Enzyme catalysis oxidation-reduction reactions

Metal Ion Catalysis Metals, whether tightly bound to the enzyme or taken up from solution along with the substrate, can participate in catalysis in several ways. Ionic interactions between an enzyme-bound metal and a substrate can help orient the substrate for reaction or stabilize charged reaction transition states. This use of weak bonding interactions between metal and substrate is similar to some of the uses of enzyme-substrate binding energy described earlier. Metals can also mediate oxidation-reduction reactions by reversible changes in the metal ion s oxidation state. Nearly a third of all known enzymes require one or more metal ions for catalytic activity. 

The oxidation-reduction reactions of C, N, S, and O in soils are, virtually without exception, catalyzed by microbial enzymes. These reaction rates without such catalysis are very slow (irreversible). Even with catalysis, the reactions are quite irreversible. The behavior of carbon and nitrogen in particular is dominated by nonequilibrium. 

The catalysis of oxidation-reduction reactions is carried out by a class of enzymes called oxidoreductases. A subclass of oxidore-ductases is given the common name dehydrogenases (such as lactate dehydrogenase), because they transfer hydrogen (hydrogen atoms or hydride atoms) from the substrate to an electron-accepting coenzyme, such as NAD. ... 

The third remarkable aspect of enzyme catalysis is the versatility of these species. They catalyze an extremely wide variety of reactions— oxidation, reduction, polymerization, dehydration, dehydrogenation, etc. Their versatility is a reflection of the range and complexity of the chemical reactions necessary to sustain life in plants and animals. 

However, these experiments may not have established a mechanism for natural flavoprotein catalysis because the properties of 5-deazaflavins resemble those of NAD+ more than of flavins.239 Their oxidation-reduction potentials are low, they do not form stable free radicals, and their reduced forms don t react readily with 02. Nevertheless, for an acyl-CoA dehydrogenase the rate of reaction of the deazaflavin is almost as fast as that of natural FAD.238 For these enzymes a hydride ion transfer from the (3 CH (reaction type D of Table 15-1) is made easy by removal of the a-H of the acyl-CoA to form an enolate anion intermediate. 

The possible role of oxygen atom transfer in molybdenum enzyme catalysis was recognized in the early 1970s (190-194). In the ensuing years, a wealth of chemistry has established molybdenum as the premier exponent of such reactions (7, 195). Importantly, related dioxo-Mo(VI) and oxo-Mo(IV) complexes are interconverted by oxygen atom transfer reactions (Eq. (13)). These reactions are effected by reductants (X) such as tertiary alkyl and aryl compounds of the group 15 elements (especially phosphines) and oxidants (XO) such as S- and N-oxides. In many cases, however, the Mo(VI) and Mo(IV) compounds participate in a comproportionation reaction yielding dinuclear Mo(V) complexes (Eq. (15)). 

In higher mammals, riboflavin is absorbed readily from the intestines and distributed to all tis.sues. It is the precursor in the biosynthesis of the cocnzyme.s flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The metabolic functions of this vitamin involve these Iwocoenzymes. which participate in numerous vital oxidation-reduction proces.ses. FMN (riboflavin 5 -phosphate) is produced from the vitamin and ATP by flavokinasc catalysis. This step con be inhibited by phcnothiazincs and the tricyclic antidepressants. FAD originates from an FMN and ATP reaction that involves reversible dinucicotide formation catalyzed by flavin nucleotide pyrophosphorylase. The.se coenzymes function in combination with several enzymes as coenzyme-en-zyme complexes, often characterized as, flavoproteins. 

In Starkeya novella, sulfite is oxidized to sulfate by the catalysis of sulfite-cytochrome c oxidoreductase [reaction (4.5)]. The enzyme catalyzes the reduction with sulfite of not only native ferricytochrome c-550 but also horse ferricytochrome c and ferricyanide (Charles and Suzuki, 1966b Yamanaka et al., 1971, 1981b). The enzyme with a molecular mass of 40 kDa has a cytochrome c-551 subunit (23 kDa) (Yamanaka et al., 1981b) and molybdenum (Toghrol and Southerland, 1983). Recently, Kappler et al. (2000) has reported that the molecular mass of the enzyme is 46 kDa and has cytochrome c subunit of 8.8 kDa. 

B. A. Palfey V. Massey, Flavin-Dependent Enzymes. In Comprehensive Biological Catalysis, Volume lll/Radical Reactions and Oxidation/Reduction] M. Sinnott, Ed. Academic Press London and SanDiego, 1998 Chapter 29, pp 83-154. 

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