Arginine, glutamine, histidine and proline are all degraded by conversion to glutamate, wich in turn is oxidized to a-ketoglutarate by glutamate dehydrogenase. Their carbon skeletons so enter the citric acid cycle.

Conversion of glutamine to glutamate involves only one reaction: hydrolysis by glutaminase.

Histidine's conversion to glutamate is more complicated: it's converted into 4-imidazolone-5-propionate. the amide bond in the ring of this intermediate is hydrolyzed to the N-formimino derivative of glutamate, wich is then converted into glutamate by transfer of its formimino group to tetrahydrofolate, a carrier of activated one-carbon units.

Both arginine and proline are converted to glutamate through the intermediate formation of glutamate-5-semialdehyde.