Degradation flavin cofactor

Degradation of DDT, dieldrin and lindane by the flavoprotein preparation was almost more efficient in the absence than in the presence of FMN (e.g. Figure 3). On the contrary, photodegradation of mexacarbate was greatly enhanced by FMN and other flavin cofactors. It is well known that flavin cofactors, such as FMN, are active photosensitizers. Hence it is possible that the mechanisms or pathways involved for the photodegradation of DDT, dieldrin and lindane and that for mexacarbate are different... 

Figure 7. Degradation pathway of mexacarbate through the flavin cofactor-fer-redoxin system...

Table 3. Degradation of C-mexacarbate by ferredoxin in the presence and absence of a flavin cofactor ...

Figure 37.6 Effect of flavin cofactor binding on the stability of the human electron-transfer flavoprotein (ETF) mutant variant Aspl28Asn. (A) Activity of the protein is affected by incubation at 39 °C (open circles) however, in the presence of 2.5-fold excess FAD the activity is preserved (black circles). (B) The stability of ETF Aspl28Asn to urea-induced chemical denaturation is higher when the flavin is bound to the protein (black circles) than in flavin-depleted ETF (open circles). (C) The presence of flavin cofactor affects the proteolytic susceptibility of ETF Aspl28Asn. Upon incubation with trypsin protease ETF Aspl28Asn is rapidly degraded (top panel), whereas in the presence of excess flavin, the protein is more resistance to proteolysis.

All bacteria where nitrate ester degradation has been characterized have very similar enzymes. The enzymes eatalyze the nicotinamide cofactor-dependent reductive eleavage of nitrate esters that produces alcohol and nitrite. Purification of the PETN reduetase from Enterobacter cloacae yielded a monomerie protein of around 40 kilo Daltons, which required NADPH as a co-faetor for aetivity. Similar enzymes were responsible for the nitrate ester-degrading activity in Agrobacterium radiobacter (Snape et al. 1997) - nitrate ester reductase - and in the strains of Pseudomonas fluorescens and Pseudomonas putida (Blehert et al. 1999) - xenobiotic reduetases . All utilize a non-covalently bound flavine mononucleotide as a redox eofactor. 

It is, however, better known that flavoenzymes (i.e., enzymes utilizing the flavin adenine dinucleotide [FAD FADH2] redox system) mediate the introduction of a,P carbon-carbon double bonds into carboxylic acids and into acetyl Coenzyme A (acetyl CoA) thioesters of long-, medium-, and short-chain fatty acids. In carboxylic acids, such as those of the tricarboxylic acid (citric acid, TCA, or Krebs) cycle (Chapter 11) the oxidation is affected by the enzyme sucdnate dehydrogenase (fumerate reductase— EC 1.3.99.1), which utilizes the cofactor flavin adenine dinucleotide (FAD) The latter is reduced to FADH2 and an ( )-double bond is introduced. The process shown in Scheme 9.105, for the conversion of succinate (1,4-butanedioic acid) to fumerate [(E)-l,4-butenedioic acid], is a fragment of the tricarboxylic acid (citric acid, TCA, or Krebs) cycle (Chapter 11), which is the pathway commonly utilized for oxidative degradation of acetate to carbon dioxide. 

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