Charge relay system

Figure 7-7. Catalysis by chymotrypsin. The charge-relay system removes a proton from Ser 195, making it a stronger nucleophile. Activated Ser 195 attacks the peptide bond, forming a transient tetrahedral intermediate. Release of the amino terminal peptide is facilitated by donation of a proton to the newly formed amino group by His 57 of the charge-relay system, yielding an acyl-Ser 195 intermediate. His 57 and Asp 102 collaborate to activate a water molecule, which attacks the acyl-Ser 195, forming a second tetrahedral intermediate. The charge-relay system donates a proton to Ser 195, facilitating breakdown of tetrahedral intermediate to release the carboxyl terminal peptide .

A charge relay system (Blow, 1976) exists in a number of hydrolytic enzymes. In such systems, an aspartate carboxylate group buried in a hydrophobic microenvironment activates a seryl hydroxyl group through hydrogen bonding. Thus, it is interesting to study the effect of carboxylate ions on other nucleophiles in aprotic media. 

The mechanism schematized above is a summary of the current knowledge. The role of Asp102 has long been controversial [10], Indeed, the catalytic triad has been depicted as a charge-relay system, meaning that the activation of the serine residue involves a concerted transfer of two protons, i.e., from serine to histidine and then to aspartic acid. More recent studies have shown that aspartic acid remains ionized and serves to stabilize the ionic transition state [6] [14-16],... 

Crystallographic studies (Blow, 1976) of the structure of the enzyme, enzyme-substrate complexes and enzyme-product complexes have identified a common feature in catalysis by the serine protease enzymes such as a-chymotrypsin. This is the well-known charge-relay system (44), in which... 

Hunkapiller et al. (1973) have recently inferred from NMR data that the pKa-value of ca. 7 in serine proteases is actually that of aspartic acid while histidine has a lower pK (< 4) so that at pH-values from 3-6 7 these residues will be neutral. A mechanism was proposed, utilizing the asp-102-hist-57-ser-195 charge relay system which would avoid charge separation. 

CHARGE RELAY SYSTEM CATASTROPHE THEORY Catastrophic depolymerization of microtubules,... 

CHARGE RELAY SYSTEM CHARGE-TRANSEER COMPLEX... 

A possible mechanism by which the peroxide is cleaved (step b in Fig. 16-14) is indicated by the arrows. His 75 and Asn 148 are directly involved, and a charge relay system below the ring may also participate. From Putnam et al.198a... 

Dunn has also proposed a mechanism involving this charge relay system in ternary complex formation, but with the substrate displacing the zinc-bound water, as shown in Scheme 9.1443 Hydride transfer from NADH, to form an alcoholate anion, has been shown to occur before protonation.1398 As well as not requiring penta-coordinate zinc, this mechanism differs from Dworschack and Plapp s in postulating the formation of an alcoholate anion. 

This mechanism, accounting for the observed pH perturbations, does not directly consider the proton charge relay system involving Ser-48 and His-51. However it is probable that this system is important in facilitating, by charge distribution, formation of the alcoholate anion and hydride transfer to NAD+, and in the reverse reaction, neutralization of the alcoholate anion and alcohol dissociation. 

The structural analysis has been carried out right up to the recognition of molecular Level (i, 2). a-Chymotrypsin is poly(amino acid) consisting of 245 amino adds, having relatively deep grooves. It catalyzes the hydrolysis of carboxylic acid derivatives such as protein, simple amides, esters, etc. The active site is composed of aspartic add, Asp (102). .. histidine, His (57). .. serine, Ser (195), and the distances between Asp. .. His and His... Ser are 2.8 A and 3.0 A, respectively. Electronic structures of these moieties depend on the pH of the reaction system. In the range of pH > 7 at which a-chymotrypsin is active, —COO" of Asp attracts N4 proton in imidazolyl of His, and Nj in the imidazolyl of His attracts the proton in OH of Ser. It is called charge-relay system . 

In addition, as stated above, the catalytic active center forms the charge-relay system and creates a high activity in the catalysis. Recent NMR studies support the concept of the charge-relay system (5, 6). By the theoretical calculation the importance of the charge-relay is also discussed (7). In both cases where Asp is absent, and where the water molecule is in place of His, the potential barrier of the reaction becomes higher and the substrate plays a role of reduction of the potential barrier of the reaction by 0.21 eV. 

Essentially, a-chymotrypsin has these characteristics the selectivity in the substrate binding, the charge-relay system in the active center and a contribution of the bound substrate to the catalysis, as cooperativities. 

Such shifts of pK values are considered in terms of the following causes. A coulombic field which is formed by charged moieties on the polymer, polarizes the catalytic moiety and changes its proton affinity. The pK value of the nucleophile would increase in an anionic field and decrease in a cationic field. The ionization of the nucleophile is promoted by base. The hydrogen bond formation in the nucleophile influences its pK value directly as the charge relay system. A hydrophobic field... 

---