Acyl-a-chymotrypsin

In like manner, increased steric bulk in the acyl group of acyl-a-chymotrypsin progressively retards the rate of deacylation [6 in the... 

Acyl-a-chymotrypsins may be isolated if the acyl function can be made sufficiently unre-active by structural variation several have been recrystallised at low pH, conditions which stabilise the acyl enzyme. The stabilised acyl enzymes can be characterised by physical methods and the first example of an X-ray crystallographic study of an enzyme intermediate is that of indolylacryloyl-o -chymotrypsin [21]. 

Firstly, not all enzymes exhibit activity in the crystal (e.g., lysozyme) because neighbouring molecules in the crystal lattice block access to the active site. Secondly, in those enzymes where a conformational change is an obligatory part of the reaction, a reduction in rate may be anticipated if these conformational changes cannot be accomplished readily in the crystal. Thirdly, there is a limitation imposed by diffusion of substrate into and products out of the crystal. Quiocho and Richards [167] showed that with carboxypeptidase Aj, crystals of 5 pm or less were required before the specific activity of the enzyme in the crystal became independent of crystal size. Rossi and Bernhard [168,169] studied the deacylation rate of co-crystallised acylated a-chymotrypsin using a chromophoric substrate. Under conditions where diffusion away of product was not essential for detection of reaction, they showed that deacylation rates were the same in the crystal as in solution. 

Fig. 26. Dependence of hydrolysis of acyl-a-chymotrypsins (t 3) on the reaction of hydroxide ion with the corresponding ethyl esters [11 la]. Deviant points 1, 3-phenylpropionyl 2, 3-(4 -hydroxyphenyl)pro-pionyt 3, 3-(3 -indolyl)propionyl.

Figure 4. Top, relative specific activity of immobilized enzyme versus the molar ratio (ratio of immobilized su ace coupling groups to enzyme immobilized). Key , diazotized lysozyme X. diazotized lipase A, acylated a-chymotrypsin. Bottom, relative specific activity of modified soluble enzymes versus the molar ratio (ratio of soluble coupling reagent to enzyme). Key O, diazobenzenesulfonic acid lysozyme X. diazobenzenesulfonic acid-lipase H, diazobenzenesulfonic acid-chymotrypsin A, acetic anhydride-chymotrypsin. Reproduced, with permission, from Ref. 20.

The 6-chloromethyl substituent (series 5 and 6) is required for the inactivation of a-chymotrypsin. Nevertheless, there is only a transient inactivation of HLE and thrombin through the formation of a stable acyl-enzyme in spite of the presence of this group as demonstrated by the spontaneous or hydroxylamine-accelerated reactivation of the treated enzymes (Scheme 11.3, pathway b).21 HLE is specifically inhibited when such an alkylating function is absent (series 7), always through the formation of a transient acyl-enzyme (Table 11.2). 

Since the imidazolide method proceeds almost quantitatively, it has been used for the synthesis of isotopically labeled esters (see also Section 3.2), and it is always useful for the esterification of sensitive carboxylic acids, alcohols, and phenols under mild conditions. This advantage has been utilized in biochemistry for the study of transacylating enzymes. A number of enzymatic transacylations (e.g., those catalyzed by oc-chymo-trypsin) have been shown to proceed in two steps an acyl group is first transferred from the substrate to the enzyme to form an acyl enzyme, which is then deacylated in a second step. In this context it has been shown[21] that oc-chymotrypsin is rapidly and quantitatively acylated by Af-fraw.s-cinnamoylimidazole to give /ra/w-cinnamoyl-a-chymotrypsin, which can be isolated in preparative quantities and retains its enzymatic activity (see also Chapter 6). 

Hubbard and Kirsch (1972) have recently proposed that histidine may act as a nucleophile in a-chymotrypsin acylation reactions of esters having a good leaving group (jO-nitrophenol). This suggestion was based on a similarity in p-value for acylation by p-substituted nitrophenyl and dinitrophenyl benzoates and nucleophilic attack on these compounds by imidazole, in contrast with less positive p-values for hydroxide ion catalysis. Hammett p-values for hydrolysis of substituted phenyl esters are given in Table 6 and show little apparent trend. The values for hydroxide ion and alcoholate ions are... 

From the second-order rate constant for imidazole-catalysed cyclization of the ethyl ester (34) (8 x 10 M s" ) and the rate constant for acylation of a-chy mo trypsin by N-acetyltyrosine ethyl ester (1600 s ), it can be calculated that, in order to attain a rate constant of the magnitude seen in the a-chymotrypsin reaction, a neighbouring imidazole would have to possess an effective molarity of 200,000 M. An effective concentration of this magnitude is not unreasonable, but it is probable that other factors are also important in the enzymatic reaction. 

In accord with the above scheme, variation of the meta-substituent altered the rate of acylation but not that of deacylation. The reaction of equation (27) is similar to reaction of a-chymotrypsin with phenolic esters. 

The shapes of the curves in Fig. 6 are consistent with a two-step pathway, analogous to that of a hydrolytic enzyme such as a-chymotrypsin,30 in which an initial acylation burst is followed by a slow deacylation reaction. Following a fast preequilibrium binding, the first kinetic step can be attributed to acylation by substrate of the polymer imidazole residue, accompanied by simultaneous release of nit-rophenol(ate). The succeeding kinetic step would then be ascribed to hydrolysis of the acylimidazole leading to carboxylate ion and regenerated imidazole. 

Figure 3. Possible mechanism for (a) formation and (b) breakdown of acyl-enzyme (chymotrypsin) intermediate (3)...

Table 7.3 Kinetic constants for the hydrolysis of S-acyl-L-amino acid esters by a-chymotrypsin at 25°C. pll 7.8, and ionic strength 0.1 M, determined by partitioning experiments" ...

