Figure 9

Degradation of histidine. The pathways glutamate fermentation (fnc1) (red) and degradation of histidine to L-glutamate (histidine2) (black) describe (amino acid) degradations producing ammonia as an end product. Due to the reversibility of all its reactions, this also includes the pathway of biosynthesis of 5-formimino-THF (blue), which - inversely followed - describes the degradation of 5-formimino-THF to glutamate (c2). All three pathways are interconnected and can be interpreted as complete degradation of L-histidine to acetate and ammonia (NH₃). Thereby, three moles of ammonia per mole of histidine are produced (green or turquoise boxes, respectively). As histidine2 includes an alternative route from L-histidine to glutamate (dashed line), one mole of ammonia is either produced by the conversion of N-carbamoyl-L-glutamate to L-glutamate or by the conversion of N-formimino-tetrahydrofolate to 5,10-methenyl-tetrahydrofolate.