Riboflavin vitamin coenzymes derived from

Although the nicotinamide nucleotide coenzymes function in a large number of oxidation and reduction reactions, thus carmot be exploited as a means of assessing the state of the body s niacin reserves, because the coenzymes are not firmly attached to their apoenzymes, as are coenzymes derived from thiamin (Section 6.5.3), riboflavin (Section 7.5.3), and vitamin Be (Section 9.5.3), but act as cosubstrates of the reactions, binding to and leaving the enzyme as... 

Flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) Coenzymes derived from vitamin B2 (riboflavin) that function as electron acceptors in enzymes that catalyze electron transfer reactions. 

Many, although not all, coenzymes are derived from vitamins—substances that an organism requires for growth but is unable to synthesize and must receive in its diet. Coenzyme A from pantothenate (vitamin B3), NAD" from niacin, FAD from riboflavin (vitamin B2), tetrahydrofolate from folic acid, pyridoxal phosphate from pyridoxine (vitamin Be), and thiamin diphosphate from thiamin (vitamin Bi) are examples (Table 19.3). We ll discuss the chemistry and mechanisms of coenzyme reactions at appropriate points later in the text. 

Note that this overall reaction requires three coenzymes that we encountered as metabolites of vitamins in chapter 15 NAD+, derived from lucotiiuc acid or nicotinamide FAD, derived from riboflavin and coenzyme A(CoASH), derived from pantothenic acid. In the overall process, acetyl-SCoA is oxidized to two molecules of carbon dioxide with the release of CoASH. Both NAD+ and FAD are reduced to, respectively, NADH and FADH2. Note that one molecule of guanosine triphosphate, GTP, functionally equivalent to ATP, is generated in the process. 

Some enzymes associate with a nonprotein cofactor that is needed for enzymic activity. Commonly encountered cofactors include metal ions such as Zn2+ or Fe2+, and organic molecules, known as coenzymes, that are often derivatives of vitamins. For example, the coenzyme NAD+contains niacin, FAD contains riboflavin, and coenzyme A contains pantothenic acid. (See pp. 371-379 for the role of vitamins as precursors of coenzymes.) Holoenzyme refers to the enzyme with its cofactor. Apoenzyme refers to the protein portion of the holoenzyme. In the absence of the appropriate cofactor, the apoenzyme typically does not show biologic activity. A prosthetic group is a tightly bound coenzyme that does not dissociate from the enzyme (for example, the biotin bound to carboxylases, see p. 379). 

Several coenzymes comprising a pyrimidine ring motif are derived from GTP (22) (Fig. 3). Specifically, this group comprises two members of the B vitamin group, riboflavin (vitamin B2) (24) and folic acid/tetrahydrofolate (33). Two other members of the group, tetrahydrobiopterin (31) and molybdopterin (8), are biosynthesized de novo in animals and do not have vitamin status (20, 21). 

Branched-chain ketoaciduria (commonly known as Maple Syrup Urine Disease MSUD) is another ailment that may be caused by thiamine deficiency. In MSUD, the oxidative decarboxylation of alpha-keto acids derived from, i.e. valine, isoleucine, and leucine, is blocked due to an inadequate supply of the coenzyme thiamine pyrophosphate (TPP). Clinical symptoms of MSUD include mental and physical retardation. Describe briefly the structure of Riboflavin (Vitamin B-2) and its biochemical role. 

Almost all the activated carriers that act as coenzymes are derived from xnta-mins. Vitamins are organic molecules that are needed in small amounts in the diets of some higher animals. Table 15.3 lists the vitamins that act as coenzymes (Figure 15.17). This series of vitamins is known as the vitamin B group. Note that, in all cases, the vitamin must be modified before it can serve its function. We have already touched on the roles of niacin, riboflavin, and pantothenate. We will see these three and the other B vitamins many times in our study of biochemistry. 

FAD is the coenzyme of a class dehydrogenases called jlavoproteins. The flavin moiety of the molecule is derived from riboflavin (vitamin B2). Reduction of FAD involves the two unsubstituted N atoms of the isoalloxazine structure. 

This single reaction requires four coenzymes made from four different vitamins, in addition to the coenzyme lipoamide. These are thiamine pyrophosphate, derived from thiamine (Vitamin B ) FAD, derived from riboflavin (Vitamin B2) ... 

Another important electron acceptor is FAD (flavin adenine dinucleotide) (Figure 15.4), which is the oxidized form of FADH. The symbol FADH explicitly recognizes that protons (hydrogen ions) as well as electrons are accepted by FAD. The structures shown in this equation again point out the electrons that are transferred in the reaction. Several other coenzymes contain the flavin group they are derived from the vitamin riboflavin (vitamin B ). 

Two coenzymes which are involved in most of the redox reactions of metabolism are nicotine adenine dinucleotide, NAD", and FAD, flavine adenine dinucleotide. Most metabolic oxidations are, in fact, dehydrogenations, and not reactions with oxygen. Nicotinamide is derived from nicotinic acid, and the isoalloxazine ring of FAD is derived from riboflavin. Thiamin is the principal cofactor in enzymatic decarboxylations. Many of the vitamins serve as coenzymes in a wide variety of cellular reactions. 

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... 

Most coenzymes are derivatives of compounds we call vitamins. For example, nicotinamide adenine dinucleotide is produced from the B vitamin niacin. Flavin adenine dinucleotide comes from riboflavin. 

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