Flavoproteins Flavin mononucleotide

FUNCTIONS. Riboflavin functions as part of a group of enzymes called flavoproteins. Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) operate at vital reaction points in the respiratory chains of cellular metabolism. The structure of these two compounds is shown in Fig. R-5. 

Flavin Mononucleotide (FMN) (C17H21N409P) is a phosphoric ester of riboflavin that constitutes the cofactor of various flavoproteins. 

Flavoprotein enzymes contain flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups. FMN and FAD are formed in the body from the vitamin riboflavin (Chapter 45). FMN and FAD are usually tighdy—but not covalendy—bound to their respecdve apoenzyme proteins. Metalloflavopro-teins contain one or more metals as essential cofactors. 

Riboflavin (vitamin B2) is a component of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), coenzymes that play a major role in oxidation-reduction reactions (see Section 15.1.1). Many key enzymes involved in metabolic pathways are actually covalently bound to riboflavin, and are thus termed flavoproteins. 

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). 

A model of a flavin-based redox enzyme was prepared.[15] Redox enzymes are often flavoproteins containing flavin cofactors flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). They mediate one- or two-electron redox processes at potentials which vary in a range of more than 500 mV. The redox properties of the flavin part must be therefore tuned by the apoenzyme to ensure the specific function of the enzyme. Influence by hydrogen bonding, aromatic stacking, dipole interactions and steric effects have been so far observed in biological systems, but coordination to metal site has never been found before. Nevertheless, the importance of such interactions for functions and structure of other biological molecules make this a conceivable scenario. 

Sevrioukova, I., Truan, G., and Peterson, J. A. 1996. The flavoprotein domain of P450BM-3 Expression, purification, and properties of the flavin adenine dinucleotide and flavin mononucleotide-binding subdomains. Biochemistry, 35,7528-7535. 

Some proteins contain other native fluorophores in addition to fluorescent amino acids. These include cofactors such as nicotinamide adenine dinucleotide (fluorescent in its reduced, NADH state) and flavin adenine dinucleotide (FAD). NADH is weakly fluorescent in water, but its fluorescence yield increases markedly on binding to a protein-binding site with an emission peak around 470 nm (3). FAD and flavin mononucleotide (FMN) are also fluorescent with an emission maximum around 520 nm, but fluorescence is quenched on binding to many flavoproteins (4). 

This electron transfer to FAD is unusual, because the common role for FAD is to receive electrons from NADH. The electron transfer potential of FAD is altered by its association with the enzyme and enables it to transfer electrons to NAD+. Proteins tightly associated with FAD or flavin mononucleotide (FMN) are called flavoproteins. 

In higher mammals, riboflavin is absorbed readily from the intestines and distributed to all tis.sues. It is the precursor in the biosynthesis of the cocnzyme.s flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The metabolic functions of this vitamin involve these Iwocoenzymes. which participate in numerous vital oxidation-reduction proces.ses. FMN (riboflavin 5 -phosphate) is produced from the vitamin and ATP by flavokinasc catalysis. This step con be inhibited by phcnothiazincs and the tricyclic antidepressants. FAD originates from an FMN and ATP reaction that involves reversible dinucicotide formation catalyzed by flavin nucleotide pyrophosphorylase. The.se coenzymes function in combination with several enzymes as coenzyme-en-zyme complexes, often characterized as, flavoproteins. 

The answer is b. (Murray, pp 627-661. Scriver, pp 3897-3964. Sack, pp 121-138. Wilson, pp 287-320.) Nicotinamide adenine dinucleotide (NAD+) is the functional coenzyme derivative of niacin. It is the major electron acceptor in the oxidation of molecules, generating NADH, which is the major electron donor for reduction reactions. Thiamine (also known as vitamin Bi) occurs functionally as thiamine pyrophosphate and is a coenzyme for enzymes such as pyruvate dehydrogenase. Riboflavin (vitamin B2) functions in the coenzyme forms of flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). When concentrated, both have a yellow color due to the riboflavin they contain. Both function as prosthetic groups of oxidation-reduction enzymes or flavoproteins. Flavoproteins are active in selected oxidation reactions and in electron transport, but they do not have the ubiquitous role of NAD+. 

Like other biologically active substances we have met, flavin is found combined as a nucleotide, linked to ribose phosphate and sometimes also to adenine nucleotide, forming flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). The flavin nucleotides, unlike NAD, are bound firmly on to proteins and cannot be removed by dialysis. Such proteins are known as flavoproteins (FP). The typical reaction in which the flavoproteins participate is the oxidation of NADH2 ... 

Redox-active cofactors are important species in biological systems, playing vital roles in redox and electron-transfer processes. Among the structurally and functionally diverse redox enzymes, flavoproteins containing the flavin cofactors flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) are involved in many different biochemical processes serving as a highly versatile redox... 

Porter, T.D. and C.B. Kasper (1986). NADPH-cytochrome P-450 oxidoreductase Flavin mononucleotide and flavin adenine dinucleotide domains evolved from different flavoproteins. Biochemistry 25, 1682-1687. 

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