Acyl group transfer

Lipoic acid is an acyl group carrier. It is found in pyruvate dehydrogenase zard a-ketoglutarate dehydrogenase, two multienzyme complexes involved in carbohydrate metabolism (Figure 18.34). Lipoie acid functions to couple acyl-group transfer and electron transfer during oxidation and decarboxylation of a-keto adds. 

Figure 3. Mitochondrial fatty acid oxidation. Long-chain fatty acids are converted to their CoA-esters as described in the text, and their fatty-acyl-groups transferred to CoA in the matrix by the concerted action of CPT 1, the acylcarnitine/carnitine exchange carrier and CPT (A) as described in the text. Medium-chain and short-chain fatty acids (Cg or less) diffuse directly into the matrix where they are converted to their acyl-CoA esters by a acyl-CoA synthase. The mechanism of p-oxidation is shown below (B). Each cycle of P-oxidation removes -CH2-CH2- as an acetyl unit until the fatty acids are completely converted to acetyl-CoA. The enzymes catalyzing each stage of P-oxidation have different but overlapping specificities. In muscle mitochondria, most acetyl-CoA is oxidized to CO2 and H2O by the citrate cycle (Figure 4) some is converted to acylcamitine by carnitine acetyltransferase (associated with the inner face of the inner membrane) and exported from the matrix. Some acetyl-CoA (if in excess) is hydrolyzed to acetate and CoASH by acetyl-CoA hydrolase in the matrix. Enzymes ...

This enolate can then react with a plethora of electrophiles, setting a new stereocenter by a diastereoface-selective reaction. The simplest electrophile to trap enolate 71 is H" ", which can, for example, originate from methanol [89] or diphenyl acetaldehyde (as a readily enolizable aldehyde) [90] leading to the acy-lated catalyst species (Fig. 38). The free catalyst is regenerated by acyl-group transfer to methanol(ate) or the aldehyde-derived enolate, producing methyl or enolesters 72/73 in good yields and enantioselectivities. 

Group-transfer reactions often involve vitamins3, which humans need to have in then-diet, since we are incapable of realizing their synthesis. These include nicotinamide (derived from the vitamin nicotinic acid) and riboflavin (vitamin B2) derivatives, required for electron transfer reactions, biotin for the transfer of C02, pantothenate for acyl group transfer, thiamine (vitamin as thiamine pyrophosphate) for transfer of aldehyde groups and folic acid (as tetrahydrofolate) for exchange of one-carbon fragments. Lipoic acid (not a vitamin) is both an acyl and an electron carrier. In addition, vitamins such as pyridoxine (vitamin B6, as pyridoxal phosphate), vitamin B12 and vitamin C (ascorbic acid) participate as cofactors in an important number of metabolic reactions. 

K. Brocklehurst, Acyl Group Transfer-Cysteine Proteases , in Enzyme Mechanisms , Eds. M. I. Page, A. Williams, The Royal Society of Chemistry, London, 1987, p. 140-158. 

D. S. Auld, Acyl Group Transfer-Metalloproteinases , in Enzyme Mechanisms , Eds. 

Unsubstituted cycloamyloses have been used to catalyze a number of reactions in addition to acyl group transfer. Brass and Bender (8) showed that cycloamyloses promoted phenol release from diphenyl and bis(p-nitro-phenyl) carbonates and from diphenyl and bis(m-nitrophenyl)methyl phos-phonates. Breslow and Campbell (10,11) showed that the reaction of anisole with HOCL in aqueous solution is catalyzed by cyclohexaamylose and cycloheptaamylose. Anisole is bound by the cyclodextrins and is chlorinated exclusively in the para position while bound. Cycloheptaamylose has been used to promote regiospecific alkylation followed by the highly selective oxidation shown in reaction (3) (95). In addition cycloheptaamylose effec-... 

