Alcohol dehydrogenase, activation energy

The rate of ethanol degradation in the liver is limited by alcohol dehydrogenase activity. The amount of NAD" available is the limiting factor. As the maximum degradation rate is already reached at low concentrations of ethanol, the ethanol level therefore declines at a constant rate (zero-order kinetics). The calorific value of ethanol is 29.4 kj g Alcoholic drinks—particularly in alcoholics—can therefore represent a substantial proportion of dietary energy intake. 

The culture Pseudomonas sp. NCIB 9872 was isolated from soil using cyclopentanol as the sole carbon source [9]. Cell-free extracts of the organism displayed alcohol dehydrogenase activity and BVase activity. To obtain energy from this carbon source, the pivotal step is ling fragmentation, made possible by oxidation of the alcohol to the cyclopentanone, followed by insertion of oxygen atom to S-valerolactone, and subsequent enzymatic hydrolysis to aliphatic hydroxy acids which are then converted to metabolites. The purified enzyme consists of four subunits (M 200,000), binds two to four molecules of FAD, and requires NADH as reductant. 

Release of the chloride substituent in 2 produces ethyl acetoacetate (1) which is an inhibitor of alcohol dehydrogenase (ADH, EC 1.1.1.1) and thereby affects energy metabolism. Above all, NAD+ regeneration is impaired. As a consequence acetaldehyde becomes an overflow metabolite which is excreted into the medium or reacts with activated positions in the molecule (Fig. 3.5). 

Cobalt most often depresses the activity of enzyme including catalase, amino levulinic acid synthetase, and P-450, enzymes involved in cellular respiration. The Krebs citric acid cycle can be blocked by cobalt resulting in the inhibition of cellular energy production. Cobalt can replace zinc in a number of zinc-required enzymes like alcohol dehydrogenase. Cobalt can also enhance the kinetics of some enzymes such as heme oxidase in the liver. Cobalt interferes with and depresses iodine metabolism resulting in reduced thyroid activity. Reduced thyroid activity can lead to goiter. 

For enzymes which are able to convert more than one substrate or compound into product, the activation energy may be dependent on the substrate. One example is alcohol dehydrogenase, an important enzyme for aroma formation in semiripened peas (Table 2.13). In this case the activation energy for the reverse reaction is only slightly influenced by substrate. 

Table 2.13. Alcohol dehydrogenase from pea seeds activation energy of alcohol dehydrogenation and aldehyde reduction...

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