Glucose entry into Krebs cycle

Figure 9.6 Sequence of electron carriers in the electron transfer chain. The positions of entry into the chain from metabolism of glucose, glutamine, fatty acyl-CoA, glycerol 3-phosphate and others that are oxidised by the Krebs cycle are shown. The chain is usually considered to start with NADH and finish with cytochrome oxidase. FMN is flavin mononucleotide FAD is flavin adenine dinucleotide.

Acyl coenzyme As are introduced into mitochondria following coenzyme A esterification in the cytoplasm. Mitochondrial entry depends upon a double membrane transport involving carnitine acyltransferases II and I. Excess acetyl CoA is used for KB synthesis. KBs are transported in the blood and ultimately metabolized via the Krebs cycle. KBs are necessary to provide energy to the brain during fasting, a true alternative substrate to glucose. 

The positions of entry of glucogenic components of the amino acids into the Krebs cycle are shown in Figure 8.10. The amino acids that can be oxidized to pyruvate can be converted to oxaloacetic acid (OAA), after which the carbon skeleton exits the mitochondrion in the form of malate and is converted to glucose. Amino acids that can be oxidized to a-ketoglutarate or to succinyl-CoA can be converted to OAA and then to glucose. 

Because not all amino acids are glucogenic, and because entry of carbon-skeleton metabolites into the Krebs cycle results in the release of carbon atoms as carbon dioxide during conversion to phosphoenolpyruvate, greater than 3 g of protein is required to make 1 g of glucose. 

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