FADH2

A compound which is a good choice for an artificial electron relay is one which can reach the reduced FADH2 active site, undergo fast electron transfer, and then transport the electrons to the electrodes as rapidly as possible. Electron-transport rate studies have been done for an enzyme electrode for glucose (G) using interdigitated array electrodes (41). The following mechanism for redox reactions in osmium polymer—GOD biosensor films has... 

Direct hydroxylation of an aromatic ring to yield a hydroxybenzene (a phenol) is difficult and rarely done in the laboratory., but occurs much more frequently in biological pathways. An example is the hydroxylation of p-hydroxyphenyl acetate to give 3,4-dihydroxyphenyl acetate. The reaction is catalyzed by p-hydroxyphenylacctate-3-hydroxylase and requires molecular oxygen plus the coenzyme reduced flavin adenine dinucleotide, abbreviated FADH2. 

This thiol-disulfide interconversion is a key part of numerous biological processes. WeTJ see in Chapter 26, for instance, that disulfide formation is involved in defining the structure and three-dimensional conformations of proteins, where disulfide "bridges" often form cross-links between q steine amino acid units in the protein chains. Disulfide formation is also involved in the process by which cells protect themselves from oxidative degradation. A cellular component called glutathione removes potentially harmful oxidants and is itself oxidized to glutathione disulfide in the process. Reduction back to the thiol requires the coenzyme flavin adenine dinucleotide (reduced), abbreviated FADH2. 

Step 1 of Figure 29.3 Introduction of a Double Bond The /3-oxidation pathway begins when a fait)7 acid forms a thioester with coenzyme A to give a fatty acyl Co A. Two hydrogen atoms are then removed from C2 and C3 of the fatty acyl CoA by one of a family of acyl-CoA dehydrogenases to yield an a,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently jn biochemical pathways and usually involves the coenzyme flavin adenine dinucleotide (FAD). Reduced FADH2 is the by-product. 

As its name implies, the citric acid cycle is a closed loop of reactions in which the product of the hnal step (oxaloacetate) is a reactant in the first step. The intermediates are constantly regenerated and flow continuously through the cycle, which operates as long as the oxidizing coenzymes NAD+ and FAD are available. To meet this condition, the reduced coenzymes NADH and FADH2 must be reoxidized via the electron-transport chain, which in turn relies on oxygen as the ultimate electron acceptor. Thus, the cycle is dependent on the availability of oxygen and on the operation of the electron-transport chain. 

The Cyclic Reaction Sequence Generates FADH2 NADH... 

In Pseudomonas putida, the hydroxylation of 4-hydroxyphenylacetate to 3,4-dihydroxy-phenylacetate is carried out by an enzyme that consists of a flavoprotein and a coupling factor (Arunachalam et al. 1992). At least in Escherichia coli, it has been suggested that the large component is an FADH2-utilizing monooxygenase (Xun and Sandvik 2000 Chaiyen et al. 2001). 

FIGURE 12.8 Structure of the FAD/FADH2 active site of glucose oxidase. 

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