Facilitated tetrahedral intermediate

Aminolysis of esters often reveals general base catalysis and, in particular, a contribution to the reaction rate fi om terms that are second-order in the amine. The general base is believed to function by deprotonating the zwitterionic tetrahedral intermediate. Deprotonation of the nitrogen facilitates breakdown of the tetrahedral intermediate, since the increased electron density at nitrogen favors expulsion of an anion ... 

When written in this way it is clear what is happening. The mechanisms of these reactions are probably similar, despite the different p values. The distinction is that in Reaction 10 the substituent X is on the substrate, its usual location but in Reaction 15 the substituent changes have been made on the reagent. Thus, electron-withdrawing substituents on the benzoyl chloride render the carbonyl carbon more positive and more susceptible to nucleophilic attack, whereas electron-donating substituents on the aniline increase the electron density on nitrogen, also facilitating nucleophilic attack. The mechanism may be an addition-elimination via a tetrahedral intermediate ... 

Figure 7-6. Mechanism for catalysis by an aspartic protease such as HIV protease. Curved arrows Indicate directions of electron movement. Aspartate X acts as a base to activate a water molecule by abstracting a proton. The activated water molecule attacks the peptide bond, forming a transient tetrahedral Intermediate. Aspartate Y acts as an acid to facilitate breakdown of the tetrahedral intermediate and release of the split products by donating a proton to the newly formed amino group. Subsequent shuttling of the proton on Asp X to Asp Y restores the protease to its initial state.

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 .

It is apparent that steric bulk and stereochemical control of mechanism operates in the alkaline hydrolysis of methyl 8-acyl-1-naphthoates. The proximity and favourable orientation of the carbonyl group at the 8-position facilitates intramolecular catalysis from this group. However, the formation of the tetrahedral intermediate at the 8-acyl carbonyl group has distinct... 

Transition metal ions cause a dramatic increase in the rate of hydrolysis of /Madam antibiotics [75][133][134], For example, copper(II) and zinc(II) ions increase the rate of alkaline hydrolysis ca. 108-fold and 104-fold, respectively [76], It has been suggested that the metal ion coordinates with both the carboxylate group and the /3-lactam N-atom of penicillins (A, Fig. 5.20). This complex stabilizes the tetrahedral intermediate and, thus, facilitates cleavage of the C-N bond catalyzed by the HO ion [74] [75], Such a model appears applicable also to clavulanic acid, imipenem, and monobactams, but it re-... 

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

The collapse of the proteolytic tetrahedral intermediate of the promoted-water pathway requires a proton donor in order to facilitate the departure of the leaving amino group. Rees and Lipscomb (1982) considered Glu-270, but favored Tyr-248 for this role, but Monzingo and Matthews (1984) fully elaborated on a role for Glu-270 of carboxypeptidase A and Glu-143 of thermolysin as intermediate proton donors. This proposal for carboxypeptidase A is corroborated by the near-normal activity observed for the Tyr-248- Phe mutant of rat carboxypeptidase A (Garden et al, 1985 Hilvert et al, 1986) and is reflected in the mechanistic scheme of Fig. 31 (Christianson and Lipscomb, 1989). Mock (1975) considered Glu-270 a proton donor in the carboxypeptidase A mechanism, but his mechanism does not favor a Glu-270/zinc-promoted water molecule as the hydrolytic nucleophile. Schepartz and Breslow (1987) observed that Glu-270 may mediate an additional proton transfer in the generation of the Pi product carboxylate. 

Hydrolysis reactions involving tetrahedral intermediates are subject to steric and electronic effects. Electron-withdrawing substituents facilitate, but electron-donating and bulky substituents retard basic hydrolysis. Steric effects in acid-catalyzed hydrolysis are similar to those in base-catalyzed hydrolysis, but electronic effects are much less important in acid-catalyzed reactions. Higher temperatures also accelerate the reaction. 

Evidence for the tetrahedral intermediate includes a Hammett p constant of+2.1 for the deacylation reaction of substituted benzoyl-chymotrypsins and the formation of tetrahedral complexes with many inhibitors, such as boronates, sulfonyl fluorides, peptide aldehydes, and peptidyl trifluoromethyl ketones. In these last the chemical shift of the imidazole proton is 18.9 ppm, indicating a good low-barrier H-bond, and the pJQ of the imidazolium is 12.1, indicating that it is stabilized by 7.3 kcal mol 1 compared to substrate-free chymotrypsin. The imidazole in effect is a much stronger base, facilitating proton removal from the serine. 

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