Enzyme-catalysed hydrolytic reactions

Hydrolases This group of enzymes catalyse hydrolytic reactions, e.g. penteases (proteins), esterases (esters) etc. 

Chymotrypsin is the most-studied member of the serine protease family of enzymes. The enzyme-catalysed hydrolytic reaction has been shown to occur in at least three kinetically distinguishable steps. The first of these consists of a very fast, diffusion-controlled formation of a non-covalent enzyme-substrate complex, followed by an acylation step. In the latter the acyl group of the substrate is covalently attached to a serine alcohol of the active site with the concomitant release of the amine of an amide substrate, or the alcohol of an ester substrate. In a final deacylation step the acyl-enzyme intermediate is hydrolysed by water, thus regenerating the free enzyme and releasing the carboxylic acid ... 

Hydrolases. Enzymes that catalyse hydrolytic reactions. 

Reactants don t have to be (very) water soluble - use different solvent, or just undissolved solids Changes in solvation alter equilibria and kinetics - e. g. readily available hydrolytic enzymes catalyse synthetic reactions (including direct reversal of hydrolysis)... 

The specificity of enzyme reactions can be altered by varying the solvent system. For example, the addition of water-miscible organic co-solvents may improve the selectivity of hydrolase enzymes. Medium engineering is also important for synthetic reactions performed in pure organic solvents. In such cases, the selectivity of the reaction may depend on the organic solvent used. In non-aqueous solvents, hydrolytic enzymes catalyse the reverse reaction, ie the synthesis of esters and amides. The problem here is the low activity (catalytic power) of many hydrolases in organic solvents, and the unpredictable effects of the amount of water and type of solvent on the rate and selectivity. 

The amino groups are replaced with oxygen. Although here a biochemical reaction, the same can be achieved under acid-catalysed hydrolytic conditions, and resembles the nucleophilic substitution on pyrimidines (see Section 11.6.1). The first-formed hydroxy derivative would then tautomerize to the carbonyl structure. In the case of guanine, the product is xanthine, whereas adenine leads to hypoxanthine. The latter compound is also converted into xanthine by an oxidizing enzyme, xanthine oxidase. This enzyme also oxidizes xanthine at C-8, giving uric acid. 

As mentioned in part 2.1.3 hydrolytic enzymes are the most frequently used enzymes in organic chemistry. There are several reasons for this. Firstly, they are easy to ttse because they do not need cofactors like the oxidoreductases. Secondly, there are a large amormt of hydrolytic enzymes available because of their industrial interest. For instance detergent enzymes comprise proteases, celltrlases, amylases and lipases. Even if hydrolytic enzymes catalyse a chemically simple reaction, many important featirres of catalysis are still contained such as chemo-, regio- and stereoselectivity and specificity. 

Hailing, P.J. (1984) Effects of water on equilibria catalysed by hydrolytic enzymes in biphasic reaction systems. Enzyme Microb. Technol., 6, 513-516. 

An enzyme that catalyses ATP-dependent 2 -phosphorylation and acetyl-CoA-dependent 6 -acetylation of the antibacterial aminoglycosides has been reported.260 Because of its complementary spectrum of two enzymic reactions, this bifunctional enzyme has a wide breadth of activity. Pentacoordinated thiophosphorane intermediates such as (292a) and (292b) are involved in the hydrolytic reactions of the monothioate analogues of 5 -O-methyluridine 2 - and 3 -dhnethylphosphates, (293) and (294), which have been studied over a wide range of HC1 acidities, //<, = —1.7 to pH9.261... 

Proton transfers are an integral part of acid and base catalysed reactions and as such they are important in chemical and biological processes (Jencks, 1969 Bender, 1971 Bell, 1973). DMSO has been found to influence many enzyme catalysed reactions, including hydrolytic processes, and its possible utility in enzymological studies has been noted (Rammler, 1971). 

Enzyme Models .—Two general mechanisms have been proposed for hydrolytic reactions catalysed by bovine pancreatic carboxypeptidase A the first involves formation of an anhydride intermediate and the second involves the residue Glu-270 as a general base. Work on model systems and the enzyme indicates that the general base mechanism is the correct one and a consistent mechanism (Scheme 4) has been proposed in which both zinc and Arg-145 interact with the substrate. 

Esters, amides, hydrazides and carbamates can all be metabolized by hydrolysis. The enzymes which catalyse these hydrolytic reactions, carboxylesterases and amidases, are usually found in the cytosol, but microsomal esterases and amidases have been described and some are also found in the plasma. The various enzymes have different substrate specificities, but carboxylesterases have amidase activity and amidases have esterase activity. The two apparently different activities may therefore be part of the same overall activity. 

Hydrogen cyanide can be produced by hydrolytic reaction catalysed by one or more enzymes from the plants containing cyanogenic glycosides. 

Amylase has been prepared from defatted hawk eye soybean flour. The enzyme-concentration dependence of the initial velocity for the hydrolytic reaction was investigated at pH 5.4 in a range of the enzyme concentrations and it was found that the initial velocity was proportional to the enzyme concentration in this range. The hydrolyses of maltodextrin (DPn = 74.4) and soluble starch catalysed by soybean /3-amylase were investigated in the pH range from 3.0 to 9.1 at 25 C, and and kjnax each substrate were determined at each pH. The pH-rate profile showed a bell-shaped curve, and the pH optimum was at 5.85. From Dixon plots of V and the pAT values were found to be 3.5 and... 

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