Respiratory chain enzyme complex

In mitochondria, more than 90 % of the respiratory phosphorylation is catalyzed by the H -ATP-synthase, an enzyme converting the respiratory chain-produced electrochemical potential difference (ApH+) into ATP [1-4]. Very small (but sometimes essential) portion of the respiratory energy is converted to GTP by succinate thiokinase [4]. Both respiratory chain enzymes (Complexes I, III and IV), catalyzing electron transfer from NAD(P)H to O2, and H -ATP-synthase are localized in the inner mitochondrial membrane. The great majority of the formed ATP molecules is exported from mitochondria by the ATP/ADP antiporter in exchange for extramitochondrial ADP (eqs. 1-3). 

Unlike other cellular organelles, mitochondria have their own DNA. The human mitochondrial DNA (mtDNA), maternally transmitted, encodes 13 subunits of the respiratory chain enzyme complexes I, III, IV and complex V. In addition to structural genes, mtDNA also codes for 22 transfer RNAs and 2 ribosomal RNAs. This makes the oxidative phosphorylation system, which includes the respiratory chain (complexes I-IV) and complex V, unique as the different components are encoded by nuclear DNA (Mende-lian inheritence) and mtDNA (maternal inheritence) with the exception of complex II, which is entirely nuclear encoded. 

Dysfunction of Complex 1 respiratory chain enzyme and ultrastructural damage in the hippocampal mitochondria is seen in kainite-induced SE in rats (Chuang et al., 2(X)4). A key player in mitochondrial oxidative phosphorylation. Complex I is a major source of superoxide and its dysfunction may increase mitochondrial reactive oxygen species (ROS) production and redox signaling (Taylor et al., 2(X)3). The perforant path stimulation model shows mitochondrial dysfunction and decreased brain glutathione (Cock et al., 2002). Pilocarpine-treated rats show selective decline in Complexes 1 and IV activity in hippocampal CAland CA3 subfields (Kudin et al., 2002). This pattern of complex 1 deficiency in CA3 region is also seen in humans (Kunz et al., 2000). 

Sumegi, B., Porpaczy, Z., Alkonyi, 1. (1991). Kinetic advantage of the interaction between the fatty acid P-oxidation enzymes and the complexes of the respiratory chain. Biochim. Biophys. Acta 1081, 121-128. 

In the EPR of mammalian cells, we do not see much in addition to the signals from the respiratory complexes. The enzyme aconitase from the citric-acid cycle can be detected, and also the protein cytoplasmic aconitase, later identified as the mRNA translation regulatory factor iron regulatory protein IRP-1, which actually started its career in biochemistry as an EPR signal that could not be assigned to the respiratory chain (Kennedy et al. 1992). 

Cytochrome bci is a multicomponent enzyme found in the inner mitochron-drial membrane of eukaryotes and in the plasma membrane of bacteria. The cytochrome bci complex functions as the middle component of the mitochondrial respiratory chain, coupling electron transfer between ubiquinone/ ubiquinol (see Figure 7.27) and cytochrome c. 

---