Flavin coenzyme analogs

Walsh, C. Jacobson, F. Ryerson, C. C. Flavin Coenzyme Analogs As... 

Flavin Coenzyme Analogs as Probes of Flavoenzyme Reaction Mechanisms... 

We began studies with flavin coenzyme analogs in 1972 to probe what structural features in the flavin ring system were requisite for specific aspects of the enzymic catalyses noted above. In particular, evaluations of the 5-carba-5-deazaflavin and the 1-carba-l-deazaflavin system were selected, given the pivotal role of these nitrogens in the redox transformations. 

Since reduced flavins, pteridine derivatives, and PQQ can be readily oxidized by oxygen to regenerate the oxidized forms [59-62], these coenzyme analogs can act as photocatalysts when the oxidation of substrates by the coenzymes occurs photochemically. No appreciable photooxidation of benzyl alcohol by oxygen occurs when aminopterin (AP), lumazine (Lu), or riboflavin-tetraacetate (FI) is used as a photocatalyst in the absence of acid in MeCN. When HC104 is added to this system, however, the flavin and pteridine derivatives are protonated as described above, and each proton-ated species (catH+) can act as an efficient photocatalyst for the oxidation of benzyl alcohol derivatives (X-C6H4CH2OH) by oxygen [70] ... 

An initial summary of the nature and scope of redox processes involving flavin coenzymes will be presented to set the framework for evaluation of the utility of the specific coenzyme analogs. Analogs altered atN(5), atN(l), or atN(l) andN(5) by synthetic substitution of carbon will be discussed before analysis of 8-demethyl-8-hydroxy analogs either with nitrogen or carbon at the 5 position. 

Although the ribitol moiety is not involved in the redox function of the flavin coenzymes, both the stereochemistry and nature of the sugar alcohol are important. Although some riboflavin analogs have partial vitamin action. 

The UV/visible spectroscopic probe consists of protein residues (e.g. Trp, Tyr), cofactors/coenzyes (e.g. pyridoxal and flavin coenzymes) and substrates or their analogs as target chromophores. For example, pyridoxal-5 -phosphate (PLP) of pyridoxal enzymes undergo different stages of transformations that can be monitored spectroscopically (Metzler, 1979) as shown in Figure 11.7. 

Last, let us consider the possibility of a mechanism other than a hydride transfer in NAD chemistry. Indeed, G. A. Hamilton argued that if a direct hydride transfer process occurs in dehydrogenase reactions, it is unique in biology since proton transfer would be more favorable (279). However, it is not a simple task to distinguish between these two possibilities. Generally, it is simpler to say that the reduction reaction is analogous to a transfer of two electrons rather than postulating a hydride ion. More will be said on this subject in Section 7.1.3 on flavin coenzyme. 

The reduction of the hydroxylamine group involves an energy barrier. The generally sluggish reduction of an hydroxylamine group compared to reduction of its nitroso analog is reflected by the ease of reduction of many nitrosoarene compounds to the corresponding arylhydroxylamines by reduced pyridine and flavin coenzymes (1,2, 65). The lack of... 

Each of the forms of ETF isolated from the different sources contain FAD as coenzyme and form an anionic semiquinone on one-electron reduction. Stopped-flow kinetic studies on the pig liver ETF showed the anionic flavin semiquinone to be formed at times faster than catalytic turnover and thus demonstrate the participation of the anionic FAD semiquinone as an intermediate in the acceptance of reducing equivalents from the dehydrogenase. These studies would also imply the intermediacy of the semiquinone form of the acyl CoA dehydrogenase which would have been expected to form a neutral flavin semiquinone at the time the studies of Hall and Lambeth were performed, however, no spectral evidence for its formation were found. Recent studies have shown that the binding of CoA analogs to the dehydrogenase results in the perturbation of the pKa of the FAD semiquinone such that an anionic (red) rather than the neutral (blue) semiquinone is formed. This perturbation was estimated to reduce the pKa by at least 2.5 units to a value of... 

Acid catalysis is also effective for enhancing the oxidizing ability of the excited states of flavins [59], which are important coenzymes in the biological redox reactions [60-62], Flavin analogs (FI) are known to be protonated at the N - 1 position in a strongly acidic aqueous solution (pK.-ca.0K63] ... 

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] ... 

Another milestone in flavin research was the characterization of the flavo-semiquinones, the first example of a stable flavin radical. In addition to riboflavin, FMN, and FAD, a number of flavin analogs with biological activities have been found in micro-organisms and plants. Among these, the coenzyme factor F420 isolated from methanogenic bacteria should be mentioned, which has 5-deazaflavin as its chromophore. 

DR Light, C Walsh, MA Marietta. Analytical and preparative high-performance hquid chromatography separation of flavin and flavin analog coenzymes. Anal Biochem 109 87-93, 1980. 

The mechanism of synthesis of flavin adenine dinucleotide was established by Schrecker and Komberg 1S6) with a purified enzyme from brewer s yeast. The coenzyme is formed by a reaction analogous to the synthesis of diphosphopyridine nucleotide (1S7). 

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