The general structure of an amino acid features a central carbon atom bonded to four key components: an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group).
Here's a breakdown of each component:
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Central Carbon (α-carbon): This is the core of the amino acid molecule.
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Amino Group (-NH₂): A basic functional group consisting of a nitrogen atom bonded to two hydrogen atoms.
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Carboxyl Group (-COOH): An acidic functional group containing a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-OH).
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Hydrogen Atom (-H): A single hydrogen atom.
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R-Group (Side Chain): This is the variable group that distinguishes each of the 20 common amino acids. The R-group can vary in size, shape, charge, hydrophobicity, and reactivity, giving each amino acid its unique properties.
The central carbon is chiral (except for glycine), meaning it is bonded to four different groups. This chirality allows amino acids to exist as two stereoisomers, L-amino acids and D-amino acids. Only L-amino acids are found in proteins.
In aqueous solution at physiological pH, the amino group is usually protonated (-NH₃⁺) and the carboxyl group is usually deprotonated (-COO⁻). This form is called a zwitterion.
Therefore, the general formula can be summarized as: H₂N–CRH–COOH (or variations reflecting the zwitterionic form). The 'R' signifies the unique side chain for each amino acid. These variations in 'R' groups cause the unique properties and functions of the 20 amino acids found in the human body.
