Enzyme-bound stereochemistry

Typically, lyases are quite specific for the nucleophilic donor component owing to mechanistic requirements. Usually, approach of the aldol acceptor to the enzyme-bound nucleophile occurs stereospedfically following an overall retention mechanism, while the facial differentiation of the aldehyde carbonyl is responsible for the relative stereoselectivity. In this manner, the stereochemistry of the C—C bond formation is completely controlled by the enzymes, in general irrespective of the constitution or configuration of the substrate, which renders the enzymes highly predictable. On the other hand, most of the lyases allow a reasonably broad variation of the electrophilic acceptor component that is usually an aldehyde. This feature... 

The stereochemistry of the ketosynthase-catalysed condensation of malonate with the enzyme-bound thioester at the start of each elongation cycle has... 

The catalytic pathway is best described as a random binding kinetic mechanism involving the formation of the ternary complex E-acetyl-P-ADP, with direct phosphoryl group transfer between enzyme-bound substrates to form the product ternary complex E-acetate-ATP. The formation and decomposition of these ternary complexes involve only noncovalent binding interactions of the enzyme with the substrates and products. The stereochemistry is inconsistent with a mechanism in which the phosphoryl group is transferred to an enzymic nucleophile as a step in the interconversion of the ternary complexes. The case of acetate kinase is one in which the stereochemical course of phosphoryl group transfer essentially discredited a double-displacement mechanism that had been reasonably well supported by other evidence. 

Scheme 10 shows a proposed scheme of the overall reaction catalyzed by IPNS [161]. Mechanistic studies have been performed using kinetic isotope effects [166-168] and substrate analogues [153, 169-178] by Baldwin et al. Based on these reactivities, it has been proposed that an enzyme-bound P-lactam intermediate attached to the iron(IV) (ferryl) center is formed and that the complex reacts to complete carbon-sulfur bond formation by a free-radical mechanism. Each ring closure inhibits a marked primary kinetic isotope effect and monocyclic products without thiazolidine ring closure are formed by using ACV analogues. It was shown that the C-S bond formation proceeds with complete retention of stereochemistry [179] and that an epoxide is formed from... 

With the support of quantum mechanics this proteolysis study has readily shown that fluorinated amino acid side chains are able to direct enzyme substrate interactions, which can have an influence on proteolytic stability. Depending on the absolute stereochemistry and on the position within the sequence, aTfm amino acids can considerably stabilize peptides against proteolysis. The unique electrostatic properties of carbon-bound fluorine, however, may also induce a contrary effect. The conformational restrictions of C -dialkylation seem to be partly dimin-ishable by the electrostatic consequences of fluorination. With this knowledge. 

The glycosidases act by two different mechanisms which is revealed by the stereochemistry at the anomeiic centre of the product (McCarter and Withers, 1994). In one type of glycosidases the anomeiic centre is directly attacked by a hydroxide to give a product with inverted stereochemistry at the anomeiic centre. In the other mechanism, the anomeric centre is attacked by the carboxylate group of a glutamic acid residue to form an intermediate in which the carbohydrate moiety is covalently bound to the enzyme similar to in epoxide hydrolases (Figure 2.16) and serine hydrolases. Attack on this intermediate by a nucleophile leads to the net result which is retention of the stereochemistry at the anomeric centre. 

This field contains cofactors which participate in the reaction but are not bound to the enzyme, and prosthetic groups being tightly bound. The commentary explains the function or, if known, the stereochemistry, or whether the cofactor can be replaced by a similar compound with higher or lower efficiency. 

The chemistry of interest when cyclodextrin or its derivatives are used as enzyme mimics involves two features. First of all, the substrate binds into the cavity of the cyclodextrin as the result of hydrophobic or lyophobic (4) forces. Then the bound substrate undergoes a reaction, which may involve the cyclodextrin as a reagent or as a catalyst. The speed of this reaction is promoted generally by the proximity induced by binding, and in addition the reactions are often selective because of geometric constraints in the transition state. This selectivity may involve the selective reaction of one potential substrate relative to another, selective production of one regiochemical isomer compared with another, or selective production of one stereoisomer relative to another. This last area, selective stereochemistry and asymmetric synthesis, is still one of the most neglected areas of cyclodextrin chemistry. 

We have drawn the product with stereochemistry even though it is not chiral. This is because one of the two enantiotopic thiol esters is hydrolysed while this intermediate is still bound to the enzyme, so a single enantiomer of the half-acid/half-thiol ester results. 

---