What are the Special Case Amino Acids?
Several amino acids possess unique structural or chemical properties distinguishing them from the rest. These "special case" amino acids frequently appear in discussions of protein structure and function. While the exact definition of "special case" can vary depending on the context, several amino acids consistently appear in this category.
The most frequently cited special case amino acids include:
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Cysteine: Cysteine's thiol (-SH) group readily oxidizes, forming a disulfide bond (S-S) with another cysteine residue. This covalent bond is crucial for stabilizing protein tertiary and quaternary structure. (Source: Chemistry LibreTexts, Jack Westin's MCAT Guide, Wikipedia)
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Glycine: Glycine has the simplest side chain—a single hydrogen atom. This small size allows for greater flexibility in protein folding, often found in regions requiring tight turns or bends. (Source: Chemistry LibreTexts, Amino Acids: Special Cases - Biochemistry Flashcards)
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Proline: Proline's side chain forms a cyclic structure with the amino group, limiting its rotational flexibility. This rigidity is important for maintaining specific protein conformations, often found in turns or loops. (Source: Reddit Biology discussion, Khan Academy video, Chemistry LibreTexts)
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Histidine: Histidine's imidazole side chain can act as both an acid and a base, making it crucial for enzyme catalysis and pH buffering within proteins. (Source: Khan Academy video, Quizlet flashcards)
While other amino acids exhibit unique properties, these four are most commonly highlighted as "special cases" due to their significant impact on protein structure and function. The specific properties discussed can influence the overall folding and stability of the protein.
