Enzymatic synthesis of X-Phe-Leu-NH2 in low water content systems: influence of the N-alpha protecting group and the reaction medium composition

Optimal alpha-chymotrypsin-catalyzed synthesis of N-Ac-Phe-Gly-NH(2)

N-Acetyl-phenylalanine-glycinamide (N-Ac-Phe-Gly-NH(2)), a type of dipeptide derivative, was synthesized from N-acetyl phenylalanine ethyl ester and glycinamide and catalyzed by alpha-chymotrypsin, a protease, in a biphasic system. Response surface methodology with a four-factor, five-level central composite rotatable design was employed to evaluate the effects of selected parameters that included incubation time, reaction temperature, enzyme activity, and pH level on the yield of the dipeptide derivative. The results indicated that pH significantly affected the yield of N-Ac-Phe-Gly-NH(2). In a ridge max analysis, the optimum condition for this synthesis included an incubation time of 30.9 min, a reaction temperature of 35.8 degrees C, an enzyme activity of 159.2 U, and a pH of 8.98. The predicted and the actual (experimental) yields were 98.0 and 95.1%, respectively.

Solid-phase peptide synthesis by ion-paired alpha-chymotrypsin in nonaqueous media

Solid-phase synthesis of dipeptides in low-water media was achieved using AOT ion-paired alpha-chymotrypsin solubilized in organic solvents. Multiple solvents and systematic variation of water activity, a(w), were used to examine the rate of coupling between N-alpha-benzyloxycarbonyl-L-phenylalanine methyl ester (Z-Phe-OMe) and leucine as a function of the reaction medium for both solid-phase and solution-phase reactions. In solution, the observed maximum reaction rate in a given solvent generally correlated with measures of hydrophobicity such as the log of the 1-octanol/water partitioning coefficient (log P) and the Hildebrand solubility parameter. The maximum rate for solution-phase synthesis (13 mmol/h g-enzyme) was obtained in a 90/10 (v/v) isooctane/tetrahydrofuran solvent mixture at an a(w) of 0.30. For the synthesis of dipeptides from solid-phase leucine residues, the highest synthetic rates (0.14-1.3 mmol/h g-enzyme) were confined to solvent environments that fell inside abruptly defined regions of solvent parameter space (e.g., log P > 2.3 and normalized electron acceptance index <0.13). The maximum rate for solid-phase synthesis was obtained in a 90/10 (v/v) isooctane/tetrahydrofuran solvent mixture at an a(w) of 0.14. In 90/10 and 70/30 (v/v) isooctane/tetrahydrofuran environments with a(w) set to 0.14, seven different N-protected dipeptides were synthesized on commercially available Tentagel support with yields of 74-98% in 24 h.

Enzymes in low water systems

Water is fundamental for enzyme action and for formation of the three-dimensional structure of proteins. Hence, it may be assumed that studies on the interplay between water and enzymes can yield insight into enzyme function and formation. This has proven correct, because the numerous studies that have been made on the behavior of water-soluble and membrane enzymes in systems with a low water content (reverse micelles or enzymes suspended in nonpolar organic solvents) have revealed properties of enzymes that are not easily appreciated in aqueous solutions. In the low water systems, it has been possible to probe the relation between solvent and enzyme kinetics, as well as some of the factors that affect enzyme thermostability and catalysis. Furthermore, the studies show that low water environments can be used to stabilize conformers that exhibit unsuspected catalytic properties, as well as intermediates of enzyme function and formation that in aqueous media have relatively short life-times. The structure of enzymes in these unnatural conditions is actively being explored.

Enzymatic peptide synthesis in low water content systems: preparative enzymatic synthesis of [Leu]- and [Met]-enkephalin derivatives

A novel total enzymatic synthesis of [Leu]- and [Met]-enkephalin derivatives was accomplished in low-water content systems at a preparative scale. alpha-Chymotrypsin, papain, thermolysin and bromelain adsorbed on Celite were used as catalysts. Organic solvents such as acetonitrile and ethyl acetate with small amounts of buffer added or at specific water activity were used as reaction media. Simple readily available amino acid ester derivatives were used as starting building blocks. This feature allowed the possibility of using the products in one step directly as acyl-donor ester, without any chemical or enzymatic modification, in the next enzymatic coupling. The optimal strategy for the synthesis of the enkephalin derivatives was different depending on the carboxy terminal group. The preparation of the carboxy-terminal amide derivatives (R-Tyr-Gly-Gly-Phe-Leu[Met]-NH2) was achieved via 4 + 1 fragment condensation catalyzed by alpha-chymotrypsin. The carboxy-terminal ethyl ester derivatives (R-Tyr-Gly-Gly-Phe-Leu[Met]-OEt) were obtained via 2 + 3 condensation catalyzed by bromelain, a quite unusual protease for peptide synthesis but more effective than papain in this coupling. Both syntheses were carried out in four enzymatic steps and one or two chemical deprotection steps routinely used in peptide synthesis. The overall yields of pentapeptide derivatives were between 40-54% of pure product.

Pepsin-catalyzed peptide synthesis in organic media: studies with free and immobilized enzyme

Pepsin-catalyzed synthesis of protected peptides was studied in two-phase systems containing up to 5% (by volume) of aqueous phase. A methodological study was carried out to determine the optimum conditions for the synthesis of the model protected peptide Z-Phe-Phe-OMe. Several parameters such as concentrations of carboxylic and amino components, pH of the aqueous phase, ratio of organic to aqueous phase volumes and nature of the organic solvent were investigated. It was observed that the most hydrophobic solvents produced the best yields, despite the low solubility of substrates in these media. The log P of the solvent could be used to predict the solvent effect over the reaction yields. Pepsin immobilized by adsorption onto the solid supports Celite and Chromosorb was employed to perform a study of secondary specificity of the enzyme in organic media through the coupling between Z-X-Phe-OH (X = Ala, Asp, Glu, Gly, Phe, Ile, Val, Trp and Tyr) and Phe-OMe. This investigation was performed in two solvent systems: (A) ethyl acetate:citrate buffer pH 4.5 (98:02, v:v) and (B) acetonitrile:citrate buffer pH 4.5 (96:04, v:v). Reaction rate data showed that pepsin had a preference for more hydrophilic substituents in the P2 position. These data are in contrast to the literature for a similar reaction performed in predominantly aqueous media. Thus, for mainly organic media, partition phenomena are very important and may cause an apparent modification of enzyme specificity.