The rearrangement of aryl and naphthyl acetates has been reported to be catalyzed by Bi(0Tf)3xH20 (Scheme 9) [68, 69]. As previously reported, only ortho-Fries products (l-hydroxy-2-acylaromatics) were produced from substrates for which ortho acylation was possible. In the case of 2,6-dimethoxyphenyl acetate, only the 3,5-dimethoxy-2-hydroxy acetophenone was produced, indicating that in this case the mechanism involves an intermolecular acyl-group transfer. As in other reactions, the nature of the true catalyst is still unclear since triflic acid also catalyzes this reaction. 

An efficient synthesis of P-lactam with D-phenylglycine or its derivatives as a side chain can be accomplished only by using a kinetically controlled approach via acyl group transfer from an activated side chain donor, and D-phenylglycine can be used as ester, usually methyl (PGM) or ethyl (PGE), or as amide (PGA). As a... 

Baldini, L., Bracchini, C., Cacciapaglia, R., Casnati, A., Mandolini, L. and Ungaro, R. (2000) Catalysis of acyl group transfer by a double-disiplacement mechanism the cleavage of aryl esters catalyzed by calixcrown-Ba complexes. Chem.-Eur. J., 6, 1322. 

The chymotiypsin reaction is one example of acyl group transfer (see Fig. 6-21). Glycosyl group transfers involve nucleophilic substitution at C-l of a sugar ring, which is the central atom of an acetal. In principle, the substitution could proceed by an SnI or Sn2 path, as described for the enzyme lysozyme (see Fig. 6-25). 

Reactions Requiring Acyl-Group Transfers Linked to Oxidation—Reduction Biotin Mediates Carboxylations Folate Coenzymes Are Used in Reactions for One-Carbon Transfers... 

The two important steps of the reaction depend on both acetyl groups being activated, one for enolization and the other for acyl-group transfer. In the first step, one of the molecules must be enolized by the intervention of a base to remove an a proton, forming an enolate ... 

Lipoic acid mediates electron transfer and acyl-group transfer in a-keto acid dehydrogenase complexes. 

Titration of the intact active site obviates problems due to inactive protein which contribute to a false molarity. Active-site titrations of acyl group transfer enzymes such as a-chymotrypsin utilise a substrate which has a good leaving group. This enables the buildup of an acyl enzyme intermediate which forms faster than it can degrade and results in... 

The scheme proposed for the reaction over HFAU was that PA dissociates in phenol (P) and ketene and that o-HAP, which was highly favoured over the para isomer, results partly from an intramolecular rearrangement of PA, partly from acyl group transfer from PA to P whereas p-HAP results from this latter reaction only. In these experiments, the zeolite deactivation was very fast, as a result of coke deposition and zeolite dehydroxylation. Catalyst stability can be considerably improved by operating at lower temperatures and especially by substituting equimolar mixtures of PA and water or P and acetic acid for PA. Much higher HAP yields were obtained by using the P - acetic acid mixture as reactants.[68]... 

Activates carboxyl Rare groups acyl group transfer... 

Acyl Group Transfer (Acyl Carrier Protein)... 

Vagelos PR (1973) Acyl group transfer (acyl carrier protein). In The enzymes. Academic,... 

Pantothenic acid Coenzyme A Acyl-group transfer Hypertension... 

An analogous acyl group transfer has been observed in acylated hydrazones of 3-hydroxybenzo [ b] thiophene-2-carboxaldehyde and 1,3-benzoyl migration and E-isomerization pathways compete on irradiation of certain 4-acyloxy-2-azabuta-1,3-dienes, the... 

Section IV,FI An elimination-addition mechanism of acyl group transfer has been proposed for the decomposition of l-(JV-methylcarbamoyl)... 

H. Al-Rawi and A. Williams, Elimination-Addition Mechanisms of Acyl Group Transfer the Hydrolysis and Synthesis of Carbamates J. Am. Chem. Soc., 1977, 99, 2671. 

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