Hydrogen reducing equivalents

NAD PI I gives up hydrogen atoms to the flavo protein NADPH— cytochrome P450 reductase and becomes NADP+. The reduced flavo protein transfers these reducing equivalents to cytochrome P450. The reducing... 

The reducing equivalents transferred can be considered either as hydrogen atoms or electrons. The driving force for the reaction, E, is the reduction/oxidation (redox) potential, and can be measured by electrochemistry it is often expressed in millivolts. The number of reducing equivalents transferred is n. The redox potential of a compound A depends on the concentrations of the oxidized and reduced species [Aqx] and [Area] according to the Nernst equation ... 

Figure 3.3. Reactions of CO with hydrogen-reduced A. vinosum hydrogenase (Ffappe et al. 1999). Starting with enzyme plus 0.8mM hydrogen (equivalent to 1 bar Nia-SR state), a transient Nia-C state was detected within 10ms when the solution was mixed with CO-saturated buffer (in the dark). Thereafter, a rapid decline of the Nia-C state was noticed (conversion into Nia-S-CO). The sample obtained at 10 ms could be converted to the Nia-L state by illumination at 30K. Raising the temperature to 200K did not reverse process instead a state was detected in which CO was directly bound to nickel (Nia -CO). Protons are not shown.

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

The N2-fixing enzyme used by the bacteria is nitrogenase. It consists of two components an Fe protein that contains an [Fe4S4] cluster as a redox system (see p. 106), accepts electrons from ferredoxin, and donates them to the second component, the Fe-Mo protein. This molybdenum-containing protein transfers the electrons to N2 and thus, via various intermediate steps, produces ammonia (NH3). Some of the reducing equivalents are transferred in a side-reaction to In addition to NH3, hydrogen is therefore always produced as well. 

Figure 9-6. Synthesis of tyrosine from phenylalanine. Hydroxylation of phenylalanine to tyrosine is one of several reactions in the body that require tetrahydrobiopterin as a cofactor to provide electrons and hydrogen as reducing equivalents.

Another general method is based on oxygen insertion into metal-hydrogen bonds (50,72,79-81) by any of several known mechanisms. Hydrogen abstraction by superoxo complexes followed by oxygenation of the reduced metal, as in the catalytic reaction of Eqs. (3)-(4) (50,72), works well but is limited by the low availability of water-soluble transition metal hydrides and slow hydrogen transfer (equivalent of reaction (3)) for sterically crowded complexes. 

Regeneration of reduced enzyme In order for ribonucleotide reductase to continue to produce deoxyribonucleotides, the disulfide bond created during the production of the 2 -deoxy carbon must be reduced. The source of the reducing equivalents is thioredoxin—a peptide coenzyme of ribonucleotide reductase. Thioredoxin contains two cysteine residues separated by two amino acids in the peptide chain. The two sulfhydryl groups of thioredoxin donate their hydrogen atoms to ribonucleotide reductase, in the process forming a disulfide bond (see p. 19). 

The reducing equivalents temporarily stored in N AD(P)H are utilized in a number of ways, all of which lead to biosynthesis of essential molecules and/or oxidative degradation of metabolites. Examples range from the simple reduction of a substrate for biosynthetic purposes (e.g. the steroid reductase mediated hydrogenation of the isolated double bond of desmosterol to give cholesterol, Table 1) (71MI11000) to complex electron transport chains that are switched on by the transfer of the electrons of N AD(P)H to the next electron carrier of the chain. These multienzyme systems are used for a number of purposes (see below). 

Fig. 13. (A) EPR spectrum, recorded at 70 K, of reduced 430 in a hydrogen-reduced cell extract from M. thermoautotrophicum (Marburg strain). (B and C) Computer simulations, assuming equal interaction with four (B) or three (C) equivalent 14N nuclei. The arrows indicate places where B gives a better fit. Reproduced, with permission, from Ref. 90.

Finally, vesicles have been used to store reducing equivalents by incorporating C MV1", formed from C14MV2+ on irradiation in the presence of EDTA and [Ru(bipy)3]2+, as multimers (monomers are obtained in the presence of CTAC micelles). These multimers are more stable to air than the monomers but generate hydrogen from water on the addition of carbowax protected colloidal platinum.343... 

During conversion of ethanol to acetaldehyde, hydrogen ion is transferred from alcohol to the cofactor nicotinamide adenine dinucleotide (NAD+) to form NADH. As a net result, alcohol oxidation generates an excess of reducing equivalents in the liver, chiefly as NADH. The excess NADH production appears to underlie a number of metabolic disorders that accompany chronic alcoholism. 

---