Glutamate-43, nucleophilic attack

Use of the relatively small cyclopropane ring drastically reduces the potential for deleterious steric bulk effects and adds only a relatively small lipophilic increment to the partition coefficient of the drug. One of the clever elements of the rolicyprine synthesis itself is the reaction of d,l tranylcypromine (67) with L-5-pyrrolidone-2-carboxylic acid (derived from glutamic acid) to form a highly crystalline diastereomeric salt, thereby effecting resolution. Addition of dicyclohexylcarbodiimide activates the carboxyl group to nucleophilic attack by the primary amine thus forming the amide rolicyprine (68). 

Nucleophilic attack by an alcohol on the y phosphate (Pig. 13-10a) displaces ADP and produces a new phosphate ester. Studies with 180-labeled reactants have shown that the bridge oxygen in the new compound is derived from the alcohol, not from ATP the group transferred from ATP is a phosphoiyl (—POf-), not a phosphate (—OPOf-). Phosphoiyl group transfer from ATP to glutamate (Fig. 13-8) or to glucose... 

Using the data of Wilson and Cannan (18), Cleaves (81) was able to show that the rate of formation of pyrrolidone carboxylic acid from glutamic acid in aqueous solution depends directly on the concentration of the ionic species of glutamic acid in solution. Thus, the reactive species are (I), (II), and (IV), while (III) is relatively unreactive. Protonation of the amino group and dissociation of the y-carboxyl group thus makes these groups less reactive carboxylate ion resonance apparently hinders nucleophilic attack by the amino nitrogen. 

Blout observed that the rate of polymerization of y-benzyl glutamic acid NCA was approximately 100-times faster when initiated with sodium methoxide rather than a primary or secondary amine.19,101 He proposed the active monomer mechanism of NCA polymerization previously formulated by Ballard and Bamford,1111 which involves the extraction of the proton from the NCA nitrogen to give anion 3, followed by nucleophilic attack of this anion on the amino acid carbonyl of a second NCA (Scheme 2). The active monomer mechanism was further studied and substantiated by Goodman.112-141... 

One possible mechanism for the hydrolysis of peptides or esters by carboxypeptidase A involves two steps with an anhydride (acyl-enzyme) intermediate.418 In the first step, the zinc(II) activates the substrate carbonyl group towards nucleophilic attack by a glutamate residue, resulting in the production of a mixed anhydride (127). Breakdown of the anhydride intermediate is rate determining with some substrates.419 An understanding of the chemistry of metal ion effects in anhydride hydrolysis is therefore of fundamental importance in regard to the mechanism of action of the enzyme. Until recently there have been few studies of metal ion-catalysed anhydride solvolysis. 

The binding of glycyl-L-tyrosine in the active site pocket of carboxypeptidase A is illustrated in Fig. 15. Tyrosine-248 and glutamic acid-270 are believed to participate in the catalytic reaction and represent the acidic and basic groups, respectively, involved in the bell-shaped pK-rate profile. In the bond-cleavage reaction, the carboxyl group of Glu-270 may act by a nucleophilic attack on the carbonyl group while Tyr-248... 

In addition to the above factors, catalysis by many enzymes proceeds via formation of covalent adducts between the substrate and a group on the enzyme. In many cases, these adducts are formed by nucleophilic attack on the substrate. Typical nucleophiles include the -OH side chains of serine and threonine, the -SH of cysteine, the -CO2" of aspartate and glutamate, the -NH2 of lysine, and the imidazole of histidine. An example of an enzyme that utilizes nucleophilic catalysis is papain, discussed above, where the CysS forms a covalent adduct with the carbonyl carbon of the amide to be cleaved, as shown in Fig. 4.74. 

Schnackerz et al. (1979). The same adduct was isolated from glutamate decarboxylase following inactivation with serine O-sulfate (Likos et al., 1982). The mechanism proposed to account for this observation involves elimination of sulfate from the inactivator followed by transimination with the active site lysine of the enzyme (Fig. 9). The released aminoacrylate then rotates in the active site and acts as a nucleophile, attacking the electrophilic imine. Under basic conditions the enzyme lysine is eliminated, generating the free cofactor adduct. This reaction is therefore unusual in that the inactivator is ultimately the nucleophilic partner in the inactivation reaction, in contrast to all of the reactions described to this point, in which the enzyme provides the nucleophile. 

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