Coupled-substrate

II. Active carrier-mediated Flux is saturable with increasing concentration. Competitive substrates. Flux may be asymmetrical. Flux can be against an electrochemical gradient. Energy dependent—directly or indirectly coupled. Substrate specificity, competition, saturation. Flux is asymmetrical. 

Figure 13 Schematic representation of the coupled substrate-enzymatic-luminol CL reaction system in a reversed micellar medium. (From Ref. 64 with permission.)...

Enzyme EC number Direct or coupled Substrate used Product detected Linking substrate Linking enzymes... 

Biocatalytic approaches to cofactor regeneration can be divided into coupled-enzyme methods and coupled-substrate methods.In the coupled-enzyme method, the oxidized cofactors (NAD+ and NADP+) are recycled in situ by performing an oxidation reaction using a second enzyme and an inexpensive auxiliary substrate. This second enzyme must employ the same cofactor, but neither enzyme should be able to accept the same substrate. 

The coupled substrate method is perhaps the simplest approach to asymmetric ketone reduction, using a single recombinant ADH to perform the oxidation of a cheap auxiliary... 

A related approach, developed almost simultaneously by Neuss and co-workers at Eli Lilly (25) and by Kutney et al. in Vancouver (26), utilized as coupling substrate the chloroindoline alkene 17, which, through equilibrium with the corresponding chloroindolenine, was readily prepared by chlorination of dihydrocleavamine 18. Condensation with the 17-deacetyl-16-carboxyhydrazide derivative of vindoline (19) gave a binary indole-indoline product (20) (Scheme 4). The undesired C-16 con-... 

The relatively poor chemical yield in the above reaction may be increased by the use of more electron-deficient coupling substrates, such as the nitro compounds below, which can be coupled under the given conditions to yield up to 80% of the chiral bisbenzoic acid derivative32. 

The importance of electron-deficient coupling substrates is furthermore illustrated in the unsuccessful attempt to couple (A/)-2,2 -bis(2-bromo-l-phenoxymethyl)-U -binaphthalene283 and in the very poor chemical yields (11 %) in coupling the inverse ester (P)-2,2 -bis(l-bro-mo-2-naphthalenoxycarbonyl)-l,T-binaphthalene33. 

HLADH was applied for the reduction of a broad variety of substrates. The regeneration of NADH can be achieved by the ethanol coupled-substrate method [239]. Cyclic ketones are good substrates for this enzyme [137, 240, 241]. Heterocyclic compounds containing oxygen or sulfur are accepted, too [242 246], but nitrogen-containing substrates are inhibitors of HLADH, because they apparently complex with zinc in the active site of the enzyme [247],... 

Figure 5.9 NAD+/NADH cofactor regeneration a the coupled enzyme approach b the coupled substrate approach c, d examples [42,43] of the two approaches.

In order to avoid these limitations, efforts have been made to find ways of coupling substrate turnover directly to a selection process. Two groups have devised a method that involves the attachment of a substrate to a phage-enzyme in a way that allows its intraphage interaction with the enzyme. Phage-displaying active enzymes are able to convert the substrate into product. They can be captured from mixtures with product specific reagents or antibodies. 

The source of the label may be the solvent or a coupled substrate. It can be introduced by a single enzyme (Scheme 57) or using a coupled enzyme system (Scheme 58). ... 

Many sodium-coupled systems located in the proximal tubule of the kidney exhibit an analogous mechanism (see Fig. 3). When the transport of both substances, the substrate going uphill and the coupled substrate going downhill, is flowing in the same direction, the transporter is termed... 

ABC MDRs (as all other members of this superfamily) are primary active transporters that couple substrate translocation to binding and hydrolysis of ATP. M D Rs in all the other superfamilies are secondary transporters that use electrochemical gradients of ions (most frequently protons but sometime sodium) to transport their diverse substrates. Both primary and secondary transporters are ubiquitous in bacteria li owcvcr, their relative presence seems to correlate with energy generation fermentative bacteria tend to rely more on the primary transporters while aerobic bacteria contain somewhat more secondary transporters in their genomes [32, 33]. 

For the production of sulcatol (3, 6-methyl-5-hepten-2-ol). 71 brockii alcohol dehydrogenase has been used as a soluble enzyme, immobilized onto solid supports (PAN or Eupergit)26 113 237 and, recently, in a continuously working membrane reactor250. Generally the coupled substrate approach is used. 

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