histidine metabolic pathway PW:0000051 Those metabolic reactions involved in the synthesis, utilization and/or degradation of histidine, an essential amino acid which can act as either a proton donor or a proton acceptor. While the synthesis and degradation pathways of histidine are present in microorganisms, histidine can be acted upon by eukaryotic enzymes. Histidine is the precursor of histamine, is involved in immune responses and also acts as a neurotransmitter. histidine metabolic process pathway hydrophilic amino acid metabolic pathway amino acid metabolic pathway (KEGG)