Liver alcohol dehydrogenase models

Liver Alcohol Dehydrogenase Models with Mixed Donor Ligands 1228... 

For a liver alcohol dehydrogenase (LADH) model an NS2O coordination sphere is required. The chelating aldehydes are ideal for the formation of this donor set when combined with bis(pentafluoro-thiophenolato)zinc. Structural data on the complexes with one equivalent of 6-methylpyridine-2-carbaldehyde, 6-methoxypyridine-2-carbaldehyde, 2-(dimethylamino)benzal-dehyde) demonstrate that the coordination sphere for LADH has been reproduced to a close approximation and the corresponding alcohol complexes have also been characterized.354 Other thiophenols have been used to form such complexes but have not been structurally characterized.304... 

We then coupled the regeneration system 1 to the horse liver alcohol dehydrogenase (HLADH) catalyzed oxidation of cyclohexanol to cyclohexanone as a model system (Fig. 9). 

Modelling the effects of Ser-48 in the hydride transfer step of liver alcohol dehydrogenase,... 

In the following year, Cleland and his coworkers reported further and more emphatic examples of the phenomenon of exaltation of the a-secondary isotope effects in enzymic hydride-transfer reactions. The cases shown in Table 1 for their studies of yeast alcohol dehydrogenase and horse-liver alcohol dehydrogenase would have been expected on traditional grounds to show kinetic isotope effects between 1.00 and 1.13 but in fact values of 1.38 and 1.50 were found. Even more impressively, the oxidation of formate by NAD was expected to exhibit an isotope effect between 1.00 and 1/1.13 = 0.89 - an inverse isotope effect because NAD" was being converted to NADH. The observed value was 1.22, normal rather than inverse. Again the model of coupled motion, with a citation to Kurz and Frieden, was invoked to interpret the findings. 

Figure 16.3 A proposed model for the productively bound ternary complex of horse liver alcohol dehydrogenase. It is suggested that the ionized alcohol displaces the zinc-bound water molecule shown in Figure 16.2. [Courtesy of C.-I. Branden.]...

Figure 3-24. A zinc(ii) complex which acts as a functional model for the hydride transfer reaction which occurs at the active site of the enzyme liver alcohol dehydrogenase.

Kimura, E., Shionoya, M., Hoshino, A., Ikeda, T., Yamada, Y., A model for catalytically active zinc(II) ion in liver alcohol-dehydrogenase - a novel hydride transfer-reaction catalyzed by zinc (II) -macrocyclic polyamine complexes. J. Am. Chem. Soc. 1992,114, 10134-10137. 

Cronin L, Walton PH (2003) Synthesis and structure of [Zn(OMe)(L)HZn(OH)(L)]-2(BPh4), L = ris.ris-1.3.5-tt i s / E)-3-(2-furyl)acrylideneamino cyclohcxanc structural models of carbonic anhydrase and liver alcohol dehydrogenase. Chem Commun 1572-1573... 

Fig. 29. The observed ternary complex of horse liver alcohol dehydrogenase, NAD(H). and p-bromobenzyl alcohol (dark lines) and the proposed productive alcohol position (dotted lines), based on model building. Stereo drawing from the work of Branden and colleagues.

Hard electrophiles like Mg(C104)2 are used to activate abiotic systems. In the enzyme liver alcohol dehydrogenase (LAD) a considerably different catalytic apparatus is present a zinc ion coordinated to two cysteines and a histidine serves as a coordinating site for the carbonyl compound/alcoholate, as illustrated in equation (10). This zinc ion has amphoteric properties consistent with the capacity to activate the reaction in both directions without being consumed, in other words to act as a catalyst. Synthetic models of this catalytically active zinc have been shown to possess some catalytic activity in analogy to the enzyme (see Section L3.3.5.1iii). 

Modelling the Substrate Binding Domain of Horse Liver Alcohol Dehydrogenase, HLADH, by Computer Aided Substrate Overlay... 

In a model reaction for liver alcohol dehydrogenase, the zinc ethoxide complex [TpBut,Me]Zn-OEt reacts stoichiometrically with -nitrobenzaldehyde in benzene to produce [TpBut,Me]Zn-0CH2C6H4N02 and acetaldehyde (Scheme 10).68 For a series of similar reactions involving the zinc isopropoxide complex [Tpph,Me]Zn-OCH(CH3)2 and various aromatic aldehydes in toluene (Scheme 10), the zinc-containing products of the reaction, [Tpph,Me]Zn-OCH2Ar, were isolated in 60-70% yield, while the acetone produced was identified using ]H NMR.70... 

Synthesis and structural characterization of synthetic analogs of liver alcohol dehydrogenase (LADH) continued to be one of the most investigated fields of research. A number of ligands suitable for modeling LADH have been reported and their hydroxo zinc complexes also described.133-135 [Zn(TmMes)(HOMe)]+, for example, exhibits a coordination environment that resembles aspects of that in LADH.136 The ethanol complex [Zn(Tm Bu)(HOEt)]+ has been also isolated.137 [NS2] donor ligands, that feature thioether donors, also provide coordination environments that mimic the active site of LADH.138,139... 

Recently, a controversial debate has arisen about whether the optimization of enzyme catalysis may entail the evolutionary implementation of chemical strategies that increase the probability of tunneling and thereby accelerate reaction rates [7]. Kinetic isotope effect experiments have indicated that hydrogen tunneling plays an important role in many proton and hydride transfer reactions in enzymes [8, 9]. Enzyme catalysis of horse liver alcohol dehydrogenase may be understood by a model of vibrationally enhanced proton transfer tunneling [10]. Furthermore, the double proton transfer reaction in DNA base pairs has been studied in detail and even been hypothesized as a possible source of spontaneous mutation [11-13]. 

Application of this methodology to this model of horse liver alcohol dehydrogenase yields the results shown in Fig. 9.4. In fact we do see strong numerical evidence for the presence of a promoting vibration - intense peaks in the spectral density for the reaction coordinate are greatly reduced at a point between the reactant and product wells. This is defined as a point of minimal coupling. As we have described, the restraint on the hydride does not impact the spectral density computation. This computation measures the forces on the reaction coordinate, not those... 

Alcohol dehydrogenase from pig liver (/ e-face selective), from Mucor javanicus (Si-face selective), and, especially, from horse liver (/fe-face selective) can be used for the reduction of cyclic ketones. The horse liver enzyme is very well characterized and models for the prediction of the stereochemical course of the reaction have been developed278,279. When subjected to horse liver alcohol dehydrogenase (HLADH) reduction, the following decalin derivatives yield products with >98% ee279,280. e.g. ... 

Liver alcohol dehydrogenase subunit viewed as a CPK model. The left-hand side of the molecule is the coenzyme binding domain and the right-hand side is the catalytic domain. The catalytic zinc ion is accessible from two channels located above (not visible) and below the coenzyme binding domain. The upper channel permits approach of the nicotinamide ring of the coenzyme. The lower channel permits approach of the substrate. The substrate channel closes up, trapping the substrate inside the molecule, when both coenzyme and substrate are present. 

Zinc ion is essential for the catalytic activities of both yeast and liver alcohol dehydrogenase. Until recently, model systems have been notably unsuccessful in accounting for the participation of Zn(II) in the enzyme-catalyzed oxidoreductive interconversion of aldehyde and alcohol. The studies of Creighton and Sigman (20) and of Shinkai and Bruice (21, 22) conclusively demonstrate that Lewis (general) acid catalysis by Zn + (and other divalent metal ions) can effectively promote aldehyde reduction by the reduced 1,4-dihydropyridine moiety. 

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