Chymotrypsin steric effect

Transition state theory, 46,208 Transmission factor, 42,44-46,45 Triosephosphate isomerase, 210 Trypsin, 170. See also Trypsin enzyme family active site of, 181 activity of, steric effects on, 210 potential surfaces for, 180 Ser 195-His 57 proton transfer in, 146, 147 specificity of, 171 transition state of, 226 Trypsin enzyme family, catalysis of amide hydrolysis, 170-171. See also Chymotrypsin Elastase Thrombin Trypsin Plasmin Tryptophan, structure of, 110... 

Fig. 2.14. Influence of the steric effect on deacylation of two acyl-chymotrypsins (according to Bender etal., 1964). a N-acetyl-L-tyrosyl-chymotrypsin, b acetyl-chymotrypsin...

There is now convincing evidence that an acyl chymotrypsin intermediate is formed from both specific and non-specific substrates (Bender and Kezdy, 1964 Bender et al., 1964). This intermediate is undoubtedly an acylserine. Acyl- and phosphorylserine derivatives have been isolated and identified. In view of evidence such as a D2 O solvent isotope effect ( h2oAd2o) 2-3 for both acylation and deacylation (Bender and Hamilton, 1962), alcohol and amine nucleophiles showing little dependence on the p/iTa-value of the nucleophile in reaction with furoyl enzyme (Inward and Jencks, 1965), and the influence of increasing steric bulk in the acyl group (Fife and Milstien, 1967 Milstien and Fife, 1968,.1969), consistent... 

Based on the known substrate specificity of a-chymotrypsin, phenylalanine has been chosen as the amino acid at the Pi position (P-nomenclature according to Schechter and Berger) [55]. The a-proton at Pi has been substituted either by methyl or trifluoromethyl. Substitutions beyond Pi contain trifluoromethyl alanine or aminoisobutyric acid. Therefore, each fluorosubstitution can be compared to its natural as well as fluorine-free a-substituted analog, thereby enabling differentiation of the steric and electronic effects. Scheme 2 summarizes the amino acids that have been used in this study. 

Most of the earlier studies on the immobilization of enzymes were directed towards the attachment of the enzymes to water-insoluble polymeric supports such as cellulose dextran derivatives, polyacrylamide and porous glass Diffusion problems and steric hindrance are two main factors affecting the application of such supports. The introduction of soluble polymers for immobilization purposes overcomes these difficulties to a greater extent. These soluble enzyme derivatives were synthesized in order to increase the effective molecular size of parent en mes this would rmit the use of ultrafiltration without any los of the enzyme. O NeiD etal. immobilized the enzyme chymotrypsin on soluble dextran for... 

Biological properties such as the reactivity of enzymes, toxicity or drug action usually involve a combination of fundamental effects that give rise to equations which include more than one parameter. The Hansch equation is usually modified by the addition of terms such as steric parameters ( J or polar substituent constants (a). Examples are found in the catalytic action of a-chymotrypsin on 4-nitrophenyl esters of aliphatic acids (Equation 35). 

Besides proximity effect and orientation effect, steric compression effect could be a third factor to consider. Nonetheless there can still be an enhancement of 10 to 10" in rate constants not accounted for by these factors. Among the likely candidates, electrostatic stabilization of the transition state and release of ground state strain should be mentioned. The notion of freezing of substrate specificity by Bender is also a factor for consideration. An example is the aromatic hole in a-chymotrypsin (see below) which allows a favorable steric situation for the amino add side chain of the substrate. 

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