Reagent active-site selective

Active Site Selective Reagents - Naturally Occurring Toxins and Laboratory Tools... 

Active site selective reagents can be classified in various manners. One way is to divide them according to how they react (Table IX). In such a classification one finds substrates that can be covalently attached by chemical treatment of the enzyme while it is catalyzing some change in the substrate. An example is the reaction of functional amino groups by the enzyme muscle aldolase acting on glyceraldehyde and reduced by cyanoboro-hydride (16). 

A second type of active site selective reagent is when there is a pseudosubstrate, such as diisopropylfluorophosphate. Acting... 

Ethoxy)-allylidenecyclopropane (136a) readily underwent Diels-Alder reaction with activated dienophiles under mild conditions (Table 14) [33]. Only one regioisomer was formed with unsymmetrically substituted dienophiles such as methyl maleic anhydride (137), and quinones 138-141 (entries 2 and 3-6). AH the cycloadducts 143-147 derive from an endo approach between the two reagents. Two site-isomers were obtained in 96 4 ratio with 3-isopropyl-6-methyl-p-quinone (141) (entry 6) and the high site-selectivity observed in this... 

Recently, Hajek and Radoiu observed that MW not only increase the rate of heterogeneous catalytic reactions, but also affect the product selectivity [85], The results were explained in terms of MW-induced polarization, involving the absorption of MW by highly polarized reagent molecules on the active site of the catalyst. On the other hand there is little, if any, activation of homogeneous catalytic reactions in polar solvents [86], presumably due to the high absorbent power of MW irradiation by the solvent. 

The overall reaction catalyzed by epoxide hydrolases is the addition of a H20 molecule to an epoxide. Alkene oxides, thus, yield diols (Fig. 10.5), whereas arene oxides yield dihydrodiols (cf. Fig. 10.8). In earlier studies, it had been postulated that epoxide hydrolases act by enhancing the nucleo-philicity of a H20 molecule and directing it to attack an epoxide, as pictured in Fig. 10.5, a [59] [60], Further evidence such as the lack of incorporation of 180 from H2180 into the substrate, the isolation of an ester intermediate, and the effects of group-selective reagents and carefully designed inhibitors led to a more-elaborate model [59][61 - 67]. As pictured in Fig. 10.5,b, nucleophilic attack of the substrate is mediated by a carboxylate group in the catalytic site to form an ester intermediate. In a second step, an activated H20... 

Evidence for the involvement of histidine residues in the active site has been obtained from a number of experiments. Treatment with the histidine-selective reagent diethyl pyrocarbonate causes complete loss of activity in prolyl 4-hydroxylase (EC 1.14.11.2) [215] and 2,-4-dichlorophenoxyacetate dioxygenase (TfdA) (EC 1.13.11 group) [211], Site-directed mutagenesis of some of the conserved His residues in prolyl 4-hydroxylase [216] and aspartyl P-hydroxylase (EC 1.14.11.16) [217] also results in the loss of enzymatic activity. More recently, direct spectroscopic evidence for His coordination has been obtained in ESEEM studies of Cu(II)-substituted TfdA [218],... 

Clearly the above scheme of liquid-phase oxidation by oxygen shows H202 and ROOH formed as intermediate products. However, they cannot be related to active sites of another, secondary reaction as is customary in, for example, conjugated processes. The reasons for making such comparisons are as follows firstly, H202 and ROOH are final products of a complex reaction and initial reagents for other thermodynamically probable reactions secondly, their formation and consumption (by the scheme selected) do not correspond to the notion of active site of conjugated reactions. 

Common catalytic systems are characterized by the presence of reagent molecules only, whereas the enzymatic system is multicomponent and possesses low concentrations of the substrates in water. The interaction between a substrate with an oxidant or a reducer is most often considered. This makes unnecessary simulation of the enzyme selectivity. However, free contact of reagent molecules with active sites preserves the possibility of various mechanism realizations which is the reason for decrease of the process selectivity. Apparently, a compromise should be found in resolving the question of selectivity in biomimics development in suggesting that, though complex gap mechanism is the effective method for distance and mutual orientation control of reactive groups in the enzyme, it may hardly be implemented in synthetic systems. 

Goeldner, Hirth and colleagues have presented evidence that the efficiency of labeling by certain photoaffinity reagents is improved in the environment provided by the receptor. They define a photosuicide inhibitor as a ligand analog of an enzyme or a receptor, the photodecomposition of which is selectively induced by the intrinsic physico-chemical properties of an active site (Goeldner et al., 1982). 

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