Biotin reactions involving

Biotin is involved in carboxylation and decarboxylation reactions. It is covalently bound to its enzyme. In the carboxylase reaction, C02 is first attached to biotin at the ureido nitrogen, opposite the side chain in an ATP-dependent reaction. The activated C02 is then transferred from carboxybiotin to the substrate. The four enzymes of the intermediary metabolism requiring biotin as a prosthetic group are pyruvate carboxylase (pyruvate oxaloacetate), propionyl-CoA-carboxylase (propionyl-CoA methylmalonyl-CoA), 3-methylcroto-nyl-CoA-carboxylase (metabolism of leucine), and actyl-CoA-carboxylase (acetyl-CoA malonyl-CoA) [1]. 

One of the more interesting reactions involving the reduction of a sulphone was recently described in a paper87 that reported on a synthetic approach to biotin. The molecule in question contains a sulphone as part of a thiolane ring that is fused to a cyclic urea. As outlined in equation (33), either functionality can be reduced depending on the selection of... 

The reaction involves biotin as a carrier of activated HCO3 (Fig. 14-18). The reaction mechanism is shown in Figure 16-16. Pyruvate carboxylase is the first regulatory enzyme in the gluconeogenic pathway, requiring acetyl-CoA as a positive effector. (Acetyl-CoA is produced by fatty acid oxidation (Chapter 17), and its accumulation signals the availability of fatty acids as fuel.) As we shall see in Chapter 16 (see Fig. 16-15), the pyruvate carboxylase reaction can replenish intermediates in another central metabolic pathway, the citric acid cycle. 

Biotin is involved in many carboxylation and decarboxylation reactions in carbohydrate, fatty acid, protein, and nucleic acid metabolism. Milk is a fairly good source of this vitamin, generally providing about 3/xg/100 g. Pasteurization has a minimal effect on the biotin content of milk. 

Biotin is the essential coenzyme for carboxylation reactions involving bicarbonate as the carboxylating agent. Several reactions have been described in which ATP-depen-dent carboxylation occurs at carbon atoms activated for enolization by ketonic or activated acyl groups. One reaction is known in which a nitrogen atom of urea is carboxyl-ated. 

Claisen reactions involving acetyl-CoA are made even more favourable by first converting acetyl-CoA into malonyl-CoA by a carboxylation reaction with CO2 using ATP and the coenzyme biotin (Figure 2.9). ATP and CO2 (as bicarbonate, HC03-) form the mixed anhydride, which car-boxy lates the coenzyme in a biotin-enzyme complex. Fixation of carbon dioxide by biotin-enzyme complexes is not unique to acetyl-CoA, and another important example occurs in the generation of oxaloacetate from pyruvate in the synthesis of glucose from non-carbohydrate sources... 

The acetyl-CoA generated from citrate is then used for fatty acid biosynthesis. In the human being, only two multifunctional enzymes are involved acetyl-CoA carboxylase (also termed malonyl-CoA synthetase) and fatty acid synthetase, with a molecular weight of 500,000, and coded by a single gene. The product of the two enzymes is palmitate. Other fatty acids may be made from palmitate by chain unsaturation, or elongation, or both (see later). The initial reaction involves the carboxylation of acetyl-CoA in two steps by acetyl-CoA carboxylase. Biotin is a cofactor, and one molecule of ATP is hydrolyzed to ADP and Pc ... 

Ribozyme-catalyzed reactions involving C-C bond formations have also been reported. Seelig and Jaschke (233) presented the in vitro selection of ribozyme catalysts for the Diels-Alder reaction between maleimide and anthracene, employing a 2 X lO -member library of 160-mer modified ONs (L28) with 120 randomized positions. The selection strategy used is shown in Fig. 10.40. Library L28 was prepared from the corresponding dsDNA sequences, and transcription initiation was performed in the presence of ternary complexes between guanosine monophosphate (10.57), PEG (10.58), and anthracene (10.59, step a. Fig. 10.40). The library obtained contained a 5 -anthracene-PEG appendage and was incubated with biotin-modified maleimide... 

The final reaction, catalyzed by biotin synthase, involves the insertion of sulfur between the unreactive methyl and methylene carbons of dethiobiotin. The enzyme has an iron-sulfur box, and requires NADPff and a ferredoxin or flavo-doxin reducing system. S-Adenosylmethionine is also required, and is cleaved to yield methionine and a 5 -deoxyadenosyl radical during the reaction. Biotin synthase is a member of the radical SAM family of enzymes, in which the catalytic 5 -deoxyadenosyl radical is formed from S-adenosylmethionine,... 

FIGURE 9,32 Structure of biotin. Biotin is used as the carrier of carboxyl groups in carboxyialion reactions. In an ATl -dcpendcnt reaction, bicarbonate is transferred to biotin, generating carboxy-biotin. The carboxyl group is bound to the I -nitrogpn. The second half of biotin-dependent carboxylation reactions involves transfer of the carboxyl group to the substrate. 

