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|Section2= |Section3= }} Histidine (abbreviated as His or H) is a proteinogenic, α-amino acid with an imidazole functional group. Initially thought essential only for infants, longer-term studies shown it's essential for adults also. It is one of the 23 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. ==Chemical properties== The conjugate acid (protonated form) of the imidazole side chain in histidine has a p''K''a of approximately 6.0. This means that, at physiologically relevant pH values, relatively small shifts in pH will change its average charge. Below a pH of 6, the imidazole ring is mostly protonated as described by the Henderson–Hasselbalch equation. When protonated, the imidazole ring bears two NH bonds and has a positive charge. The positive charge is equally distributed between both nitrogens and can be represented with two equally important resonance structures. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「histidine」の詳細全文を読む スポンサード リンク
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