Esterases acyl transfer

Clearly, the oxyanion hole is now as significant a feature of the binding site of such acyl transfer abzymes as it is already for esterases and peptidases — and not without good reason. Knossow has analysed the structures of three esterase-like catalytic antibodies, each elicited in response to the same phosphonate TSA hapten (Charbonnier et al., 1997). Catalysis for all three is accounted for by transition state stabilization and in each case there is an... 

Kinetic optical resolution of racemic alcohols and carboxylic acids by enzymatic acyl transfer reactions has received enormous attention in recent years56. The enzymes generally employed are commercially available lipases and esterases, preferentially porcine liver esterase (PLE) or porcine pancreatic lipase (PPL). Lipases from microorganisms, such as Candida cylindracea, Rhizopus arrhizus or Chromobacterium viscosum, are also fairly common. A list of suitable enzymes is found in reference 57. Standard procedures are described in reference 58. Some examples of the resolution of racemic alcohols are given39. 

One particularly interesting area which has not been subjected to detailed study is metal-promoted acyl transfer. Studies on esterases and peptidases have shown that acyl transfer occurs to a nucleophilic group of the enzyme within an enzyme-substrate complex and the acyl group is then hydrolyzed in the second step. Many of these enzymes, e.g. carboxypeptidase, contain zinc(II). 

Until the last decade or so, the only synthetically useful catalytic asymmetric acyl transfer processes were biotransformations using hydrolase enzymes particularly lipases and esterases [24]. Various lipases and esterases provide high levels of stereoselectivity (s) for the acylative KR and ASD of a wide variety of sec-alcohols and some amines, although the latter transformations have been less thoroughly explored [25-28]. However, the preparative use of enzymes is associated with a number of well-documented limitations, including their generally... 

Comprehensive Biological Catalysis—a Mechanistic Reference Volume has recently been published. The fiiU contents list (approximate number of references in parentheses) is as follows S-adenosylmethionine-dependent methyltransferases (110) prenyl transfer and the enzymes of terpenoid and steroid biosynthesis (330) glycosyl transfer (800) mechanism of folate-requiring enzymes in one-carbon metabohsm (260) hydride and alkyl group shifts in the reactions of aldehydes and ketones (150) phosphoenolpyruvate as an electrophile carboxyvinyl transfer reactions (140) physical organic chemistry of acyl transfer reactions (220) catalytic mechanisms of the aspartic proteinases (90) the serine proteinases (135) cysteine proteinases (350) zinc proteinases (200) esterases and lipases (160) reactions of carbon at the carbon dioxide level of oxidation (390) transfer of the POj group (230) phosphate diesterases and triesterases (160) ribozymes (70) catalysis of tRNA aminoacylation by class I and class II aminoacyl-tRNA synthetases (220) thio-disulfide exchange of divalent sulfirr (150) and sulfotransferases (50). 

Other thermophilic enzymes from archaea may be used for analytical and preparative purposes. For instance, the glucose dehydrogenase from S. solfataricus [70] may serve as a suitable tool for glucose determination, whereas the relatively broad substrate spectrum of the esterase from S. acidocaldarius (catalyzes the acyl transfer to various alcohols and amines [71]) or of the alcohol dehydrogenase from S. solfataricus (oxidizes various aliphatic and aromatic alcohols [72]) makes these enzymes rather attractive for... 

The esterases and lipases are members of a still larger group of enzymes that catalyze acyl transfer, either in the direction of solvolysis or by acylation of the substrate. Both types of enzymes are called hydrolases. In water, hydrolysis occurs, but in the presence of alcohols, transesterification can occur. Reactions in the acylation direction are done in the presence of acyl donors. Esters of enols such as vinyl acetate or isopropenyl acetate are often used as sources of the acyl group. These enol esters are more reactive than alkyl esters, and the enol that is displaced on acyl transfer is converted to acetaldehyde or acetone. To avoid side products arising from these carbonyl compounds, one can use 1-ethoxyvinyl esters, which give ethyl acetate as the by-product. ... 

