Flavoprotein dehydrogenation reactions

In flavin-dependent monooxygenases, a flavin-oxygen intermediate reacts with the substrate, also producing water in a second step, and requiring cofactors for regeneration of the flavin moiety. The unusual flavoprotein vanillyl-alcohol oxidase (EC 1.1.3.38), in which the flavin moiety is covalently bound, catalyzes the oxidation of p-substituted phenols as well as deamination, hydroxylation and dehydrogenation reactions [10]. 

Fig. 60. The respiratory chain of higher plants. Ubiquinone appears to serve as an electron reservoir. = probable site of ATP formation. SD = succinate dehydrogenase. It used to be assumed that, with the exception of the reaction catalyzed by SD, the hydrogen acceptor in dehydrogenation reactions was NAD+ and that the hydrogen then entered the respiratory chain in the form of NADH+H+. In reality the situation is more complicated since the lipoic acid oxidizing flavoproteid of the pyruvate dehydrogenase and the a-ketoglutarate dehydrogenase complexes—in both cases the same flavoproteid is involved—can establish direct contact with the flavoproteins of the respiratory chain just like succinate dehydrogenase. associated with encircled flavoproteins means that ATP can be formed as a result of transitions between the various flavoproteins, except those involving SD.

Pyruvate oxidase. The soluble flavoprotein pyruvate oxidase, which was discussed briefly in Chapter 14 (Fig. 14-2, Eq. 14-22), acts together with a membrane-bound electron transport system to convert pyruvate to acetyl phosphate and C02.319 Thiamin diphosphate is needed by this enzyme but lipoic acid is not. The flavin probably dehydrogenates the thiamin-bound intermediate to 2-acetylthiamin as shown in Eq. 15-34. The electron acceptor is the bound FAD and the reaction may occur in two steps as shown with a thiamin diphosphate radical intermediate.3193 Reaction with inorganic phosphate generates the energy storage metabolite acetyl phosphate. 

Searls and Sanadi (1960a) determined the reduction potential of the pig heart dihydrolipoic dehydrogenase from the extent of its reduction at different DPNH DPN ratios. The value is between —0.332 and —0.320 volt at pH 7.0 and 25°C. Thus the reduction potential of the flavoprotein is close to that of the DPNH-DPN system, —0.320 volt at pH 7.0 and 25°C (Burton and Wilson, 1953), and the Lip(SH)2-LipS2 system, —0.325 volt at pH 7.0 and 25°C (see Section II, A). This is consistent with the ready reversibility of reaction (26) as well as the high initial reaction rates in both directions. It would appear that DPN is the physiological electron acceptor for the a-keto acid dehydrogenation complexes in vivo and that the DPNH formed is reoxidized by way of the electron transport chain. However, the... 

Three enzyme activities forming a relatively stable complex participate in the overall reaction (Fig. 22) -ketoacid dehydrogenase, dihydrolipoamide acyl-transferase and the flavoprotein dihydrolipoamide reductase. Thiamine pyrophosphate (D 10.4.5), lipoic acid (Table 16) and coenzyme A (D 11) are involved. Acids shortened by one C-atom and activated by binding to CoA are the final products. The activated bond is formed during the dehydrogenation of intermediate I by formation of the acylated lipoic acid (intermediate II). The last steps are transfer of the acyl moiety to coenzyme A and regeneration of the lipoic acid. 

In the first of these three reactions two hydrogen atoms are removed from succinate and fumarate is formed. In this dehydrogenation the hydrogen acceptor is not NAD but a flavo-protein (FF). The flavoprotein is in fact the enzyme succinate dehydrogenase which contains flavin adenine dinucleotide (FAD) as a covalently bound prosthetic group and also non-haem iron. It differs from other enzymes of the citrate cycle in being an integral constituent of the inner mitochondrial membrane and directly linked to the electron-transport chain. 

The sequence of reactions from 3 to 7, Fig. 5, was established in the following manner. Dehydrogenation of isobutyryl CoA was demonstrated with dialyzed liver extract, employing the oxidation-reduction indicator, triphenyltetrazolium chloride, as the hydrogen acceptor (87). Only the CoA derivate would serve as a substrate. Furthermore, Crane et al. (97) observed that the flavoprotein, butyryl CoA dehydrogenase, specific for straight chain acyl CoA derivatives from C4 to Cg, is reduced by isobut3rryl CoA. 

Dehydrogenation of succinate introduces a C—C double bond to give rise to the frans-compound fumarate (step 7). The enzyme succinate dehydrogenase is a flavoprotein and is a member of the respiratory chain (cf. Chapt. X-4). This reaction is inhibited by malonate—the classical example of competitive inhibition The wrong substrate is attached to the enzyme, but cannot undergo the reaction for simple chemical reasons. 

Massey, V., S. Ghisla, D. Ballou, C. T. Walsh, Y. F. Cheng, and R. H. Abeles Rapid Reaction Studies on Dehydrogenation and Elimination Reactions of D-Amino Acid Oxidase and Lactate Oxidase. In Flavins and Flavoproteins (T. P. Singer, ed.). Amsterdam Elsevier, in press. 

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