Flavin mononucleotide redox reactions

It is well known that the selective transport of ions through a mitochondrial inner membrane is attained when the oxygen supplied by the respiration oxidizes glycolysis products in mitochondria with the aid of such substances as flavin mononucleotide (FMN), fi-nicotinamide adenine dinucleotide (NADH), and quinone (Q) derivatives [1-3]. The energy that enables ion transport has been attributed to that supplied by electron transport through the membrane due to a redox reaction occurring at the aqueous-membrane interface accompanied by respiration [1-5],... 

III. SELECTIVE ION TRANSFER AT THE W/0 INTERFACE COUPLED WITH REDOX REACTIONS BETWEEN FLAVIN MONONUCLEOTIDE IN W AND A FERROCENE DERIVATIVE IN O AND CO2 EVOLUTION [19,21]... 

Riboflavin (vitamin Bj) is chemically specified as a 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine (Eignre 19.22). It is a precnrsor of certain essential coenzymes, such as flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) in these forms vitamin Bj is involved in redox reactions, such as hydroxylations, oxidative carboxylations, dioxygenations, and the reduction of oxygen to hydrogen peroxide. It is also involved in the biosynthesis of niacin-containing coenzymes from tryptophan. 

Riboflavin (vitamin B2 6.18) consists of an isoalloxazine ring linked to an alcohol derived from ribose. The ribose side chain of riboflavin can be modified by the formation of a phosphoester (forming flavin mononucleotide, FMN, 6.19). FMN can be joined to adenine monophosphate to form flavin adenine dinucleotide (FAD, 6.20). FMN and FAD act as co-enzymes by accepting or donating two hydrogen atoms and thus are involved in redox reactions. Flavoprotein enzymes are involved in many metabolic pathways. Riboflavin is a yellow-green fluorescent compound and, in addition to its role as a vitamin, it is responsible for the colour of milk serum (Chapter 11). 

In the following, the outline of the redox reaction between O2 in W and decamethyl-ferrocene in O catalysed by flavin mononucleotide (FMN) in W evolving CO2, which was elucidated by applying VCTIES, will be introduced [12,13]. The selective ion transfer controlled by the redox reaction will also be discussed. The respiration mimetic reactions such as the redox reactions between -nicotinamide adenine dinucleotide and ascorbic acid or oxygen in W and quinone derivatives or hydroquinone derivatives, respectively, were also investigated with the aid of VCTIES [14,15], though they are not introduced here. 

Flavoenzymes are widespread in nature and are involved in many different chemical reactions. Flavoenzymes contain a flavin mononucleotide (FMN) or more often a flavin adenine dinucleotide (FAD) as redox-active prosthetic group. Both cofactors are synthesized from riboflavin (vitamin B2) by microorganisms and plants. Most flavoenzymes bind the flavin cofactor in a noncovalent mode (1). In about 10% of aU flavoenzymes, the isoalloxazine ring of the flavin is covalently linked to the polypeptide chain (2, 3). Covalent binding increases the redox potential of the flavin and its oxidation power, but it may also be beneficial for protein stability, especially in flavin-deficient environments. 

Riboflavin, also known as vitamin B2, is an essential component of FAD and flavin mononucleotide (FMN)—coenzymes tliat are involved in many redox reactions. 

Whereas redox reactions on metal centres usually only involve electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer, but hydrogen transfer as well — hence the frequently used denomination dehydrogenase . Note that most of these dehydrogenase reactions are reversible. Redox reactions in biosynthetic pathways usually use NADPH as their source of electrons. In addition to NAD and NADP+, which intervene in redox reactions involving oxygen functions, other cofactors like riboflavin (in the form of flavin mononucleotide, FMN, and flavin adenine dinucleotide, FAD) (Figure 5.3) participate in the conversion of [—CH2—CH2— to —CH=CH—], as well as in electron transfer chains. In addition, a number of other redox factors are found, e.g., lipoate in a-ketoacid dehydrogenases, and ubiquinone and its derivatives, in electron transfer chains. 

