Oxidases and Reductases

Cytochrome P450s are iron-containing enzymes that function in steroid biosyntheses and xenobiotic oxidative metabolism. P450cam is a water-soluble P450 isolated from the bacterium Pseudomonas putida that catalyzes the hy-droxylation of camphor. P450cam was the first member of this family to have its crystal structure solved. 

To explain the rank order of binding of three phenylimidazole (PI) isomers, 1-PI, 2-PI, and 4-PI, to cytochrome P450cam, Harris and Loew used AMBER 4.0 to calculate the relative free energy of solvation. The (and 

The enzyme dihydrofolate reductase (DHFR) catalyzes the nicotinamide adenine dinucleotide phosphate (NADPH) reduction of folate to dihydrofolate and tetrahydrofolate, the class of cofactors used in the biosynthesis of thymidylate and hence DNA. Inhibition of DHFR prevents cell growth and kills cells, so 

DHFR inhibitors such as methotrexate (MTX) play an important role in cancer therapy. X-ray crystallographic studies s gf binary and ternary complexes of DHFR have provided valuable insight into the binding of both substrates and inhibitors. 

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Figure 16.7 Oxidase and reductase inhibitors from virtual screening.

Smith, L., Davies, H. C., Reichlin, M., Margoliash, E. Separate oxidase and reductase reaction sites on cytochrome c demonstrated with purified site-specific antibodies. J. Biol. Chem. 248, 237—243 (1973). 

A recent study, however, has shown that aminopeptidase activity is present on the surface of porcine buccal mucosa, and that various aminopeptidase inhibitors, including amastatin and sodium deoxycholate, reduce the mucosal surface degradation of the aminopeptidase substrate, leucine-enkephalin [149], Since the peptidases are present on the surface of the buccal mucosa, they may act as a significant barrier to the permeability of compounds which are substrates for the enzyme. In addition to proteolytic enzymes, there exist some esterases, oxidases, and reductases originating from buccal epithelial cells, as well as phosphatases and carbohydrases present in saliva [154], all of which may potentially be involved in the metabolism of topically applied compounds. 

Although molybdenum and tungsten enzymes carry the name of a single substrate, they are often not as selective as this nomenclature suggests. Many of the enzymes process more than one substrate, both in vivo and in vitro. Several enzymes can function as both oxidases and reductases, for example, xanthine oxidases not only oxidize purines but can deoxygenate amine N-oxides [82]. There are also sets of enzymes that catalyze the same reaction but in opposite directions. These enzymes include aldehyde and formate oxidases/carboxylic acid reductase [31,75] and nitrate reductase/nitrite oxidase [83-87]. These complementary enzymes have considerable sequence homology, and the direction of the preferred catalytic reaction depends on the electrochemical reduction potentials of the redox partners that have evolved to couple the reactions to cellular redox systems and metabolic requirements. 

Enantioselective oxidation of racemic alcohols as well as reduction of racemic ketones and aldehydes have been widely applied to obtain optically active alcohols.25 27 The enzymes catalyzing these reactions are alcohol dehydrogenase, oxidases, and reductases etc. Coenzymes (NADH, NADPH, flavine etc) are usually necessary for theses enzymes. For example, for the oxidation of alcohols, NAD(P)+ are used. The hydride removed from the substrate is transferred to the coenzyme bound in the enzyme, as shown in Figure 24. There are four stereochemical patterns, but only three types of the enzymes are known. 

The oxidoreductase class of biocatalysts is one of the most common of all biological reactions, comprising dehydrogenases, oxidases, and reductases. All these enzymes act on substrates through the transfer of electrons with various co-factors or co-enzymes serving as acceptor molecules. Only a select group of reactions will be discussed because of space limitations, so the reader is referred to other texts for more in-depth discussions of other oxidation-reduction reactions.17 20 25-28... 

Another potential application is the coupling of oxidases and reductases to convert cholesterol to coprostanol, which is not absorbed and is not atherogenic (61,62). 

