Flavin derivatives, structures

Oxidation of P-nicotinamide adenine dinucleotide (NADH) to NAD+ has attracted much interest from the viewpoint of its role in biosensors reactions. It has been reported that several quinone derivatives and polymerized redox dyes, such as phenoxazine and phenothiazine derivatives, possess catalytic activities for the oxidation of NADH and have been used for dehydrogenase biosensors development [1, 2]. Flavins (contain in chemical structure isoalloxazine ring) are the prosthetic groups responsible for NAD+/NADH conversion in the active sites of some dehydrogenase enzymes. Upon the electropolymerization of flavin derivatives, the effective catalysts of NAD+/NADH regeneration, which mimic the NADH-dehydrogenase activity, would be synthesized [3]. 

Phosphoric acid molecules can form acid-anhydride bonds with each other. It is therefore possible for two nucleotides to be linked via the phosphate residues. This gives rise to dinucleotides with a phosphoric acid-anhydride structure. This group includes the coenzymes NAD(P) " and CoA, as well as the flavin derivative FAD (1 see p. 104). 

Chemical reduction of activated flavin by DTT or NADH as donors of protons, apparently does not result in the modifications of the structure of the flavin derivate molecule. When using a chemically reduced substrate we have found that the affinity of the aldehyde for luciferase differed insignificantly from the native form. Thus it is... 

Flavin-dependent oxynitrilase has many properties common to flavoprotein oxidases. The enzyme binds sulfite in an N5 adduct, and one-electron reduction produces anionic semiquinone.The sulfite adduct and one- or two-electron reduced enzyme are inactive, as is apoenzyme, suggesting that the flavin is involved in catalysis. When the flavin is replaced by 5-carba-5-deaza FAD, a low level of activity is retained. " " However, when the enzyme containing the artificial flavin is exposed to H2O2, the isoalloxazine ring system is partially degraded to a redox-inactive heterocyclic system. " After the formation of this redox-inactive flavin derivative, enzyme activity actually increases dramatically, suggesting that the flavin plays a structural role. It is possible that the redox state of the flavin serves a regulatory role, since enzyme activity decreases when the natural flavin is reduced however, this has yet to be proven. 

O. Tillberg L. Torbjornsson, X-Ray Structure Investigation of Flavin Derivatives. In Elavins and Elavoproteins] H. Kamin, Ed. University Park Press Baltimore, 1971 pp 1-22. 

NAD), which are adsorbed or form ad-layers at the surface, have been extensively studied. Information concerning both the structure of the molecule itself, the orientation of molecules on the surface and the potential-dependent behavior has been provided. In the second group, molecules such as heme proteins and protoporphyrin IX, photosynthetic membranes and components, porphyrins, flavoenzymes and flavin derivatives, are used. The enhancement factor for these molecules on SERS substrates can reach up to 10. Thus, the detection limit for these molecules can be nanomolar to picomolar, and this also... 

Several flavin derivatives have also been studied as molecular complexes. The structure of the complex formed between lumiflavinium bromide (52) ... 

Flavin Coenzymes, Although many flavin derivatives have been suspected of functioning in oxidizing enzymic systems as prosthetic groups, only two—riboflavin 5-phosphate (flavin mononucleotide, FMN) and flavinade-nine dinucleotide—have been definitely established in enzymic systems. Riboflavin 5 -phosphate (FMN) was identified by Warburg and Christian m ) as a constituent of the old yellow enzyme and its structure elucidated by several workers in different laboratories. Riboflavin, also known as vitamin B2 or lactoflavin, has been synthesized by the following procedure which establishes its structure (147) ... 

The activation of molecular oxygen for oxygenation reactions can also be achieved within enzymes by flavin coenzymes such as FAD 21 and flavin mononucleotide (FMN) 22 (Figure 8.9). Flavins, derived from riboflavin (vitamin Bj) possess a characteristic tricyclic isoalloxazine structure which, when reduced to FADH 23, usually by NAD(P)H, can react with molecular oxygen to form... 

