Citric acid cycle, amino acids linked

The glutamate family of transaminases is very important because the ketoacid corresponding to glutamate is a-ketoglutarate, one of the citric acid cycle intermediates. This provides a link between the citric acid cycle and amino acid metabolism. These transaminases provide amino groups for amino acid synthesis and collect amino groups during catabolism of amino acids. 

II. AMINO ACIDS LINKED WITH THE CITRIC ACID CYCLE... 

In addition to its role in oxidation of acetyl CoA, the citric acid cycle is an important central metabolic pathway, providing the link between carbohydrate, fat and amino... 

II. Amino Acids Linked with the Citric Acid Cycle... 

Porphyrins are a small but important group of secondary plant substances. We dealt with their structure when we discussed the chlorophylls. The biosynthesis of the porphyrins is interlinked with the citric acid cycle and amino acid metabolism. (Fig. 132). The precursor succinyl CoA is derived from the citric acid cycle and the precursor glycine from amino acid metabolism. The two are combined to give an unstable intermediate which loses CO2 to form 8-aminolevulinic acid. Two molecules of 8-aminolevulinic acid are combined to yield porphobilinogen, a pyrrole system, which is the building block of the porphyrins. Four such porphobilinogen molecules become linked in a series of steps to form the porphyrin system. The first porphyrin to appear in the course of biosynthesis is uroporphyrinogen III. Of the subsequent intermediates protoporphyrin IX is worthy of mention. This is because introduction of Mg " on the one hand leads to Mg-protoporphyrin IX and then further to the chlorophylls, whereas introduction of Fe++ on the other yields Fe-... 

Pyridoxal phosphate is a required coenzyme for many enzyme-catalyzed reactions. Most of these reactions are associated with the metabolism of amino acids, including the decarboxylation reactions involved in the synthesis of the neurotransmitters dopamine and serotonin. In addition, pyridoxal phosphate is required for a key step in the synthesis of porphyrins, including the heme group that is an essential player in the transport of molecular oxygen by hemoglobin. Finally, pyridoxal phosphate-dependent reactions link amino acid metabolism to the citric acid cycle (chapter 16). 

The interconnected cycles have been called the "Krebs bicycle." The pathways linking the citric acid and urea cycles are called the aspartate-argininosuccinate shunt these effectively link the fates of the amino groups and the carbon skeletons of amino acids. The interconnections are even more elaborate than the arrows suggest. For... 

This transfer of reducing equivalents is essential for maintaining the favorable NAD+/NADH ratio required for the oxidative metabolism of glucose and synthesis of glutamate in brain (McKenna et al., 2006). The malate-aspartate shuttle is considered the most important shuttle in brain. It is particularly important in neurons. It has low activity in astrocytes. This shuttle system is fully reversible and linked to amino acid metabolism with the energy charge and citric acid cycle of neuronal cells. 

Like all the other amino acids in proteins, the L isomer of alanine is exclusively used, though D-alanine can be found as a component of bacterial cell walls. Another variant of alanine, called / -alanine, has the amino group on the carbon and is found in pantothenic acid. Alanine can be formed by transamination of pyruvate and thus has close links to pathways such as glycolysis, gluconeogenesis, and the citric acid cycle. 

The urea cycle, which has links to the citric acid cycle, plays a central role in nitrogen metabolism. It is involved in both the anabolism and the catabolism of amino acids. 

Recall Which amino acids in the urea cycle are the links to the citric acid cycle Show how these links occur. 

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