Tricarboxylate cycle energetics

Lactate and alanine enter as pyruvate following the activities of lactate dehydrogenase (Figure 11.4) and alanine aminotransferase (Section 16.3). The first of the bypass reactions, the objective of which is to overcome the unfavourable energetics of a reversal of the pyruvate kinase reaction, seems a tortuous route (Figure 11.11). The reaction sequence relies on two important enzymes pyruvate carboxylase and phosphoenolpyruvate carboxykinase. Since pyruvate carboxylase is located exclusively in the mitochondrion, pyruvate must cross the inner mitochondrial membrane (Section 12.2). Oxaloacetate produced by pyruvate carboxylase cannot traverse the inner membrane and is reduced by malate dehydrogenase into l-malate. This step is the reversal of the tricarboxylate cycle reaction (Section 12.4). Malate may, of... 

This chapter is mainly concerned with the contribution of the tricarboxylic acid cycle to carbohydrate metabolism. The TCA cycle is the main source of electrons for oxidative phosphorylation, and thereby the major energetic sequence in the metabolism of aerobic cells or organisms. It serves as the main distribution center of metabo-... 

Fig. 3 Model for regulation of mcl-PHA at the enzymatic ievel in P. putida GPol. Alkanoic acids of mcl (C -Cj ) are taken up by the cell and enter the P-oxidation cycle. The mcl-PHA synthase preferentially polymerizes only (/f)-3-hydroxyacyl-CoA of mcl. It is proposed that during accumulation degradation tilso teikes place (Ren et al. 2009 Tinn 1998). Depending on the energetic status of the cell, the 3-hydroxytilktmoate can be excreted or recycled to (/J)-3-hydroxyacyl-CoA by the acyl-CoA synthetase. Consequently, (R)-3-hydroxyacyl-CoA can be polymerized to mcl-PHA again or channeled back to the p-oxidation cycle and subsequently to the tricarboxylic acid cycle for energy gtiin...

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