Catabolic Pathways for Arginine Histidine Glutamate Glutamine, and Proline

Glutamine is converted to glutamate by glutaminase or several other enzymes by the removal of the amide nitrogen. Proline is first converted to a Schiff base and then converted by hydrolysis to glutamate-5-semialdehyde. All of these changes occur on the same carbon. Arginine and histidine contain 5adjacent carbons and a sixth carbon attached through a nitrogen attom. The catabolism of these amino acids is thus slightly more complicated than glutamine or proline. Arginine is converted to ornithine and urea. Ornithine is furthere transaminated to produce glutamate-5-semialdehyde. Glutamate-5-semialdehyde is converted to glutamate. The enzymes involved in the steps of the histidine pathway are listed in the box in the lower right corner of the diagram. Tetrahydrofolate is the cofactor in the final step converting histidine to glutamate. Transamination or deamination of glutamate produces a-ketoglutarate which feeds into the citric acid cycle.