Studies of the chymotrypsinogen family of proteins. IX. Steady-state kinetics of the chymotryptic hydrolysis of N-acetyl-L-tryptophan ethyl ester at pH 8.0

Chiral phosphine lewis bases catalyzed asymmetric aza-Baylis-Hillman reaction of N-sulfonated imines with activated olefins

In the aza-Baylis-Hillman reaction of N-sulfonated imines (N-arylmethylidene-4-methylbenzenesulfonamides and others) with methyl vinyl ketone, ethyl vinyl ketone, and acrolein, we found that, in the presence of a catalytic amount of chiral phosphine Lewis base such as (R)-2'-diphenylphosphanyl-[1,1']binaphthalenyl-2-ol LB1 (10 mol %) and molecular sieve 4A, the corresponding aza-Baylis-Hillman adducts could be obtained in good yields with good to high ee (70-95% ee) at low temperature (approximately -30 to -20 degrees C) or at room temperature in THF, respectively. In CH2Cl2 upon heating at 40 degrees C, the aza-Baylis-Hillman reaction of N-sulfonated imines with phenyl acrylate or naphthyl acrylate gave the adducts in good to high yields (60-97%) with moderate ee (52-77%). The mechanistic insight has been investigated by 31P and 1H NMR spectroscopic measurements. The key enolate intermediate, which has been stabilized by intramolecular hydrogen bonding, has been observed by 31P and 1H NMR spectroscopy. An effective bifunctional Lewis base and Bronsted acid phosphine Lewis base system has been disclosed in this catalytic, asymmetric aza-Baylis-Hillman reaction.

The specificity in the elementary steps of alpha-chymotrypsin catalysis. A temperature study with a series of N-acetyl-L-amino acid methyl esters

The method of 'added nucleophile' (methanol) was employed to study the steady-state kinetics of the alpha-chymotryptic hydrolysis of methyl esters of N-acetyl-L-amino acids of the RCH(NHCOCH3)-C(O)OCH3 type, which are derivatives of alpha-aminobutyric acid, norvaline, norleucine, phenylalanine, alpha-aminoheptanoic acid and alpha-aminooctanoic acid. In the 5 to 30 degrees C temperature range at pH 8.0 the elementary rate and binding constants for both hydrolysis (Ks, k2, k3) and methanolysis (k-2) of the intermediate acylenzyme have been determined. The following results are discussed: 1. The enthalpy-entropy compensation phenomenon, which has previously been established to take place in alpha-chymotryptic catalysis, is characterized by two values of isokinetic temperatures (Tc), i.e., about 200 K at the step of binding the substrate and 430--460 K at the chemical steps of the enzymatic reaction. 2. It is demonstrated how the free energy and enthalpy, as well as entropy, change along all the reaction coordinate. To explain the results obtained, a mechanism for enzyme-substrate interaction is suggested according to which the sorption of substrate group R on the enzyme is different in the metastable intermediates and in the transition states of the reaction: (i) in the enzyme-substrate complex and in the acylenzyme, substrate group R merely emerges from water to be buried in the protein. Two facts point to the hydrophobic nature of the E-R interaction. First, the change in the free energy is exactly delta GRextr, that is the free energy of the transfer (extraction) of substrate group R from water to the organic solvent. Second, the motive force of the E-R interaction is the change in the entropy only. (ii) In the transition state the situation is different, i.e., the E-R interaction becomes thermodynamically still more favourable (the free energy change is 2 delta GRtrans) and involves thermodynamically favourable changes not only in the entropy (as is the case in the intermediates), but also in the enthalpy. Hence the E-R interaction in the transition states is accompanied by conformational and solvational changes of the enzyme-substrate system and, first and foremost, of the entire protein.

Thermodynamics of binding to native alpha-chymotrypsin and to forms of alpha-chymotrypsin in which catalytically essential residues are modified; a study of "productive" and "nonproductive" associations

The standard free energy (deltaG degrees), enthalpy (deltaH degrees), and entropy (deltaS degrees) of association for proflavin and D- and L-N-AcTrp have been obtained at pH 7.8 for native alpha-chymotrypsin (Cht) and for forms of Cht in which essential catalytic residues of the active site are modified. The modified Cht forms studied are dehydroalaninyl-195-alpha-Cht and N-methylhistidinyl-57-alpha-Cht. Associations to native Cht (pH 7.8) are characterized by negative deltaH degrees and deltaS degrees values (i.e., for L-AcTrp deltaH degrees = -9.1 kcal/mol and deltaS degrees = -21 eu at T = 25 degreesC). In contrast, we found associations to modified Chts to be characterized by an enthalpy near zero and a positive entropy of association, the values of the deltaH degrees and deltaS degrees for association to the modified Cht forms being similar to those expected for transfer of small aromatic molecules from water to a nonpolar solvent phase. Differences in deltaH degrees and deltaS degrees observed for binding of substrate analogues and inhibitors to modified and native Cht (pH 7.8) are approximately + 10 kcal/mol and +30 eu, respectively. Data from D. D. F. Shiao ((1970), Biochemistry 9, 1083) similarly show differences of comparable magnitude between binding of substrate analogues to active alpha-Cht (pH 7.8) and the His-57 protonated form of alpha-Cht (pH 5.6). The negative deltaH degrees and deltaS degrees values of associations for binding to active alpha-Cht indicate that a substrate-induced conformational change occurs on substrate association with the primary binding site (S1), which does not occur in Ser-195 and His-57 modified Cht. From these differences we infer a linkage between binding of substrate into S1 and the catalytic residues in the nucleophilic subsite (S1-S1'). Our data also show that associations of substrate analogues into potentially productive Michaelis complexes S1 cannot be easily differentiated from associations that are nonproductive (i.e., nonactivated) from their deltaG degrees obsd, but may be differentiated by their respective deltaH degrees obsd and deltaS degrees obsd for association. Accordingly, it is indicated that the probable substrate association-activation process, characterized thermodynamically in this work, occurs in the substrate binding step and leads to lowered free energies of activation in catalytic steps succeeding binding however, the process does not influence the observed strength of substrate binding.

Molecular orbital studies of enzyme activity: I: Charge relay system and tetrahedral intermediate in acylation of serine proteinases

The charge relay ststem and its role in the acylation of serine proteinases is studied using the partial retention of diatomic differential overlap (PRDDO) technique to perform approximate ab initio molecular orbital calculations on a model of the enzyme-substrate complex. The aspartate in the charge relay system is seen to act as the ultimate proton acceptor during the charging of the serine nucleophile. A projection of the potential energy surface is obtained in a subspace corresponding to this charge transfer and to the coupled motions of active site residues and the substrate. These results together with extended basis set results for cruder models suggest that a concerted transfer of protons from Ser-195 to His-57 and from His-57 to Asp-102 occurs with an energy barrier of 20-25 kcal/mole (84-105 kJ/mole). The subsequent nucleophilic attack on the scissile peptide linkage by the charged serine is then seen to proceed energetically downhill to the tetrahedral intermediate. The formation of the tetrahedral intermediate from the Michaelis complex is calculated to be nearly thermoneutral.