NADH enzyme, model systems

Figure 2 Effect of enzyme immobilization on luminescent image spatial resolution evaluated using coupled enzymatic reactions on nylon net as a model system, (a) Immobilized 3a-hydroxysteroid dehydrogenase (b) immobilized 3a-hydroxysteroid dehydrogenase and FMN-NADH oxidoreductase (c) immobilized 3a-hydroxysteroid dehydrogenase, FMN-NADH oxidoreductase, and bacterial luciferase. (From Ref. 47. Copyright John Wiley Sons Ltd. Reproduced with permission.)...

The function of the enzymes of the mitochondrial respiratory chain is to transform the energy of redox reactions into an electrochemical proton gradient across the hydrophobic barrier of a coupling membrane. Isolated oligoenzyme complexes of the respiratory chain of mitochondria, cytochrome c oxidase, succinate ytochrome c reductase, and NADH CoQ reductase, are able to catalyze charge transfer in model systems, e.g., at a water/octane interface, which can be followed by a change in the interfacial potential at this interface [20-... 

Calixarenes similarly to natural detergents easily embed the micelles and bilayers formed by other amphiphiles [54] and can exhibit highly selective catalytic activity as enzyme-mimicking systems. Thus, water-soluble calix[6]arenes 8-11 catalyzed the hydration of 1,4-dihydronicotinamide derivatives, important model compound used to study the properties of the reduced form of the coenzyme nicotinamide adenine dinucleotide (NADH) (Scheme 4.8) [55], The reaction proceeded according to the Michaelis-Menten kinetics. 

For instance, an NAD" analogue bound to a water-soluble dextran polymer has been applied in the preparation of an enzyme electrode (127) and a model enzyme reactor. Medicinal applications in this field are obvious. One example will illustrate the principle. The oxidoreduction reaction, NAD -I-substrate- NADH-I-H + product, can be coupled to immobilized enzymes. In this model system, the substrate is pumped into a chamber containing the dextran-bound NAD" and two NAD -linked dehydrogenases. At the other end the product of the reaction is removed at the same rate by ultrafiltration. Hence, the process can be recycled. 

A different redox system model - the model for NADH - was also described by our group. [16] As electron transfer mediators, FMN and FAD accept two electrons from NAD(P)H and transfer one electron to metal centres in heme-containing proteins, nonheme iron, or molybdenum sites. However, the transfer of electrons between reduced pyridine - dinucleotide cofactors is slow under physiological conditions and must be catalysed by enzymes. Function of these enzymes was mimicked by a modification of the cofactor by a recognition site for its counterpart and, thus, efficient electron transfer was enabled directly. Functionalised 1,4-dihydronicotinamides bearing a recognition unit for flavins were synthesised (Scheme 18). 

The problem of biomimetic model design simulating the action mechanism of corresponding enzymes is based on the idea of structural-functional conformity. In 1971, alcohol dehydrogenase was primarily synthesized [123], In this biomimetic system the product is formed due to direct electron transfer from the reduced co-factor (NADH) analog to aldehyde. Note that the display of alcohol dehydrogenase catalytic activity requires the presence of zinc (II) ion. 

If both MT-1 and HO-1 mRNA induction by heme-hemopexin involves a copper-redox enzyme in both heme transport (and consequent induction of HO-1 mRNA) and the signaling pathway for MT-1 expression, a plausible working model can be formulated by analogy with aspects of the yeast iron uptake processes and with redox reactions in transport (Figure 5-6). First, the ferric heme-iron bound to hemopexin can act as an electron acceptor, and reduction is proposed to be required for heme release. The ferrous heme and oxygen are substrates for an oxidase, possibly NADH-dependent, in the system for heme transport. Like ferrous iron, ferrous heme is more water soluble than ferric heme and thus more suitable as a transport intermediate between the heme-binding site on hemopexin and the next protein in the overall uptake process. The hemopexin system would also include a copper-redox protein in which the copper electrons would be available to produce Cu(I), either as the copper oxidase or for Cu(I) transport across the plasma membrane to cytosolic copper carrier proteins for incorporation into copper-requiring proteins [145]. The copper requirement for iron transport in yeast is detectable only under low levels of extracellular copper as occur in the serum-free experimental conditions often used. 

Kojima M, Toda F, Hattori K (1980) The cyclodextrin-nicotinamide compound as a dehydrogenase model simulating apoenzyme-coenzyme-substrate ternary complex system. Tetrahedron Lett 21 2721-2724 Kojima M, Toda F, Hattori K (1981) 3 Cyclodextrin-nicotinamide as a model for NADH dependent enzyme. J Chem Soc Perkin Trans 1 1647-1651... 

An additional feature in Fig. 2 worth noting is the amino-terminal 160 amino acids of mercuric reductase that lacked a fixed position in the crystal and therefore were not part of the solved structure. These contain the sequence that is homologous to MerP and postulated to be a mercurybinding domain. This region is drawn in Fig. 2 as an extension from the protein perhaps it functions like a baseball mitt that catches Hg from the membrane transport proteins and delivers Hg " to the carboxyl-terminal catalytic binding site, so that, as in the bucket brigade model above, Hg " is never found free within the cell. Mutant strains with the transport system but lacking the MerA detoxification enzyme are hypersensitive to mercury salts, as they accumulate Hg " but cannot get rid of it. After reduction by NADH (via FAD and the active site cysteine pair), metallic Hg is released... 

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