Transition states amide hydrolysis

Tetrhedral intermediate, 172 Thermodynamic cycles, 186 Thermolysin, zinc as cofactor for, 204 Thrombin, 170 Torsional potential, 111 Transition states, 41-42,44, 45,46, 88, 90-92 in amide hydrolysis, 219-221 oxyanion hole and, 181 stabilization of, 181,181 carbonium ion, 154,155,156-161, 167-169 for gas-phase reactions, 43... 

Transition state theory, 46,208 Transmission factor, 42,44-46,45 Triosephosphate isomerase, 210 Trypsin, 170. See also Trypsin enzyme family active site of, 181 activity of, steric effects on, 210 potential surfaces for, 180 Ser 195-His 57 proton transfer in, 146, 147 specificity of, 171 transition state of, 226 Trypsin enzyme family, catalysis of amide hydrolysis, 170-171. See also Chymotrypsin Elastase Thrombin Trypsin Plasmin Tryptophan, structure of, 110... 

The first substrate analogue inhibitors of FAAH were reported in 1994. The anandamide analogues prepared represented three elasses of putative transition-state inhibitors a-trifluoromethyl ketones, a-ketoesters and a-ketoamides [62], In the initial sereening studies, it was found that the trifluoromethyl ketone eompounds tested were effeetive inhibitors of AEA hydrolysis. A selected set of a-keto esters also inhibited hydrolysis, while a-keto amides were ineffective. In particular, arachidonyl trifluoromethyl ketone (32), gave almost 100% inhibition of anandamide hydrolysis. A detailed investigation of the structural requirements for FAAH inhibition with a-trifluoromethyl ketones has been carried out by Roger and co-workers [63]. 

The need for a good understanding of the mechanism of the reaction is well illustrated by the case of amide hydrolysis. Many early enterprises sought to employ transition state analogues (TSAs) that were based on a stable anionic... 

Amide hydrolysis at alkaline pH involves a tetrahedral anionic intermediate, which was mimicked by the transition state analogue [49], an /V-aryl arylphosphonamidate, appropriately related to substrate anilide [50] (Fig. 18) (Appendix entry 2.8). 

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... 

In order to generate antibodies which catalyse the hydrolysis of carbonates (6, 10), carboxylic esters (9) and amides with a certain degree of specificity, the phosphates (7a. lOai and phosphonates 9a were used as haptens that mimic the tetrahedral negatively charged transition state of the spontaneous hydrolysis reaction (see Scheme 11.3) [27] [29]. 

The intermediate can, however, also be trapped by an amine to form an amide although at pH 7 in aqueous solution primary amines are predominantly proto-nated and only poorly reactive. Intramolecularity will, however, improve the poor reactivity of a lysine residue towards an acyl intermediate provided that the His and the Lys residues are close in space. The net reaction under these conditions is therefore an amidation of the lysine side chain by the active ester that is more efficient than the direct acylation of a lysine residue by at least three orders of magnitude (Fig. 10). The lysine residue will also improve the reactivity of the His side chain by electrostatic transition state stabilization and the wasteful reaction with other His residues that gives rise to hydrolysis is therefore suppressed. 

The highly ordered cyclic transition state of the Diels-Alder reaction permits design of reaction parameters which lead to a preference between the transition states leading to diastereomeric or enantiomeric adducts. (See Part A, Section 2.3, to review the principles of diastereoselectivity and enantioselectivity.) One way to achieve this is to install a chiral auxiliary.56 The cycloaddition proceeds to give two diastereomeric products which can be separated and purified. Because of the lower temperature required and the greater stereoselectivity observed in Lewis acid-catalyzed reactions, the best enantioselectivity is often observed in catalyzed reactions. Chiral esters and amides of acrylic acid are particularly useftd because the chiral auxiliary can be easily recovered upon hydrolysis of the adduct to give the enantiomerically pure carboxylic acid. 

The structural analysis of the trypsin inhibitor from bovine pancreas (BPTI) in complex with trypsin shows that the inhibitor occupies and blocks the substrate binding pocket in a highly complementary maimer (fig. 2.9). In the trypsin-BPTI complex, the catalytically essential Ser-OH of trypsin contacts a CO group of the inhibitor in a manner very similar to the tetrahedral transition state of amide or ester bond hydrolysis (see fig. 2.9b). The inhibitor can be likened to a pseudo-substrate and, as such, is bound with high affinity. The cleavage of the peptide bond is, however, not possible due to other circumstances, such as the fact that water is prevented from reaching the active site with the inhibitor boimd. 

Phosphonates have been widely used as analogues of carboxylic acids. They have been particularly effective as analogues of tetrahedral transition states that occur in the course of enzyme-catalyzed reactions such as hydrolysis of the amide (peptide) bond. As such, they may be used as inhibitors of enzymes (e.g., 82, 83) or as haptens for producing antibodies that are catalytic (e.g., 84). A notable example is H203P— CH2—CH2—CH(—NH2)—COOH, which has effects that are likely to be due to its interference with glutamate as a neurotransmitter (85). 

Furthermore, the sulfonamide bond is expected to possess enhanced metabolic stability with structural similarities to the tetrahedral transition state involved in amide bond enzymatic hydrolysis, thus making sulfonamide peptides interesting candidates in the development of protease inhibitors and new drugs. The oligomers and polymers should also be interesting molecular scaffolds, with specific secondary structures enforced by hydrogen bonding)100,101 ... 

A different type of stereoelectronic control has been found in the breakdown in solution of tetrahedral addition intermediates that arise in ester and amide hydrolysis and other reactions of carboxyl and carbonyl groups. In the case of an intermediate such as structure 8.47, in which there are two atoms with non-bonded electrons (generally O or N), the lowest-energy transition state for breakdown is a conformation in which nonbonded electrons of each are anti to the group being expelled (structures 8.48).50... 

Figure 25-20 Possible energy profiles for different pathways for hydrolysis of a simple amide. The dashed line represents an enzyme-catalyzed formation of R(CO)OCOCH3, which is here hypothesized to have a lower-energy transition state than the reaction of the anhydride with water (see text).

---