Reversal of an esterase-type reaction was the only mechanism demonstrated for wax ester biosynthesis until 1967 (Kolattukudy, l%7a). Under physiological conditions an acyl transfer mechanism would be expected to be involved in such ester synthesis. Because of the high thioesterase activ-... 

When A -nucleophiles such as ammonia, amines or hydrazine are subjected to acyl-transfer reactions, the corresponding Af-acyl derivatives - amides or hydrazides - are formed through interception of the acyl-enzyme intermediate by the iV-nucleophile (Schemes 2.1 and 3.21) [262]. Due to the pronounced difference in nucleophiU-city of the amine (or hydrazine) as compared to the leaving alcohol (R -OH) (Scheme 3.21), aminolysis reactions can be regarded as quasi-irreversible. Any type of serine hydrolase which forms an acyl-enzyme intermediate (esterases, lipases, and most proteases) is able to catalyze these reactirms. Among them, proteases such as subtilisin and peniciUin acylase and lipases from Candida antarctica and Pseudomonas sp. have been used most often. 

This is the most common application of biocatalysis in organic synthesis and represents the majority of published examples. Enzymes that catalyze acyl transfer reactions of esters and amides are widely distributed in nature and belong to the lipase/esterase and protease/amidase families, respectively. They play key roles in the metabolism of lipids and proteins and the choice of names, lipase versus esterase, is subject to debate. Normally, acyl ttansfer occurs almost exclusively to water, resulting in hydrolysis. This is particularly valuable for amide hydrolysis that normally requires forcing conditions and strong acid or... 

Hydrolases catalyze cleavage of substrates by the addition of water. This chapter focuses on hydrolases that cleave carboxylic acid derivatives such as esters and amides lipases, esterases, and proteases. By carrying out reactions in organic solvents without water, hydrolases can also catalyze acyl transfer reactions to make esters and amides. 

The enzyme works optimally at pH > 9 with N-protected amino acid esters as acyl donors, and amino acids or amino acid amides as nucleophiles (acceptors). At that pH value it has high esterase activity an amino acyl-enzyme complex will be formed rapidly which transfers the acyl residue to the acceptor molecule (transpeptidation), and since at alkaline pH the rate of peptide cleaving is minimal, the product formed will be released in excellent yield. As an example the last coupling step in a synthesis of the opioide peptide Met-enkephalin that started with benzoylarginine ethylester may be shown (Fig. 8). 

Thrombin attacks synthetic esters, namely tosyl l-arginine methyl ester (TAMe), and when thrombin is treated with DIPT, the loss of esterase activity is proportional to the loss of clotting activity. Studies with synthetic esters have suggested that, as with trypsin, the reaction between thrombin and its substrates occurs in three steps an enzyme-substrate complex is formed the acyl portion of the ester is transferred to the enzyme, which becomes acylated while the alcoholic portion of the ester is released and the enzyme is deacylated and the peptide is transferred to water. The natural substrate of thrombin is fibrinogen. In addition to converting fibrinogen to fibrin, thrombin releases the contractile protein from platelets, activates fibrinase, and may participate in the conversion of prothrombin to thrombin. 

Several systems for formation and hydrolysis of cholesteryl esters in rat liver are known. Microsomes contain an acyl-CoA cholesterol acyltransferase, which requires coenzyme A and ATP for fatty add activation, and operates at neutral pH. Enzymic transfer of fatty adds from lecithin to cholesterol occurs in the soluble fraction of rat liver. A third enzyme, cholesterol esterase, occurs in rat liver and its main function is probably hydrolytic. Although human liver apparently does not have acyl-CoA cholesterol acyltransferase activity, it does have a reversible cholesterol esterase (E.C.3.1.1.13) with optimal... 

A. Fatty Acid Oxidation. B. Acyl Activation and Transfer. C. Lipases and Esterases. D. Phospholipid and Steroid Enzymes. 

Esterases of LAB can also catalyze the direct synthesis of esters from glycerides and alcohols via a transferase reaction (alcoholysis) in aqueous systems, that is the transfer of fatty acyl groups from glycerides to alcohols. This topic requires further research (Holland et al. 2005), as well as the pathways of formation of other lipid-derived aroma compounds in LAB. 

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