Respiratory chain catalytic cycle

Figure 7-2. Reactions of the pyruvate dehydrogenase (PDU) multienzyme complex (PDC). Pyruvate is decarboxylated by the PDH subunit (I ,) in the presence of thiamine pyrophosphate (TPP). The resulting hydroxyethyl-TPP complex reacts with oxidized lipoamide (LipS3), the prosthetic group of dehydrolipoamide transacetylase (Ii2), to form acetyl lipoamide. In turn, this intermediate reacts with coenzyme A (CoASH) to yield acetyl-CoA and reduced lipoamide [Lip(SH)2]. The cycle of reaction is completed when reduced lipoamide is reoxidized by the flavoprotein, dehydrolipoamide dehydrogenase (E3). Finally, the reduced flavoprotein is oxidized by NAD+ and transfers reducing equivalents to the respiratory chain via reduced NADH. PDC is regulated in part by reversible phosphorylation, in which the phosphorylated enzyme is inactive. Increases in the in-tramitochondrial ratios of NADH/NAD+ and acetyl-CoA/CoASH also stimulate kinase-mediated phosphorylation of PDC. The drug dichloroacetate (DCA) inhibits the kinase responsible for phosphorylating PDC, thus locking the enzyme in its unphosphory-lated, catalytically active state. Reprinted with permission from Stacpoole et al. (2003).

Fi>2 is a member of a family of homologous flavoproteins that catalyze the oxidation of a-hydroxy acids. It is located in the intermembrane space of yeast mitochondria and provides pyruvate for the Krebs cycle as well as participates in a short electron-transfer chain involving cytochrome c and cytochrome oxidase, making it an important respiratory enzyme. Unlike many other flavoproteins that oxidize a-hydroxy acids, F 2 has very poor reactivity toward oxygen. Fi>2 is a homotetramer with each monomer containing both an FMN and a heme The catalytic cycle has five redox steps (Scheme 7). In the first step, lactate is oxidized to... 

From the previous section it is evident that our knowledge about the respiratory chain is still quite incomplete. We know which prosthetic groups participate (cf. diagram in Section 4). It remains to be clarified, however, to what proteins they are bound and what role the metals and any new cofactors might play. The reason for this unsatisfactory state of knowledge is that the enzymes under consideration are bound very firmly to the mitochondrial structure (cf. Chapt. XIX-3). Only very recently have techniques been developed to subdivide the mitochondria in such a manner that most of their activity is retained. The subunits thus obtained have been called electron-transport particles (Green and co-workers). Some of the catalytic capabilities have been sacrificed (e.g. the enzymes of the citric acid cycle). But they are still able to oxidize NADHs or succinate with consumption of Oj and formation of ATP (see below). With the further destruction of these subunits, the capacity for oxidative phosphorylation disappears. 

For the cycle to run in this fashion—i.e., anaerobically—there would always have to be a fresh supply of NAD to enter the cycle as a true substrate. But actually the coenzyme is present only in catalytic amounts and is regenerated continuously. The citrate cycle runs only in conjunction with the respiratory chain, and the calculation of energy yields must be revised. Indeed, of a total of 216 kcal of chemical energy released 191 are due to the respiratory chain. This should make it quite clear that the energy is really derived from the formation of water and not from CO2 production ... 

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