The kinetic and structural data may be combined to give the following qualitative description of the mechanism of acylation of chymotrypsin by a good polypeptide substrate.116... 

Table 12.7 Yields of a-chymotrypsin-catalyzed peptide synthesis in water and ice using maleoyl-L-tyrosine methyl ester as acyl donor and various peptides, amino acid amides, and amino acids as nucleophiles (Schuster, 1990).

Figure 12.7 Left dependence of the yield of the a-chymo-trypsin-catalyzed synthesis of the peptides maleoyl-Tyr-D-Leu-NH2 (o) and maleoyl-Tyr-/3-Ala-Gly-OH ( ) in ice upon the relative amount of nucleophile free base (for conditions, see Table 12.7). Right influence of temperature on the yield of the a-chymotrypsin-catalyzed peptide synthesis in ice using maleoyl-L-Tyr methyl ester as acyl donor and H-D-Leu-NH2 (o), H-Gly-Gly-Gly-OH ( ), and H-Ala-Ala-OH (A) as amino components (for conditions, see Table 12.7) (Schuster, 1990).

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... 

In Equation 11.13, A = k2k3[E]0[S]0/(k2 + k3) / [S]0 + k3Ks/(k2 + k3), which has the form of a Michaelis-Menten equation, B = [E]0[S]0 /(fe + 3) 2/([S]0+ Km(apparent)), and b is a composite rate constant describing the build-up of the acyl enzme intermediate (or, in the general case, the covalently bound enzyme intermediate). The non-linear plot of [Lg ] against time is shown in Fig. 11.10A for a typical substrate of a-chymotrypsin extrapolation of the linear portion gives the intercept shown which allows evaluation of B. 

With regard to the use of protease in the synthetic mode, the reaction can be carried out using a kinetic or thermodynamic approach. The kinetic approach requires a serine or cysteine protease that forms an acyl-enzyme intermediate, such as trypsin (E.C. 3.4.21.4), a-chymotrypsin (E.C. 3.4.21.1), subtilisin (E.C. 3.4.21.62), or papain (E.C. 3.4.22.2), and the amino donor substrate must be activated as the ester (Scheme 19.27) or amide (not shown). Here the nucleophile R3-NH2 competes with water to form the peptide bond. Besides amines, other nucleophiles such as alcohols or thiols can be used to compete with water to form new esters or thioesters. Reaction conditions such as pH, temperature, and organic solvent modifiers are manipulated to maximize synthesis. Examples of this approach using carboxypeptidase Y (E.C. 3.4.16.5) from baker s yeast have been described.219... 

We have made several artificial enzymes that use cyclodextrin to bind a substrate and then react with it by acylating a cyclodextrin hydroxyl group. This builds on earlier work by Myron Bender, who first studied such acylations [83]. We added groups to the cyclodextrin that produced a flexible floor, capping the ring [84]. The result was to increase the relative rate of cyclodextrin acylation by m-t-butylphenyl acetate from 365 relative to its hydrolysis rate in the buffer to a Complex/ buffer of 3300. We changed the substrate to achieve better geometry for the intracomplex acylation reaction, and with a p-nitrophenyl ester of ferroceneacrylic acid 10 we achieved a relative rate for intracomplex acylation of ordinary [3-cyclodextrin vs. hydrolysis of over 50 000 and a Vmax comparable to that for hydrolysis of p-nitrophenyl acetate by chymotrypsin... 

In addition, as is shown in Table 13, the deacylation which is represented in kd is increased by a factor of more than 103 in poly(PHA-IM-am), compared with poly(PHA-am). As is observed in the deacylation which is catalysed by imidazole moiety in the case of a-chymotrypsin, the catalysis by imidazole moiety in the deacylation of the acylated hydroxamic acid is considered. The turnover number, turnover hi poly(PHA-IM-am) is also high. The deacylation rate of poly(PHA-IM-am)... 

However, most nucleophiles attack 5-oxazolones at the carbonyl group and the products are derivatives of a-amino acids formed by acyl-oxygen fission. Thus the action of alcohols, thiols, ammonia and amines leads, respectively, to esters, thioesters and amides orthophosphate anion gives acyl phosphates (Scheme 18). The use of a-amino acids in this reaction results in the establishment of a peptide link. Cysteine is acylated at the nitrogen atom in preference to the sulfur atom. Enzymes, e.g. a-chymotrypsin and papain, also readily combine with both saturated and unsaturated azlactones. A useful reagent for the introduction of an a-methylalanine residue is compound (202). Both the trifluoroacetamido and ester groups in the product are hydrolyzed by alkali to give a dipeptide. The alkaline hydrolyzate may be converted into the benzyloxycarbonyl derivative, which forms a new oxazolone on dehydration. Reaction with an ester of an amino acid then yields a protected tripeptide (equation 45). 

Figurel.10. Stereo diagram of the acyl moiet of the spin-labeled tryptophanyl-acylenzyme reaction intermediate of a-chymotrypsin. Active site residues close to the acyl moiety are labeled (Makinen et al., 1998). Reproduced with permission.

Asymmetrization of a prochiral dicarboxylic acid diester catalyzed by lipases, where the stereo center of the product is located on the acyl side, becomes a single-step process because the polar carboxylic acid and/or amide formed are not well accepted as substrates by the Upase. One example is the enantioselective hydrolysis or ammonolysis of diethyl 3-hydroxyglutarate, as shown in Scheme 7.4, a reaction which leads to the formation of a precursor for the important chiral side chain of atorvastatin, lipitor [40, 41]. The S-enantiomer was formed with high e.e. (98%), but unfortunately this is the undesired enantiomer for the production of the pharmaceutically important product. Only a-chymotrypsin gave a predominance of the... 

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