FMN

FMN See flavin mononucleotide, foamed plastics See cellular plastics. 

Bacterial concentrations have also been determined by using the enzyme-catalyzed chemiluminescent reaction of reduced flavin mononucleotide (FMN) with oxygen and aldehydes. The detection limit was reported to be 10 ceUs of E. coli, which contains 7 x 10 g of FMN per ceU (303). 

In contrast to the nicotinamide nucleotide dehydrogenases, the prosthetic groups FMN and FAD are firmly associated with the proteins, and the flavin groups are usually only separated from the apoen2yme (protein) by acid treatment in water. However, in several covalently bound flavoproteins, the enzyme and flavin coen2ymes are covalently affixed. In these cases, the flavin groups are isolated after the proteolytic digestion of the flavoproteins. 

Riboflavin-5 -Phosphate. Riboflavin-5 -phosphate [146-17-8] (vitamin B2 phosphate, flavin mononucleotide, FMN, cytoflav), C2yH22N402P,... 

Flavin mononucleotide (Na, 2H2O salt, FMN) [ 130-40-5JM 514.4, pKj 2.1 (PO4H2), pK2 6.5 (PO4H ), pKj 10.3 (CONH), fluorescence Xmax 530nm (870nm for reduced form). 

Figure 4.14 Examples of different types of open twisted a/p structures. Both schematic and topological diagrams are given. In the topological diagrams, arrows denote strands of p sheet and rectangles denote a helices, (a) The FMN-binding redox protein flavodoxln. (b) The enzyme adenylate kinase, which catalyzes the reaction AMP +...

The first structure, flavodoxin (Figure 4.14a), has one such position, between strands 1 and 3. The connection from strand 1 goes to the right and that from strand 3 to the left. In the schematic diagram in Figure 4.14a we can see that the corresponding a helices are on opposite sides of the p sheet. The loops from these two p strands, 1 and 3, to their respective a helices form the major part of the binding cleft for the coenzyme FMN (flavin mononucleotide). 

ELAVOPROTEINS. Flavin is an essential substance for the activity of a number of important oxidoreductases. We discuss the chemistry of flavin and its derivatives, FMN and FAD, in the chapter on electron transport and oxidative phosphorylation (Chapter 21). 

Riboflavin, or vitamin B2, is a constituent and precursor of both riboflavin 5 -phosphate, also known as flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). The name riboflavin is a synthesis of the names for the molecule s component parts, ribitol and flavin. The structures of riboflavin. 

FIGURE 18.22 The redox states of FAD and FMN. The boxes correspond to the colors of each of these forms. The atoms primarily involved in electron transfer are indicated by red shading in the oxidized form, white in the semiqninone form, and bine in the reduced form. 

Reduction potentials can also be quite sensitive to molecular environment. The influence of environment is especially important for flavins, such as FAD/FADHg and FMN/FMNHg. These species are normally bound to their respective flavoproteins the reduction potential of bound FAD, for example, can be very different from the value shown in Table 21.1 for the free FAD-FADHg couple of —0.219 V. A problem at the end of the chapter addresses this... 

As its name implies, this complex transfers a pair of electrons from NADH to coenzyme Q a small, hydrophobic, yellow compound. Another common name for this enzyme complex is NADH dehydrogenase. The complex (with an estimated mass of 850 kD) involves more than 30 polypeptide chains, one molecule of flavin mononucleotide (FMN), and as many as seven Fe-S clusters, together containing a total of 20 to 26 iron atoms (Table 21.2). By virtue of its dependence on FMN, NADH-UQ reductase is a jlavoprotein. 

NADH-UQ reductase 850 >30 FMN Fe-S NADH (matrix side) UQ (lipid core)... 

Although the precise mechanism of the NADH-UQ reductase is not known, the first step involves binding of NADH to the enzyme on the matrix side of the inner mitochondrial membrane, and transfer of electrons from NADH to tightly bound FMN ... 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

Three protein complexes have been isolated, including the flavoprotein (FP), iron-sulfur protein (IP), and hydrophobic protein (HP). FP contains three peptides (of mass 51, 24, and 10 kD) and bound FMN and has 2 Fe-S centers (a 2Fe-2S center and a 4Fe-4S center). IP contains six peptides and at least 3 Fe-S centers. HP contains at least seven peptides and one Fe-S center. 

TOrfla ((mm H ((nn HMeiOT Hy.fieBOH nOpflflOK HO p, a (fmn — nepBbiH HOpaflOK. HoaTOMy B CHjiy ypaBHeHHfl... 

Draw two different chair conformations of fmns-l,4-dimethylcyclohexane. and label all positions as axial or equatorial. 

The two isomers cis- and fmns-4-tert-butyl-2-methylcyclohexanone are iivterconverted by base treatment. Which isomer do you think is more stable, and why ... 

Flavin adenine dinucleotide (FAD) Scheme 10.15 Chemical structures of FMN and FAD. 

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