Enzyme dehydrogenases

Oxidoreduciases. Enzymes catalysing redox reactions. The substrate which is oxidized is regarded as the hydrogen donor. This group includes the trivially named enzymes, dehydrogenases, oxidases, reductases, peroxidases, hydrogenases and hydroxylases. 

It is possible to use isolated, partially purified enzymes (dehydrogenases) for the reduction of ketones to optically active secondary alcohols. However, a different set of complications arises. The new C H bond is formed by delivery of the hydrogen atom from an enzyme cofactor, nicotinamide adenine dinucleotide (phosphate) NAD(P) in its reduced form. The cofactor is too expensive to be used in a stoichiometric quantity and must be recycled in situ. Recycling methods are relatively simple, using a sacrificial alcohol, or a second enzyme (formate dehydrogenase is popular) but the real and apparent complexity of the ensuing process (Scheme 8)[331 provides too much of a disincentive to investigation by non-experts. 

We conclude this chapter by describing some chemical features of nucleotide coenzymes and some of the enzymes (dehydrogenases and flavoproteins) that use them. The oxidation-reduction chemistry of quinones, iron-sulfur proteins, and cytochromes is discussed in Chapter 19. 

Amino acids NAD(P)H Pyridoxamine 5 -phosphate Pyruvate Amines Pyridoxal 5-phosphate dependent enzymes Dehydrogenases Transaminases Pyridoxal 5-phosphate dependent enzymes Amino acid decarboxylases... 

Enzymes are proteinaceous catalysts peculiar to living matter. Hundreds have been obtained in purified and crystalline form. Their catalytic efficiency is extremely high—one mole of a pure enzyme may catalyze the transformation of as many as 10,000 to 1,000,000 moles of substrate per minute. While some enzymes are highly specific for only one substrate, others can attack many related substrates. Avery broad classification of enzymes would include hydrolytic enzymes (esterases, proteases), phosphorylases, oxidoreductive enzymes (dehydrogenases, oxidases, peroxidases), transferring enzymes, decarboxylases and others. 

Consider the NAD -dependent enzyme dehydrogenase, which catalyzes the oxidation of a substrate, RH ... 

The cyanuric chloride/PEG method seems to work for all classes of enzymes, including hydrolases (lipases [469], proteases, glucosidases [470]) and redox enzymes (dehydrogenases, oxidases [471]). The residual activities are usually high (50-80%), and for most enzymes about five to ten PEG chains per enzyme molecule are sufficient to render them soluble in organic solvents. Care has to be taken to avoid extensive modification which leads to deactivation. PEG-modified enzymes may be recovered from a toluene solution by precipitation upon the addition of a hydrocarbon such as petroleum ether or hexane [472]. 

The NAD+ and NADH molecules (including sugars, phosphate, and adenine) are coenzymes in the enzyme dehydrogenase. Dehydrogenase oxidizes a substrate by removing two electrons and a proton to the nicotine amide acceptor or, alternatively, to a flavin. The proton and two electrons originate from foodstuff, for example, lipids, as mentioned previously. NADH dehydrogenase is the first enzyme in the electron transport chain. 

Co enzyme dehydrogenases (diaphorasea) activate co-enzymes to effect indirect oxidation of the primary substrate by cytochrome or flavoprotein. 

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