Activation of drugs to give toxic products is common. Apart from non-enzymatic activation (e.g., via autoxidation), activation by enzymatic one-electron oxidation or reduction frequently occurs. Several non-specific oxidases and reductases are encountered in mammalian tissues. Enzyme systems that have been studied in detail are peroxidases and microsomal oxidases and reductases. Xanthine oxidase also has received some attention. In many insta .ces the end products of the reaction are critically dependent upon the presence of oxygen in the system. This is because oxygen is an excellent electron acceptor, i.e., it can oxidize donor radicals, forming superoxide in the process. In this way a redox cycle is set up in which the xenobiotic substrate is recovered. The toxic effects of the xenobiotic often can be attributed to the oxidative stress arising from such a cycle. However, it seems that for some substrates, oxidative stress of this kind can be less damaging than anaerobic reduction. Anaerobic reduction can lead to formation of further reduced products with additional toxicity. 

Oxidoreductases are enzymes that catalyze oxidation-reduction (redox) reactions. Lactate dehydrogenase is an oxidoreductase that removes hydrogen from a molecule of lactate. Other subclasses of the oxidoreductases include oxidases and reductases. 

Cytochrome c interacts easier with the bacterial oxidase and reductase than does Csm with the corresponding eukaryotic proteins. 

Bacterial C2 and csso behave in much the same way toward the eukaryotic oxidase and reductase. 

Cytochrome C2 reacts much better with the oxidase and reductase from bacterial Cbso than from eukaryotic c. 

All of these alterable regions are less promising candidates for binding sites to oxidase and reductase than are the unchanging front and top— the regions nearest the viewer in Figs. 21-23. 

The preference of C2 for the bacterial oxidase and reductase mentioned in point 4 above can be explained in terms of the greater similarity of structure between C2 and csso than c. The essential equivalence o.f the reactions of C2 and Csso with eukaryotic oxidase and reductase (point 3) undoubtedly has the same origin. The greater ease of reaction of c with the oxidase and reductase from csso than the reverse (point 2) may arise from the extra bulk of the C550 molecule. If the cm, oxidase and reductase evolved to accommodate a large molecule, then the smaller c might be... 

The one feature of the Ca proposal that still appears valid—heme crevice approach by both oxidase and reductase—has also been strongly... 

The face of the molecule as shown in Figs. 21-24 is probably the one seen by reductase and oxidase molecules, with entry and egress of electrons through the exposed edge of the heme. This, at least, is the simplest assumption in the absence of compelling evidence to the contrary. Which part of the crevice encounters each of the two molecules—oxidase and reductase—or whether or not they bind in the same region, are questions which can only be answered by selective chemical modification of residues on the surface of the cytochrome molecule or by physical chemical characterization of intermolecular complexes with cytochrome c. 

A wide variety of different cytochrome-linked electron-transfer systems is encountered in bacteria respiratory chains with oxygen, nitrate or sulphate as electron acceptors, fumarate reductase systems and light-driven cyclic electron-transfer systems (Fig. 3). All these systems are composed of several electron-transfer carriers, the nature of which varies considerably in different organisms. Electron carriers which are most common in bacterial electron-transfer systems are flavoproteins (dehydrogenases), quinones, non-heme iron centres, cytochromes and terminal oxidases and reductases. One common feature of all electron-transfer systems is that they are tightly incorporated in the cytoplasmic membrane. Another important general property of these systems is that electron transfer results in the translocation of protons from the cytoplasm into the external medium. Electron transfer therefore... 

The oxidation of ascorbic acid still attracts attention and a mechanism has been proposed for the reaction with photochemically produced H02 radicals. An analogous reaction with hydrogen peroxide affords a means of access to L-threonic acid. Heating D-isoascorbic acid in aqueous pyridine in the presence of boric acid gives the decarboxylation products D-arabinose and D-ribose. Two Russian papers have reported on the oxidation of ascorbic acid with copper(ll) in the presence and absence of oxygen and light. As part of a basic study of the action of oxidase and reductase enzymes, the reduction of Methylene Blue by ascorbic acid has been examined, in the presence of surfactants added to mimic the effects of the protein parts of the enzymes. ... 

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