Coenzymes in Biologically Important Oxidation-Reduction Reactions

B FIGURE 1B.2 Comparison of the state of reduction of carbon atoms in biomolecules  

In catabolism, large molecules are broken down to smaller products, releasing energy and transferring electrons to acceptor molecules of various sorts. The overall process is one of oxidation. 

In anabolism, small molecules react to give rise to larger ones this process requires energy and involves acceptance of electrons from a variety of donors. The overall process is one of reduction. 

5 Coenzymes in Biologically Important Oxidation-Reduction Reactions 

Chapter 15 The Importance of Energy Changes and Electron Transfer In Metabolism 

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Indicine IV-oxide (169) (Scheme 36) is a clinically important pyrrolizidine alkaloid being used in the treatment of neoplasms. The compound is an attractive drug candidate because it does not have the acute toxicity observed in other pyrrolizidine alkaloids. Indicine IV-oxide apparently demonstrates increased biological activity and toxicity after reduction to the tertiary amine. Duffel and Gillespie (90) demonstrated that horseradish peroxidase catalyzes the reduction of indicine IV-oxide to indicine in an anaerobic reaction requiring a reduced pyridine nucleotide (either NADH or NADPH) and a flavin coenzyme (FMN or FAD). Rat liver microsomes and the 100,000 x g supernatant fraction also catalyze the reduction of the IV-oxide, and cofactor requirements and inhibition characteristics with these enzyme systems are similar to those exhibited by horseradish peroxidase. Sodium azide inhibited the TV-oxide reduction reaction, while aminotriazole did not. With rat liver microsomes, IV-octylamine decreased... 

In addition to their role in the formation of DNA and RNA (see Section 27.2), nucleotides have other important biological functions. For example, adenosine triphosphate (ATP) is an important energy carrier in biochemical reactions, and nicotinamide adenine dinucleotide is a coenzyme that is often involved in biochemical oxidation-reduction reactions. 

What are the reactions of key oxidation-reduction coenzymes Many biologically important redox reactions involve coenzymes, such as NADH and FADH. These coenzymes appear in many reactions as one of the half reactions that can be written for a redox reaction. 

The conversion (19) of thiols to disulphides coupled with reduction of flavin (vitamin B2 family) is a topic of import in connection with coenzyme reactivity in flavoenzymes. Since flavin oxidation of thiols involves nucleophilic attack of thiolate ion in the rate-determining step (Loechler and Hollocher, 1975 Yokoe and Bruice, 1975), this biologically important reaction would be markedly affected by hydrophobic environments. 

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