Amine oxidases catalyze the oxidative deamination of both xenobiotic and biogenic amines, and thus have many critical biological functions. Two distinct classes differ in the nature of their prosthetic groups [1]. The flavin-(FAD flavin adenine dinucleotide)-dependent amine oxidases include monoamine oxidases (MAO A and B) and polyamine oxidases. Amine oxidases not containing FAD, the so-called semicarbazide-sensitive amine oxidases (SSAO), include both plasma amine oxidases and tissue amine oxidases. These contain quinonoid structures as redox cofactors that are derived from posttranslationally modified tyrosine or tryptophan side chains, topaoquinone frequently playing this role [2]. 

Aromatic compounds arise in several ways. The major mute utilized by autotrophic organisms for synthesis of the aromatic amino acids, quinones, and tocopherols is the shikimate pathway. As outlined here, it starts with the glycolysis intermediate phosphoenolpyruvate (PEP) and erythrose 4-phosphate, a metabolite from the pentose phosphate pathway. Phenylalanine, tyrosine, and tryptophan are not only used for protein synthesis but are converted into a broad range of hormones, chromophores, alkaloids, and structural materials. In plants phenylalanine is deaminated to cinnamate which yields hundreds of secondary products. In another pathway ribose 5-phosphate is converted to pyrimidine and purine nucleotides and also to flavins, folates, molybdopterin, and many other pterin derivatives. 

Structures of the vitamin riboflavin (a) and the derived flavin coenzymes (b). Like NAD+ and NADP+, the coenzyme pair FMN and FAD are functionally equivalent coenzymes, and the coenzyme involved with a given enzyme appears to be a matter of enzymatic binding specificity. The catalytically functional portion of the coenzymes is shown in red. 

However, recent x-ray studies on p-hydroxybenzoate-p-hydroxybenzoate hydroxylase binary complex crystals clearly show the aromatic substrate is bound at the flavin 4a-5 edge and orthogonal to the isoalloxazine plane (29). Unless this binary complex structure is highly misinformative, it can be inferred that in the 02, p-hydroxy-benzoate, enzyme ternary active complex, oxygen transfer is in the 4a,5 region, not the la, 1 region of the bound FAD, which rules out la-OOH derivatives as important oxygenating intermediates for this enzyme. 

Table III. Correlation of Structure and Redox Activity for Flavin and Its N ( 5 )-modified Derivatives...

Trivalent hydrogen-bonding systems have been used very extensively for guiding and influencing the structuring of polymers. As mentioned in Sect. 1, Fig. 5, the most important triple-hydrogen bonds derive from 2,6-diamino-pyridines, 2,6-diamino-1,3,5-triazines and their complexes with flavine- and... 

Early crystallographic studies of TMADH provided data from two derivatives at 6 resolution that revealed the domain structure and certain elements of secondary structure (Lim et al., 1982 Lim et al., 1984). Higher resolution data at 2.4 resolution have been collected and the structure solved by the multiple isomorphous replacement method with anomolous scattering (Lim et al., 1986). Analysis of the diffraction pattern lead to the identification of ADP as the third cofactor in TMADH. At the time the 2.4 data set was analysed, there was no sequence information available for TMADH (Lim et al., 1986), except for a 12 residue peptide which contained the covalently bound flavin (Kenney et al., 1978). Gas-phase sequencing of isolated peptides initially provided 80% of the primary sequence of... 

Reduction of the heme iron of cytochromes P450 to the ferrous state 3 is necessary for the binding and subsequent activation of atmospheric dioxygen. Initially, two electrons are derived from NAD(P)H by flavin adenine dinucleotide (FAD)-containing proteins and then are used sequentially via one-electron transfers. AU cytochromes P450 can be divided into two main classes with respect to the reduction mechanism and the structure of their immediate redox partner. The first class includes most soluble... 

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