is reflected in the uncertainity that has accompanied efforts to understand mechanisms. For flavoproteins at least four mechanistic possibilities must be considered.1533 233 (a) A reasonable hydride-transfer mechanism can be written for flavoprotein dehydrogenases (Eq. 15-23). The hydride ion is donated at N-5 and a proton is accepted at N-l. The oxidation of alcohols, amines, ketones, and reduced pyridine nucleotides can all be visualized in this way. Support for such a mechanism came from study of the nonenzymatic oxidation of NADH by flavins, a reaction that occurs at moderate speed in water at room temperature. A variety of flavins and dihydropyridine derivatives have been studied, and the electronic effects observed for the reaction are compatible with the hydride ion mecha-nism.234 236... 

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 three-dimensional structure of the complex of D-amino acid oxidase with the substrate analog benzoate has been determined. The carboxyl group of the inhibitor is bound by an arginine side chain (Fig. 15-11) that probably also holds the amino acid substrate. There is no basic group nearby in the enzyme that could serve to remove the a-H atom in Eq. 15-26 but the position is appropriate for a direct transfer of the hydrogen to the flavin as a hydride ion as in Eq. 15-23.161/162/257 In spite of all arguments to the contrary the hydride ion mechanism could be correct However, an adduct mechanism is still possible. 

The catalytic effect of metal ions such as Mg2+ and Zn2+ on the reduction of carbonyl compounds has extensively been studied in connection with the involvement of metal ions in the oxidation-reduction reactions of nicotinamide coenzymes [144-149]. Acceleration effects of Mg2+ on hydride transfer from NADH model compounds to carbonyl compounds have been shown to be ascribed to the catalysis on the initial electron transfer process, which is the rate-determining step of the overall hydride transfer reactions [16,87,149]. The Mg2+ ion has also been shown to accelerate electron transfer from cis-dialkylcobalt(III) complexes to p-ben-zoquinone derivatives [150,151]. In this context, a remarkable catalytic effect of Mg2+ was also found on photoinduced electron transfer reactions from various electron donors to flavin analogs in 1984 [152], The Mg2+ (or Zn2+) ion forms complexes with a flavin analog la and 5-deazaflavins 2a-c with a 1 1 stoichiometry in dry MeCN at 298 K [153] ... 

Several of the reductases mentioned here belong to the same structural family (the FNR family), and they are mechanistically related to each other (9). A two-electron reduction of the flavin by NAD(P)H in these enzymes typically involves the transient formation of an oxidized flavin-reduced pyridine nucleotide charge-transfer complex, which is followed by hydride transfer. After reduction, the flavin can transfer its electrons to different redox partners. With NADH cytochrome b5 reductase, this transfer occurs in separate single-electron transfer steps. With... 

In the oxidation of DHO, the C5 pro-S hydrogen is removed as a proton by an active site base, while the C6 hydrogen is transferred to N5 of the isoalloxazine ring of the flavin as a hydride. Structures of product complexes of all DHODs show that C6 of OA is in van der Waals contact with N5 of the flavin and that C5 of OA is positioned correctly for proton abstraction by the active site base (Figure 4), either serine in Class 2... 

A useful feature of flavins is that their absorption spectra are altered by changes in their reduction state. Thus, the reduction state can be examined by measuring changes in the visible absorbance range (Figure 4.2). This unique property stimulated research into the redox properties , of the enzyme and the complex processes of hydride/ electron transfer from NADPH, across the flavins and on to P450, discussed later in this chapter. 

NADH directly transfers a hydride ion to flavins and analogs (e.g., deazaflavin), indicating hydrogen transfer within a molecular complex (Scheme 7.2.27). Hydrophobic NADH derivatives with an ammonium side chain have been bound to a PEG substituent of a flavin and are then oxidized much faster to NAD than without complex formation. Chiral deazaflavino-... 

The basis of the catalysis of the splitting of the disulfide is presumably the formation of a charge-transfer complex between the two-electron donor NADPH (equivalent to a hydride anion) and the acceptor flavin combined with proximity effects. Both coenzymes, NADPH and FAD, are bound to the protein by adenosine phosphate-protein interactions, the substrate is loosely bound at the cleft between the units of a protein dimer (Fig. 9.6.12) (Schulz, 1983 Douglas, 1987). 

Fig. 2.6 A cartoon representation of a model for POR-P450 complex formation in the endoplasmie reticulum (ER) membrane. Flavin mononucleotide (FMN) domain, flavin adenine dinueleotide FAD) domain, and P450s are shown in blue, yellow, and red balls, respeetively. (1) Multiple P450s exist in the ER membrane. Nucleotide binding favors formation of the elosed form, similar to the one found in the erystal strueture [36]. (2) Upon binding to pyridine nueleotide (NADPH), the enzyme adopts the elosed form. In the elosed form, hydride transfer, inter-...

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