Pyridine nucleotides electron transfer between

Electron transfer between pyridine nucleotides and disulfide compounds is catalyzed by several fiavoproteins and three of these are well characterized. Lipoamide dehydrogenase functions in the oxidative decarboxylation of a-keto acids catalyzing the reoxidation of reduced lipoate by NAD+ (18, 19). Glutathione reductase catalyzes electron transfer between NADPH and glutathione ZO-22). Thioredoxin reductase catalyzes the reduction of thioredoxin by NADPH (5) thioredoxin is a protein of 12,000 molecular weight containing a single cystine residue which is the electron acceptor S3). 

FerredoxinrNADP oxidoreductase (FNOR, EC 1.18.1.2) is a flavoenzyme that catalyzes electron transfer between the redox protein, ferredoxin, and the pyridine nucleotide coenzymes, NADP(H) and/or NAD(H). Enzymes of this type have been characterized from many organisms, including from both the bacterial and eukaryotic domains. " However, only one such enzyme has been purified from... 

A con alent modification of electrode surfaces by mediators N,N -di-aminopropyl-4,4 -bypiridinium (DAPV) or cobalt diaminosarcophagine (CoD) makes it possible to observe a direct electron transfer between the electrode and a viologen accepting pyridine nucleotide oxidoreduc-tase (VAPOR) or dihydrolipoamide dehydrogenase (Lip-DH) [227]. In this case the electron transfer does not involve a true mediation step since the mediator is a component of the electrode. In this way it forms a molecular wire between the electrode and the electron accepting prosthetic group of the enzyme [227]. 

An important difference between animal and plant electron transferring proteins is the non-exchangeability in their physiological functions adrenodoxin is inactive for photosynthetic pyridine nucleotide reduction and spinach ferredoxin does not function in adrenal steroid hydroxy-... 

The pyridine nucleotide-disulfide oxidoreductases, lipoamide dehydrogenase (4), glutathione reductase (5), and thioredoxin reductase (6-8) share so many properties in common that they will be compared and contrasted before being considered separately. As their group name implies, they catalyze the transfer of electrons between pyridine nucleotides and disulfides. In spite of their similarities they function in widely divergent metabolic roles. 

The principal features, in addition to EHj, common to lipoamide dehydrogenase and glutathione reductase deserve emphasis the formation of complexes between the oxidized enzymes and their respective oxidized pyridine nucleotides the formation of complexes between EHj and both oxidized and reduced pyridine nucleotides the formation of charge transfer complexes between 4-electron-reduced enzymes and oxidized pyridine... 

Flavins are unique coenzymes that are able to catalyze both one- and two-electron transfers. Because of this, many flavoproteins are involved in transferring electrons between other proteins. Often, flavoproteins are reduced by two-electron donors, such as pyridine nucleotides, and then pass those electrons one at a time to a single-electron acceptor, such as an iron-sulfur cluster in another protein. Conversely, some enzymes accept single electrons from reduced enzymes. In either case, the flavoenzymes are transferring single electrons thus, flavin semiquinone is frequently stabilized and observed during turnover. 

Diaphorase and Cytochrome Reductase. Enzymes have been isolated from animal sources that have many of the properties of the various yeast enzymes. The first, and simplest, was liberated from particulate structures by Straub, who employed dilute ethanol and ammonium sulfate at 43 C. The enzyme could then be purified and was named a diaphorase. Diaphorase was coined to identify a widespread group of enzymes that transfer electrons from DPNH to dyes. Many of the purified flavoproteins have been found to oxidize pyridine nucleotides, and almost all of the flavoproteins can reduce dyes. Straub s diaphorase reduces methylene blue but not cytochrome c. Slight modification of the isolation procedure was found by workers at the Enzyme Institute of the University of Wisconsin to yield a cytochrome reductase. The relation between these preparations is not known, but it is possible that one is derived from the other. Cytochrome reductase contains 4 atoms of iron for each flavin, whereas Straub s preparation contains little iron. Both proteins have molecular weights around 75,000, and contain 1 equivalent of FAD. 

Hemin as a Prosthetic Group. The cytochromes often are associated with the flavoproteins. They are oxidoreductases because they transfer electrons. Iron is involved in the electron transport by a reversible change of its valency between Fe++ and Fe+++. The cytochromes contain a complex porphyrin system, which is either identical with or closely related to the heme of hemoglobin. Because of their importance, a separate chapter (Chapt. IX) has been devoted to hemo-proteins. The cooperation between pyridine nucleotides and the flavoproteins in the respiratory chain is discussed in Chapt. X-4. 

Flavins can serve as either one-electron or two-electron transfer reagents, acting at the interface between the cytochromes and NADH. The vitamin, riboflavin, is converted by the body to the catalytically active coenzyme forms, FMN or FAD. Flavoproteins are involved in oxidation reactions with substrates which include the pyridine nucleotides, a-amino acids and a-hydroxy acids, and compounds containing saturated carbon-carbon bonds convertible into olefins. When oxidation occurs, the isoalloxazine ring of FMN (Fig. 8) or FAD becomes reduced. 

---