Biotin coenzyme function

The chemistry of a fourth coenzyme was at least partially elucidated in the period under discussion. F. Lynen and coworkers treated P-methylcrotonyl coenzyme A (CoA) carboxylase with bicarbonate labelled with 14C, and discovered that one atom of radiocarbon was incorporated per molecule of enzyme. They postulated that an intermediate was formed between the enzyme and C02, in which the biotin of the enzyme had become car-boxylated. The carboxylated enzyme could transfer its radiolabelled carbon dioxide to methylcrotonyl CoA more interestingly, they found that the enzyme-COz compound would also transfer radiolabelled carbon dioxide to free biotin. The resulting compound, carboxybiotin [4], was quite unstable, but could be stabilized by treatment with diazomethane to yield the methyl ester of N-carboxymethylbiotin (7) (Lynen et al., 1959). The identification of this radiolabelled compound demonstrated that the unstable material is N-carboxybiotin itself, which readily decarboxylates esterification prevents this reaction, and allows the isolation and identification of the product. Lynen et al. then postulated that the structure of the enzyme-C02 compound was essentially the same as that of the product they had isolated from the reaction with free biotin, but where the carbon dioxide was inserted into the bound biotin of the enzyme (Lynen et al., 1961). Although these discoveries still leave significant questions to be answered as to the detailed mechanism of the carboxylation reactions in which biotin participates as coenzyme, they provide a start toward elucidating the way in which the coenzyme functions. 

Vitamins are chemically unrelated organic compounds that cannot be synthesized by humans and, therefore, must must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water-soluble, whereas four vitamins (vitamins A, D, K, and E) are termed fat-soluble (Figure 28.1). Vitamins are required to perform specific cellular functions, for example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat soluble vitamin (vitamin K) has a coenzyme function. These vitamins are released, absorbed, and transported with the fat of the diet. They are not readily excreted in the urine, and significant quantities are stored in Die liver and adipose tissue. In fact, consumption of vitamins A and D in exoess of the recommended dietary allowances can lead to accumulation of toxic quantities of these compounds. 

The coenzyme biotin, which functions as a C02 carrier, is covalently bound to the enzyme through the side chain amino group of a lysine residue. OAA... 

The first examples of mechanism must be divided into two principal classes the chemistry of enzymes that require coenzymes, and that of enzymes without cofactors. The first class includes the enzymes of amino-acid metabolism that use pyridoxal phosphate, the oxidation-reduction enzymes that require nicotinamide adenine dinucleotides for activity, and enzymes that require thiamin or biotin. The second class includes the serine esterases and peptidases, some enzymes of sugar metabolism, enzymes that function by way of enamines as intermediates, and ribonuclease. An understanding of the mechanisms for all of these was well underway, although not completed, before 1963. 

Biotin enzymes are believed to function primarily in reversible carboxvlahon-decarboxylation reactions. For example, a biotin enzyme mediates the carboxylation of propionic acid to methylmalonic add, which is subsequently converted to succinic acid, a dtric acid cycle intermediate. A vitamin Bl2 coenzyme and coenzyme A are also essential to this overall reaction, again pointing out the interdependence of the B vitamin coenzymes. Another biotin enzyme-mediated reaction is the formation of malonyl-CoA by carboxylation of acetyl-CoA ( active acetate ). Malonyl-CoA is believed lo be a key intermediate in fatly add synthesis. 

Vitamin Biotin RDA/AI Men women 30 MQ/d Physiological Function Coenzyme in the synthesis of fat, glycogen, and amino acids Adverse Effects of Excessive Consumption No adverse effects have been reported ... 

Water-soluble vitamins. Water-soluble vitamins include vitamin C, and those of the B-complex group biotin, folate, niacin, pantothenic acid, riboflavin, thiamine, vitamin Bg and vitamin B12. They function mainly as coenzymes and prosthetic groups. 

The main metabolic function of vitamin K is as the coenzyme in the carboxyla-tion ofprotein-incorporated glutamate residues to yield y -carboxyglutamate -a unique type of carboxylation reaction, clearly distinct from the biotin-dependent carboxylation reactions (Section 11.2.1). 

Biotin is the coenzyme in a small number of carboxylation reactions in mammalian metabolism and some decarboxylation and transcarboxylation reactions in bacteria. Although the biotin-dependent enzymes are cytosolic and mitochondrial, about 25% of tissue biotin is found in the nucleus, much of it bound as thioesters to histones. Biotin has two noncoenzyme functions induction of enzyme synthesis and regulation of the cell cycle. 

To activate carbon dioxide for chemical reactions, it is advantageous to fix and destabilize this rather inert molecule. An important strategy to control and influence the reactivity of CO2 is its coordination to amines, metal-bound imido moieties, or metal centers (144), which leads to a decrease of the CO bond order, while the molecule in most cases becomes considerably bent. In biochemical pathways, the CO2 adduct of the coenzyme biotin (17) is involved in various carboxylation and transcarboxylation reactions (146). Similar structural motifs could become important functional building blocks for bioinspired photoreactions involving carbon dioxide activation steps (Fig. 18). 

The important function of biotin is its role as coenzyme for carboxylase, which catalyses carbon dioxide fixation or carboxylation reaction. The epsilon amino group of lysine in carboxylase enzymes combines with the carboxyl group of biotin to form covalently linked biotinyl carboxyl carrier protein (BCCP or biocytin) (Figure 6.8). This serves as an intermediate carrier of carbon dioxide. The carboxylation of acetyl CoA to malonyl CoA in presence of acetyl CoA carboxylase requires biotin as coenzyme. Propionyl carboxylase and pyruvate carboxylase are also associated with biotin. 

