Cysteine groups esterases

In enzymes, the most common nucleophilic groups that are functional in catalysis are the serine hydroxyl—which occurs in the serine proteases, cholinesterases, esterases, lipases, and alkaline phosphatases—and the cysteine thiol—which occurs in the thiol proteases (papain, ficin, and bromelain), in glyceraldehyde 3-phosphate dehydrogenase, etc. The imidazole of histidine usually functions as an acid-base catalyst and enhances the nucleophilicity of hydroxyl and thiol groups, but it sometimes acts as a nucleophile with the phos-phoryl group in phosphate transfer (Table 2.5). 

The preparation of protected (/ )-2-methyl-cysteine by Fukuyama starts with the enantio-selective discrimination of the prochiral ester groups in 6 with pig liver esterase (Scheme 3) [5]. The ester function of the resulting product 7 is selectively reduced (7 16). Cyclization to the )9-lactone gives compound 17. Attack of the thioacetate at the )9-lactone methylene carbon atom provides the (/f)-compound 18. Selective reduction of the carboxylic acid function in 7 gives the (S)-compound 19 in an analogous fashion. 

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). 

As the knowledge of protein structure increases, we may expect to know more about the mechanisms of enzyme action. Already information on the amino acid sequences around the active centres of quite a few enzymes has been obtained, and it has been observed that there are striking similarities amongst the active centres of various different enzymes within the same general class, such as phosphatases and esterases. Most notably, nearly all of these enzymes seem to possess at their active centres one or more serines whilst other classes of enzyme make use of the sulphur-containing amino acid cysteine, suggesting that —SH groups play a role in complex formation in these cases. It may well be that... 

Many pesticides are esters or amides that can be activated or inactivated by hydrolysis. The enzymes that catalyze the hydrolysis of pesticides that are esters or amides are esterases and amidases. These enzymes have the amino acid serine or cysteine in the active site. The catalytic process involves a transient acylation of the OH or SH group in serin or cystein. The organo-phosphorus and carbamate insecticides acylate OH groups irreversibly and thus inhibit a number of hydrolases, although many phosphorylated or carbamoylated esterases are deacylated very quickly, and so serve as hydrolytic enzymes for these compounds. An enzyme called arylesterase splits paraoxon into 4-nitrophenol and diethyl-phosphate. This enzyme has cysteine in the active site and is inhibited by mercury(ll) salts. Arylesterase is present in human plasma and is important to reduce the toxicity of paraoxon that nevertheless is very toxic. A paraoxon-splitting enzyme is also abundant in earthworms and probably contributes to paraoxon s low earthworm toxicity. Malathion has low mammalian toxicity because a carboxyl esterase that can use malathion as a substrate is abundant in the mammalian liver. It is not present in insects, and this is the reason for the favorable selectivity index of this pesticide. 

Yet another example of the catalytic triad has been found in carboxy-peptidase II from wheat. The structure of this enzyme is not significantly similar to either chymotrypsin or subtilisin (Figure 9.15). This protein is a member of an intriguing family of homologous proteins that includes esterases such as acetylcholine esterase and certain lipases. These enzymes all make use of histidine-activated nucleophiles, but the nucleophiles may be cysteine rather than serine. Finally, other proteases have been discovered that contain an active-site serine or threonine residue that is activated not by a histidine-aspartate pair but by a primary amino group from the side chain of lysine or by the N-terminal amino group of the polypeptide chain. 

The mechanism of amide- and ester-hydrolyzing enzymes is very similar to that observed in the chemical hydrolysis by a base. A nucleophilic group from the active site of the enzyme attacks the carbonyl group of the substrate ester or amide. This nucleophilic chemical operator can be either the hydroxy group of a serine (e.g., pig fiver esterase, subtifisin, and the majority of microbial lipases), a carboxyl group of an aspartic acid (e.g., pepsin) [3], or the thiol functionality of cysteine (e.g., papain) [4-6]. 

The thiol enzyme for which the most detailed mechanistic formulations have been proposed is papain . In this enzyme a cysteine thiol group appears to function in the same manner as the serine hydroxyl of other proteases and esterases. In the hydrolysis of proteins by this plant protease there is an intermediate formation of an acyl thiol, which is subsequently cleaved by water. 

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