Pathways of Amino Acid Degradation:- Five Amino Acids Are Converted to α-Ketoglutarate
The carbon skeletons of five amino acids (proline, glutamate, glutamine, arginine, and histidine) enter the citric acid cycle as -ketoglutarate (Fig. 18–26). Proline, glutamate, and glutamine have five-carbon skeletons. The cyclic structure of proline is opened by oxidation of the carbon most distant from the carboxyl group to create a Schiff base, then hydrolysis of the Schiff base to a linear semialdehyde, glutamate -semialdehyde. This intermediate is further oxidized at the same car bon to produce glutamate. The action of glutaminase, or any of several enzyme reactions in which glutamine donates its amide nitrogen to an acceptor, converts glutamine to glutamate. Transamination or deamination of glutamate produces -ketoglutarate. Arginine and histidine contain five adjacent car bons and a sixth carbon attached through a nitrogen atom. The catabolic conversion of these amino acids to glutamate is therefore slightly more complex than the path from proline or glutamine (Fig. 18–26). Arginine is converted to the five-carbon skeleton of ornithine in the urea cycle (Fig. 18–10), and the ornithine is transaminated to glutamate -semialdehyde. Conversion of histidine to the five-carbon glutamate occurs in a multistep pathway; the extra carbon is removed in a step that uses tetrahydrofolate as cofactor.
FIGURE 18–26 Catabolic pathways for arginine, histidine, glutamate, glutamine, and proline. These amino acids are converted to -ketoglutarate. The numbered steps in the histidine pathway are catalyzed by 1 histidine ammonia lyase, 2 urocanate hydratase, 3 imidazolonepropionase, and 4 glutamate formimino transferase.