Most vitamins function either as a hormone/ chemical messenger (cholecalciferol), structural component in some metabolic process (pantothenic acid), or a coenzyme (phytonadi-one, thiamine, riboflavin, niacin, pyridoxine, biotin, folic acid, cyanocobalamin). At least one vitamin has more than one biochemical role. Vitamin A as an aldehyde (retinal) is a structural component of the visual pigment rhodopsin and, in its acid form (retinoic acid), is a regulator of cell differentiation. The precise biochemical functions of ascorbic acid and a-tocopherol still are not well defined. 

In 1936, a growth-promoting factor termed vitamin B was isolated from bovine (cow) milk. There are now several different types of vitamin B known and chemically characterized, and they are collectively described as B complex vitamins because of relative similarities in their properties, physiological functions, and distribution in natural resources. Mostly recognized as coenzymes, the eight B complex vitamins currently include Bj (thiamine), B2 (riboflavin), niacin (nicotinamide), B (pyridoxine), pantothenic acid, biotin, B22 (cyanocobalamin), and folacin (folic acid). 

Biotin is one of about a dozen water-soluble factors of the vitamin B complex and is a coenzyme for enzymes involved in carboxylation reactions. The carboxyl group of the valeric acid side chain of (-f )-biotin is covalently linked through an amide bond to the s-amino group of a lysyl residue of those enzymes. The antibacterial function of avidin by binding biotin and inhibiting all biotin-enzymes is plausi-... 

The vitamins cannot be synthesised in the organism in certain cases the organism is capable of carrying out the last step from provitamin to vitamin (y -carotene to retinol). Some vitamins function as redox-catalysts in the metabolism (vitamins A, C, E and K), whereas others function as coenzyme or activators of enzymes (vitamin Bl, B2, B6, B12, folk acid, biotin and vitamin D). 

Coenzyme A (CoA), biotin, and pyridoxal phosphate are also activation-transfer coenzymes synthesized from vitamins. CoA (CoASH), which is synthesized from the vitamin pantothenate, contains an adenosine 3, 5 -bisphosphate which binds reversibly, but tightly, to a site on an enzyme (Fig. 8.12A). Its functional group, a sulfhydryl group at the other end of the molecule, is a nucleophile that always... 

Fig. 9.2. Schematic representation of a plastidic homomeric acetyl coenzyme A carboxylase (ACC) showing the three functional domains (BC, biotin carboxylase BCC, biotin carboxyl-carrier and CT, carboxyl transferase) and the transit peptide (TP) that...

Many C. in the wider sense are synthesized from vitamins. The relationships of some C. to vitamins and metabolic function are listed in the table. Strictly speaking, ATP, which commands a special position in metabolism, does not fit the definition of a CThe C. of C -unit transfer are S-Adenosylmethionine (see), Tetrahydrofolic acid (see) and Biotin (see). The C. of C2"transfer are Coenzyme A (see) and Thiamin pyrophosphate (see). Vitamin B[j is involved in various metabolic reactions, in free form, as methyl-vitamin B,2 and as S -Deoxyadenosylcobalamine (see). 

Vitamins are a well-known group of compounds that are essential for human health. Water-soluble vitamins include folate (vitamin B9) to create DNA. Folate also plays an important role in preventing birth defects during early pregnancy. Thiamine is the first vitamin of the B-complex (vitamin Bl) that researchers discovered. It allows the body to break down alcohol and metabolize carbohydrates and amino acids. Like many other B vitamins, riboflavin (vitamin B2) helps the body to metabolize carbohydrates, proteins, and fat. Niacin (vitamin B3) protects the health of skin cells and keeps the digestive system functioning properly. Pantothenic acid (vitamin B5) and biotin allow the body to obtain energy from macronutrients such as carbohydrates, proteins, and fats. Vitamin B6 (pyridoxine) acts as a coenzyme, which means it helps chemical reactions to take place. It also plays a vital role in the creation of nonessential amino acids. 

Biotin and folic acid (FA) (Figure 23.1) are two members of the water-soluble B complex vitamins. Biotin plays an important role in gene expression, cell signalling and histone biotinylation, and functions as a coenzyme in the tricarboxylic acid (TCA) cycle. It also functions in the metabolism of fatty... 

Mammals cannot synthesize biotin and depend on a regular dietary supply of this water-soluble vitamin (Zempleni et al., 2009). The Adequate Intake for biotin in adults is 30 pg/d (National Research Council, 1998). The classical role of biotin in mammalian intermediary metabolism is to serve as a covalently bound coenzyme in five carboxylases (Zanpleni et al., 2D09). Both the cytoplasmic acetyl-CoA carboxylase 1 (ACCl) and the mitochondrial acetyl-CoA carboxylase 2 (ACC2) catalyze the binding of bicarbonate to acetyl-CoA to generate malonyl-CoA, but the two isoforms have distinct functions in intermediary metabolism (Kim et al., 1997). ACCl produces malonyl-CoA for the synthesis of fatty acid synthesis in the cytoplasm ACC2... 

Thiamine, biotin, pantothenic acid, riboflavin and vitamin B12 are involved in propionic acid fermentation. Biotin forms the prosthetic group of methyl-malonyl-CoA transcarboxylase pantothenate is a constituent of CoA thiamine is not the coenzyme (co-carboxylase) of the enzyme carboxylase like in other organisms, for acetaldehyde has not been detected in propionibacteria (although traces were recently found), but it may function as a component of dehydrogenases in oxidative phosphorylation of a-keto acids. Riboflavin is a constituent of FAD and FMN. Propionibacteria can synthesize vitamins B2 and B in considerable amounts (see below), but the other three vitamins must be supplied. Some strains can grow in synthetic media without thiamine (Silverman and Workman, 1939 Delwiche, 1949), in some other strains thiamine can be replaced by / -aminobenzoic acid. 

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