Flavin adenine dinucleotide electron-transfer

A number of molybdenum-containing hydroxylases catalyzing the first hydrox-ylation step of N-containing compounds have been characterized thoroughly (e.g., carbazole [314], quinoline [327], and indole [350]). The enzyme s redox-active has been described as a molybdenum ion site coordinated to a distinct pyranopterin cofactor (two different [2Fe2S] centers) and in most cases, flavin adenine dinucleotide centers. This active center transfers electrons from the N-heterocyclic substrate to an electron acceptor, which for many molybdenum hydroxylases is still unknown [350],... 

Figure 9.6 Sequence of electron carriers in the electron transfer chain. The positions of entry into the chain from metabolism of glucose, glutamine, fatty acyl-CoA, glycerol 3-phosphate and others that are oxidised by the Krebs cycle are shown. The chain is usually considered to start with NADH and finish with cytochrome oxidase. FMN is flavin mononucleotide FAD is flavin adenine dinucleotide.

Riboflavin is the redox component of flavin adenine dinucleotide FAD. It is derived from FAD by hydrolysis of a phosphate ester link. The fully oxidised form of FAD is involved in many dehydrogenaze reactions during which it is converted to the fully reduced form. The fully oxidised state is restored either by another redox enzyme or by interaction with oxygen and hydrogen peroxide is liberated. The one-electron reduced, semiquinone form of FAD, is involved in some electron transfer steps. 

It is well known that the flavin adenine dinucleotide redox centers of many oxidases are electrically inaccessible due to the insulating effect of the surrounding protein thus, direct electron transfer from the reduced enzyme to a conventional electrode is negligible. In the present work, a variety of polymeric materials have been developed which can facilitate a flow of electrons from the flavin redox centers of oxidases to an electrode. Highly flexible siloxane and ethylene oxide polymers containing covalently attached redox moieties, such as ferrocene, are shown to be capable of rapidly re-oxidizing the reduced flavoenzyme. 

Amperometric biosensors based on flavin-containing enzymes have been studied for nearly 30 years. These sensors typically undergo several chemical or electrochemical steps which produce a measurable current that is related to the substrate concentration. In the initial step, the substrate converts the oxidized flavin adenine dinucleotide (FAD) center of the enzyme into its reduced form (FADH2). Because these redox centers are essentially electrically insulated within the enzyme molecule, direct electron transfer to the surface of a conventional electrode does not occur to a substantial degree. The classical" methods (1-4) of indirectly measuring the amount of reduced enzyme, and hence the amount of substrate present, rely on the natural enzymatic reaction ... 

Willner et al. [52] have created some elegant interfacial supramolecular assemblies to address this issue by removing the non-covalently bound flavin adenine dinucleotide (FAD) redox center from glucose oxidase and immobilizing the enzyme on a tether consisting of cystamine chemisorbed on a gold surface, a pyrroloquinoline quinone (PQQ) link and FAD. The mediator potential and electron transfer distances of this assembly were carefully chosen so that transfer of electrons from the FAD to the PQQ and to the electrode is very fast. A maximum rate of 900 150 s-1 for the enzymatic reaction within this monolayer assembly was obtained, which is indistinguishable from the value of about 1000 s-1 obtained for the enzyme in solution. While monolayers can offer molecular-level control of the interfacial structure, the... 

Hale et al. reported the use of an enzyme-modified carbon paste for the determination of acetylcholine [21], The sensor was constructed from a carbon paste electrode containing acetylcholineesterase and choline oxidase, and the electron transfer mediator tetrathiafulvalene. The electrode was used for the cyclic voltammetric determination of acetylcholine in 0.1 M phosphate buffer at +200 mV versus saturated calomel electrode. Tetrathiafulvalene efficiently re-oxidized the reduced flavin adenine dinucleotide centers of choline oxidase. The calibration graph was linear up to 400 pM acetylcholine, and the detection limit was 0.5 pM. 

There are many carrier molecules for electrons one is called the nicotinamide adenine dinucleotide (NAD+) and another is the flavin adenine dinucleotide FAD+. The reduced cofactors NADH and FADH2 transfer electrons to the electron transport chain. FMN receives electrons from NADH and passes them to coenzyme Q through Fe-S systems. Coenzyme Q receives electrons from FMN and FADH2 through Fe-S systems. Cytochromes receive electrons from the reduced form of coenzyme Q. Each cytochrome consists of a heme group, and the iron of the heme group is reduced when the cytochrome receives an electron Fe3+ Fe2+. At the end of the electron transfer chain, oxygen is reduced to water. 

---