Vitamin B2 forms the basis of two coenzymes, flavin mononucleotide (FMN) (Fig. lc). and flavin adenine dinucleotide (FAD) (Fig. Id). Both coenzymes are involved in redox reactions. The general scheme is ... 

Flavins (El) catalyze many different bioreactions of physiological importance [7-9]. Riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) have the 7,8-dimethyl isoalloxazine ring in common but differ in the side chain attached to NIO. With their five redox states, fully oxidized, one-electron reduced semiquinoid (F1H and F1 ), and fully reduced hydroquinone (FIH2 and F1H ), flavins are involved in one-electron and two-electron transfer reactions [10]. 

Flavin molecule by its redox properties plays an important role in energy providing reactions. Flavin occurs as riboflavin or as a nucleotide in flavin mononucleotide (FMN) and combined to adenine nucleotide in flavin adenine dinucleotide (FAD) Very recently it was shown by Spiro et al. that free fluorescence SERRS spectra from flavoproteins adsorbed at silver colloids (average size of 7.5 nm) can be obtained... 

Flavine mononucleotide (FMN) forms a part of the first complex of the respiratory chain of mitochondria, and its effect on the transmembrane potential value during redox reactions in the system NADH-Q6-O2 was shown by Ismailov et al. taking as an example bilayer lipid membranes.54 55 57,75... 

Figure 31. An electron transport system and redox potentials in mitochondria. FMN refers to Flavin mononucleotide in NADH2 dehydrogenase, FAD refers to Flavin adenine dinucleotide in succinate dehydrogenase, I, II, and III correspond to the reaction processes which may be involved in phosphorylation, Fe—S non-heme iron, Cyt Heme in cytochromes (after ref. 171).

Instead, biochemical redox reactions involving the oxidation of alkane to alkene require the participation of a coenzyme such as flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). The reduced forms FADH2 and FMNHj act as hydride donors in the alkene hydrogenation reactions (Figure 1.41). 

Riboflavin and other flavinoids are found in dairy produce and meat and to a lesser extent in cereals. The RDA is 1.6-2.0 mg. Flavins are stable to heat and acid but destroyed by exposure to light. UV irradiation of riboflavin in acid or neutral solution gives rise to the fluorescent compound lumichrome, whereas in alkaline solutions irradiation produces lumiflavin. Flavins are required in the body as their coenzymes flavin mononucleotide and flavin adenine dinucleotide, which are involved in redox reactions involving one- and two-electron transfers and linked to many energy-dependent processes in the body. 

Many proteins and another coenzyme called flavin mononucleotide participate in this process, which involves five redox reactions. These redox reactions generate many resonance-stabilized free radicals. 

Riboflavin (vitamin B ) plays an essential role in cellular metabolism, being the precursor of the co-enzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) that both act as hydrogen carriers in biological redox reactions involving enzymes such as NADH dehydrogenase. Once riboflavin is absorbed in the human body, the synthesis of these flavin co-enzymes is controlled by thyroid hormones that regulate the activities of the flavin biosynthetic enzymes (Rucker et al. 2001). 

Flavins are a group of natural enzyme cofactors with interesting redox and photochemical properties that can participate in a wide set of reactions [12]. The most common flavin cofactors are flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). Enzymes harboring one of these cofactors are called flavoenzymes. A large number of flavoenzymes have been extensively studied for their structural and mechanistic properties, and they are gaining momentum in industrial biocata-lytic applications [13,14], Flavoenzymes have evolved to become powerful oxidative biocatalysts they can catalyze not only simple alcohol oxidations but they were also foimd to be efficient in catalyzing, for example, oxidative C—C bond formation and enantioselective sulfoxidations. 

So what does riboflavin do As such riboflavin does nothing. Like thiamine, riboflavin must undergo metabolic change to become effective as a coenzyme. It fact, it undergoes two reactions. The first converts riboflavin to riboflavin-5-phosphate (commonly known as flavin adenine mononucleotide, FMN), about which we will say no more, and the second converts it to flavin adenine dinucleotide, FAD. The flavins are a class of redox agents of very general importance in biochemistry. FAD is the oxidized form and FADH2 is the reduced form. ... 

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