Esters oxygenase

In the intestinal mucosal cells, /3-carotene is cleaved via an oxygenase (an enzyme that introduces molecular 02 into organic compounds) to frans-retinal (aldehyde form of trans-retinol, as shown in Table 6.2), which in turn is reduced to frans-retinol, vitamin Av Retinol is then esterified with a fatty acid, becomes incorporated into chylomicrons, and eventually enters the liver, where it is stored in the ester form until it is required elsewhere in the organism. The ester is then hydrolyzed, and vitamin Ax is transported to its target tissue bound to retinol-binding protein (RBP). Since RBP has a molecular weight of only 20,000 and would be easily cleared by the kidneys, it is associated in the bloodstream with another plasma protein, prealbumin. 

A single enzyme is sometimes capable of many various oxidations. In the presence of NADH (reduced nicotinamide adenine dinucleotide), cyclohexanone oxygenase from Acinetobacter NCIB9871 converts aldehydes into acids, formates of alcohols, and alcohols ketones into esters (Baeyer-Villiger reaction), phenylboronic acids into phenols sulfides into optically active sulfoxides and selenides into selenoxides [1034], Horse liver alcohol dehydrogenase oxidizes primary alcohols to acids (esters) [1035] and secondary alcohols to ketones [1036]. Horseradish peroxidase accomplishes the dehydrogenative coupling [1037] and oxidation of phenols to quinones [1038]. Mushroom polyphenol oxidase hydroxylates phenols and oxidizes them to quinones [1039]. 

Biodegradability The general biochemistry of microbial attack on esters is well known and has been thoroughly reviewed. The main steps of ester hydrolysis [50], P-oxidation of long-chain hydrocarbons [51] and oxygenase attack on aromatic nuclei [52] have been extensively investigated. The main structural features which slow or reduce microbial breakdown are the following ... 

Inhibition of the cholinesterase enzymes depends on blockade of the active site of the enzyme, specifically the site which binds the ester portion of acetylcholine (figure 7.31). The organophosphorus compound is thus a pseudosubstrate. However, in the case of some compounds such as the phosphorothionates (parathion and malathion for example), metabolism is necessary to produce the inhibitor. In both cases metabolism by the microsomal mono-oxygenase enzymes occurs in which the sulphur atom attached to the phosphorus is replaced by an oxygen (figure 5,10). 

The provitamin A, /8-carotene, is cleaved in the enterocyte by a soluble oxygenase w hich requires bile salts for its activity [90]. The product, retinal, is reduced by another soluble enzyme to retinol which is esterified chiefly with palmitic acid by a microsomal enzyme, acyl-CoA retinol acyltransferase which is inhibited by taurocholate in vitro this enzyme is very similar to ACAT [91]. Since j8-carotene is taken up by the intestine and rather efficiently converted to retinyl esters which appear in lymph, it must be inferred that the cytosolic oxygenase is exposed to sufficiently high concentrations of bile salts for the cleavage to occur. 

According to the Warwick Biotransformation Club Database in 1987/88 [60], approximately 65% of all applications reported fell into the classes of esterolytic reactions (ester hydrolysis, synthesis or transesterification) (40%) and dehydrogenase reactions (25%). Next in importance were oxygenase-mediated reactions, peptide and oligosaccharide synthesis, which together comprise 24% of the total. Reports of enzymatic procedures for carbon-carbon bond formation were very few in number (2%). All other reaction types comprised less than 10% of the total. 

Enzymatic conversion of ketones to esters is commonly encountered in microbial degradation. A typical transformation in the enzymatic Baeyer-Villiger oxidation, which converts cyclohexanone into the lactone (Scheme 10) using a purified cyclohexanone oxygenase enzyme. Some more examples of Baeyer-Villiger oxidation are given (see pages 95 and 96). 

In a related approach, nonactivated terminal carbons were directly aminated by using a recombinant whole-cell catalyst [33]. In the key steps, an oxygenase and an m-TA were coupled in vivo within a single Escherichia coU host (BL21) (Scheme 4.8). For the oxidation of the alcohol to the respective aldehyde, the NADH-dependent oxygenase AUcBGT from Pseudomonas putida was used, which allowed the oxyfunctionalization of medium-chain-length alkanes, fatty acids [34], and selected fatty acid methyl esters [35]. Subsequent reductive amination was achieved... 

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