Coenzyme FAD

As a rule, the anabolic pathway by which a substance is made is not the reverse of the catabolic pathway by which the same substance is degraded. The two paths must differ in some respects for both to be energetically favorable. Thus, the y3-oxidation pathway for converting fatty acids into acetyl CoA and the biosynthesis of fatty acids from acetyl CoA are related but are not exact opposites. Differences include the identity of the acvl-group carrier, the stereochemistry of the / -hydroxyacyl reaction intermediate, and the identity of the redox coenzyme. FAD is used to introduce a double bond in jS-oxidalion, while NADPH is used to reduce the double bond in fatty-acid biosynthesis. 

Figure 5.3 The flavin coenzymes FAD and FMN. Note that in contrast to NAD+, flavins can be half-reduced to the stable radical FADH or fully reduced to the dihydroflavin shown.

In the biological oxidation-reduction system, reduced NAD (i.e., NADH) is reoxidized to NAD by the riboflavin-containing coenzyme FAD flavin-adenine dinucleotide). 

Biological autofluorescence in mammalian cells due to flavin coenzymes (FAD and FMN absorption, 450 nm emission, 515 nm) and reduced pyridine nucleotides (NADH absorption, 340 nm emission, 460 nm) can be problematic in the detection of fluorescence probes in tissues and cells. Fixation with aldehydes, particularly glutaraldehyde, can result in high levels of autofluorescence. This can be minimized in fixed cells by washing with 0.1% sodium borohydride in phosphate-buffered saline (5) prior to antibody incubation. Problems due to autofluorescence can be minimized by selecting probes and optical filters that maximize the fluorescence signal relative to the autofluorescence. Other factors that limit IF include the performance of the detection instrument (i.e. how well the microscope has been calibrated and set), the specificity of the antibodies, and the specimen preparation. 

Vitamin B2 (riboflavin) acts mainly as the coenzyme FAD (flavin adenine nucleotide) rmd FMN (flavin mononucleotide), which are used in many oxidation-reduction reactions in which hydrogen atoms are received or donated. Particularly noteworthy examples are their uses in... 

At the E3 active site, the lipoamide acid is oxidized by coenzyme FAD. 

Vitamin B2 is better known as riboflavin and is widely distributed in many foods. Riboflavin is used to form a coenzyme FAD important in ttie utilization of oxygen in the cells. 

The first reaction in p-oxidation of acyl CoA is the formation of trans A2-enoyl CoA or a, p-unsaturated acyl CoA in presence of acyl-CoA dehydrogenase and the coenzyme, FAD. 

STEP 6 Succinate is dehydrogenated by the coenzyme FAD to give fumarate. 

Vitamin B complex is the collective term for a number of water-soluble vitamins found particularly in dairy products, cereals and liver.Vitamin B (thiamine) is used by mouth for dietary supplement purposes and by injection in emergency treatment of Wernicke-Korsakoff syndrome. Vitamin B2 (riboflavin) is a constituent of the coenzyme FAD (flavine adenine dinucleotide) and FMN (flavine mononucleotide) and is therefore important in cellular respiration. Vitamin Be (pyridoxine) is a coenzyme for decarboxylases and transamination, and is concerned with many metabolic processes. Overdose causes peripheral neuropathy. It may be used medically for vomiting and radiation sickness and for premenstrual tension. Pyridoxine has a negative interaction with the therapeutic use of levodopa in parkinsonism by enhancing levodopa decarboxylation to dopamine in the periphery, which does not then reach the brain. The antitubercular drug isoniazid interferes with pyridoxine, and causes a deficiency leading to peripheral neuritis that may need to be corrected with dietary supplements. Vitamin B ... 

A. niger enzyme is a dimer with two very tightly bound coenzyme (FAD) molecules per dimer (Swoboda and Massey, 1965). The most salient properties of GOase are listed in Table 10.11. Only a small... 

ORD and CD of D-amino acid oxidase from pig kidney showed visible-region Cotton effects, which were dependent on the oxidation state of the coenzyme FAD (281). An inversion in the sign of the dichroic band near 372 nm was observed when free FAD was bound to the apoenzyme of D-amino acid oxidase, indicating a conformational change in FAD upon binding (282), while optical activity was induced in substrates or inhibitors of this oxidase [Ref. (283) and refs, cited therein]. Cotton effects of L-amino acid oxidase were also observed in the absorption region of FAD (284). 

Aflavoprotein is an enzyme that contains either flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) as a coenzyme. FAD and FMN, like NAD and NADP, are coenzymes used in oxidation reactions. As its name indicates, FAD is a dinucleotide in which one of the heterocyclic components is flavin and the other is adenine. FMN contains flavin but not adenine—it is a mononucleotide. (Flavin is a bright yellow compound flavus is Latin for yellow. ) Notice that instead of ribose, the flavin nucleotide has a ribitol group (a reduced ribose). Flavin plus ribitol is called riboflavin. Riboflavin is also known as vitamin B2. A vitamin B2 deficiency causes inflammation of the skin. 

Fig. 19.13. Cytochrome P450 mono-oxygenases. Electrons are donated by NADPH to O2 and the substrate. The flavin coenzymes FAD and FMN in one subunit transfer single electrons to cytochrome P450, which is an Fe-heme containing protein that absorbs hght at a wavelength of 450 nm. The enzyme is embedded in a membrane, usually the endoplasmic reticulum.

Ann O Rexia has been malnourished for some time, and has developed subclinical deficiencies of many vitamins, including riboflavin. The coenzymes FAD (flavin adenine dinucleotide) and FMN (flavin mononucleotide) are synthesized from the vitamin riboflavin. Riboflavin is actively transported into cells, where the enzyme flavokinase adds a phosphate to form FMN. FAD synthetase then adds AMP to form FAD. FAD is the major coenzyme in tissues and is generally found tightly bound to proteins, with about 10% being covalently bound. Its turnover in the body is very slow, and people can live for long periods on low intakes without displaying any signs of a riboflavin deficiency. 

A competitive inhibitor binds to the active site of an enzyme and thus competes with snbstrate molecnles for the active site. Competitive inhibitors often have molecular structures that are similar to the normal substrate of the enzyme. The competitive inhibition of snccinate dehydrogenase by malonate is a classic example. Succinate dehydrogenase catalyzes the oxidation of the substrate succinate to form fumarate by transferring two hydrogens to the coenzyme FAD ... 

Figure 10.3 Stereostructure of the complex between (i )-amphetamine and 2BXR MAO-A. For the ligand (R-Asp) and coenzyme (FAD), small white, large gray, black, and dark gray balls represent hydrogen, carbon, oxygen, and nitrogen. Phosphorus is separately indicated.

Vitamin B2 (riboflavin) occurs in green vegetables, yeast, liver, and milk, it is a constituent of the coenzymes FAD and FMN, which have an important role in the metabolism of all major nutrients as well as in the oxidative phosphorylation reactions of the electron transport chain. Deficiency of B2 causes inflammation of the tongue and lips, mouth sores, and conjimctivitis. 

It is a curious coincidence that the first isolation of a crystalline vitamin from natural sources corresponded in time to the first isolation of a crystalline enzyme. The close relationship between the vitamins and the coenzymes was clearly demonstrated in the 1930 s when in rapid succession riboflavin was shown to be a constituent of the coenzyme FAD, and cocar-... 

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