The biosynthetic pathway of (+)-biotin (1) involves a five-step sequence starting from pimelic acid as shown in Fig. (2) [20-45] to four consecutive reactions leading to dethiobiotin (DTB), a direct precursor to 1. The pathway from pimelic acid to DTB has already been well investigated, and enzymes responsible for each step have been fully characterized. However, it has remained to be observe(i how the last transformation from... 

These carbanions can be formed (Figure 5.8) by proton abstraction from ketones resulting in aldol condensations, by proton abstraction from acetyl CoA, leading to Claisen ester condensation, and by decarboxylation of p-keto acids leading to a resonance-stabilised enolate, which can likewise add to an electrophilic centre. It should be noted that the reverse of decarboxylation also leads to formation of a carbon—carbon bond (this is again a group transfer reaction involving biotin as the carrier of the activated CO2 to be transferred). 

This very interesting reaction involves the formal insertion of a sulfur atom into two unactivated CH bonds. The purified protein contains a [2Fe-2S] cluster [131]. A defined system, consisting of biotin synthase, flavodoxin, llavodoxin reductase, fructose 1,6-bisphosphate, cysteine, DTT, NADPH, ferrous ion, and SAM, capable of catalyzing the conversion of dethiobiotin to biotin has been characterized [132-134]. This system is still incomplete and gives a maximum of two moles of biotin per mole of biotin synthase. 

This reaction is analogous to the sulfur insertion reaction involved in biotin biosynthesis, and LipA and BioB show significant sequence similarity [145,146]. 8-Mercapto-octanoic acid and 6-mercapto-octanoic acid are intermediates... 

Biotin is involved in which of the following types of reactions ... 

The a-oxoamine synthases family is a small group of fold-type I enzymes that catalyze Claisen condensations between amino acids and acyl-CoA thioesters (Figure 16). Members of this family are (1) 8-amino-7-oxononanoate (AON) synthase (AONS), which catalyzes the first committed step in the biosynthesis of biotine, (2) 5-aminolevulinate synthase (ALAS), responsible for the condensation between glycine and succinyl-CoA, which yields aminolevulinate, the universal precursor of tetrapyrrolic compounds, (3) serine palmitoyltransferase (SPT), which catalyzes the first reaction in sphingolipids synthesis, and (4) 2-amino-3-ketobutyrate CoA ligase (KBL), involved in the threonine degradation pathway. With the exception of the reaction catalyzed by KLB, all condensation reactions involve a decarboxylase step. 

Each ACC half-reaction is catalyzed by a different protein sub-complex. The vitamin biotin is covalently coupled through an amide bond to a lysine residue on biotin carboxyl carrier protein (BCCP, a homodimer of 16.7-kDa monomers encoded by accB) by a specific enzyme, biotin-apoprotein ligase (encoded by birA), and is essential to activity. The crystal and solution structures of the biotinyl domain of BCCP have been determined, and reveal a unique thumb required for activity (J. Cronan, 2001). Carboxylation of biotin is catalyzed by biotin carboxylase (encoded by accC), a homodimeric enzyme composed of 55-kDa subunits that is copurified complexed with BCCP. The accB and accC genes form an operon. The three-dimensional structure of the biotin carboxylase subunit has been solved by X-ray diffraction revealing an ATP-grasp motif for nucleotide binding. The mechanism of biotin carboxylation involves the reaction of ATP and CO2 to form the shortlived carboxyphosphate, which then interacts with biotin on BCCP for CO2 transfer to the I -nitrogen. 

Groups containing a single carbon atom can be transferred from one compound to another. These carbon atoms may be in a number of different oxidation states. The most oxidized form, CO2, is transferred by biotin. One-carbon groups at lower levels of oxidation than CO2 are transferred by reactions involving tetrahydrofolate (FHf, vitamin B12, and S-adenosylmethionine (SAM). 

The two more conventional mechanisms for the first half-reaction of biotin carboxylases involve direct activation of bicarbonate by ATP. The first mechanism, proposed originally by Ochoa, may be subdivided into a stepwise mechanism (Scheme 40b) and a concerted mechanism (Scheme 40c). The high-energy carboxyphosphate molecule is an intermediate in the stepwise mechanism. 

The reactivity of biotin with several reagents can be exploited in some chemical assays like its reaction with diazo derivatives, or the reaction of the ureido ring with p-dimethylaminocinnamaldehyde in an acidic medium, but the sensitivity of these assays is relatively poor. Several methods described for biotin assay involve a competitive complex formation between biotin and avidin bound with a chromophore/ fluorophore probe. In these assays, biotin, which has a higher affinity for avidin, quantitatively displaces the probe from the complex. 

The most highly oxidized form of a single C atom is carbon dioxide and in certain reactions involving its assimilation, for example the formation of malonyl-CoA (page 2SS) and oxaloacetate (page 248), biotin acts as a carrier. Biotin deficiency has been found to occur in people who eat raw eggs in quantity. It results from the presence in egg white of avidin, a small protein that binds biotin. This property is lost on heating. 

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