Complexes lactone-enzyme

Inhibition of Glycosidases by AIdono-l,S-lactones and Aldohexoses Expressed by the Dissociation Constant K of the Enzyme-Inhibitor Complex... 

The mechanism of the enzymatic polymerization is shown in Fig. 25 and can be decomposed into three main steps. First, a complex is formed between the enzyme and the lactone. The second step is based on the nucleophihc activation of the lactone by the hydroxyl function belonging to a serine residue of the active... 

Figures for K, the dissociation constant of the enzyme-inhibitor complex, obtained under strictly comparable conditions with mouse-liver preparations, are shown in Table VIII for D-glucaro-1,4-lactone and some closely-related compounds, all of which act competitively. Table IX gives figures obtained with the enzyme from female-rat preputial gland and various substrates within the limits of these experiments, Kt for boiled D-glucarate or boiled galactarate solutions was little affected by changes in the purity of the enzyme or the pH of test. In accordance with theory, similar values of Ki for any one inhibitor were obtained with different substrates, with the notable exception of o-nitrophenyl /8-galacturonide.

Modular PKS enzymes are responsible for the synthesis of a wide diversity of structures and seem to have more relaxed specificities in several of the enzymatic steps. Their enormous appeal for combinatorial purposes, though, derives from the presence of multiple modules that can be manipulated independently, allowing the production of rings of different sizes and with potential stereochemical variation at each PK carbon. The higher complexity of these pathways has somewhat hindered their exploitation, but recently, several have been fully characterized. Among them, by far the most studied modular multienzyme complex is 6-deoxyerythronolide B synthase (DEBS 240,266,267), which produces the 14-member macrolide 6-deoxyerythronolide B (10.70, Fig. 10.45). DEBS contains three large subunits each of which contains two PKS enzyme modules. Each module contains the minimal PKS enzyme vide supra) and either none (M3), one (ketoreductase KR Ml, M2, MS, and M6), or three (dehydratase DH-enoyl reductase ER-ketoreductase KR, M4) catalytic activities that produce a keto (M3), an hydroxy (Ml, M2, MS and M6), or an unsubstituted methylene (M4) on the last monomeric unit of the growing chain (Fig. 10.45). A final thioesterase (TE) activity catalyzes lactone formation with concomitant release of 10.70 from the multienzyme complex. Introduction of TE activity after an upstream module allows various reduced-size macrolides (10.71-10.73, Eig. 10.45) to be obtained. 

Sirohmus is a macrocychc lactone produced by the bacteria Streptomyces hygroscopious. Like the calcineurin inhibitors cyclosporine and tacrolimus its mechanism of action involves formation of a complex with an immunophiUn, in this case, FKBP-12. Unlike cyclosporine and tacrolimus, sirohmus does not affect calcineurin activity but binds to and inhibits the mammalian kinase, target of rapamycin (mTOR.). mTOR is a key enzyme in cell-cycle progression. When inhibited this kinase blocks cell cycle progression at the G1 to S phase transition (Dumont and Su, 1996 Sehgal, 2003). 

The flavor and aroma of many foodstuffs are formed as a result of the action of enzymes and microorganisms. This knowledge can be exploited to obtain both complex flavor mixtures and individual flavor compounds via microbial fermentation and/or enzyme technology. Many different classes of compound can be obtained this way including esters, lactones, aldehydes, ketones and acids. 

Few, if any, simple glycosidases have been crystallized, and, indeed, for many hydrolyses, extremely complex mixtures of enzymes, for example, almond emulsin, are often used. The specificity of such complex mixtures may be increased not only by fractionation, but by the addition of appropriate inhibitors. Thus, commercial hemicellulase normally hydrolyzes both the L-arabinose-D-xylose and the n-xylose-n-xylose link in arabi-noxylans. Addition of L-arabinono-1,4-lactone to the enzyme digest, however, inhibits hydrolysis of the former links (see Conchie and Levvy ) and... 

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