Amino acids are differentiated primarily by their side chains, also known as R groups. These unique side chains give each amino acid its distinct chemical properties, influencing how it interacts with other molecules and its role in protein structure and function.
Key Differentiating Factors of Amino Acids
Several characteristics distinguish one amino acid from another:
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Side Chain (R Group) Structure: This is the most crucial factor. R groups vary widely in size, shape, charge, and polarity. Some are hydrophobic (water-repelling), while others are hydrophilic (water-attracting). Some carry a positive charge, others a negative charge, and some are neutral. This diversity is fundamental to the wide range of functions proteins can perform. For example, glycine has a simple hydrogen atom as its R group, while tryptophan has a large, bulky, aromatic ring.
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Isoelectric Point (pI): This is the pH at which an amino acid carries no net electrical charge. The pI varies among amino acids due to differences in their R group's ionization properties. This influences their behavior in different pH environments.
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Acidic, Basic, or Neutral Character: Based on the nature of their R groups, amino acids are classified as acidic (negatively charged at physiological pH), basic (positively charged at physiological pH), or neutral (no net charge at physiological pH).
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Aromatic or Aliphatic Side Chains: Amino acids can also be categorized based on whether their R groups are aromatic (containing a benzene ring) or aliphatic (containing only carbon and hydrogen atoms arranged in a linear or branched chain).
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Polarity: The R groups determine the polarity of each amino acid. Polar amino acids readily interact with water, while nonpolar amino acids tend to cluster together within protein structures, away from water.
Examples
- Glycine: Has a small, nonpolar hydrogen atom as its R group, making it highly flexible.
- Alanine: Has a methyl group (CH3) as its R group, slightly larger and still nonpolar.
- Aspartic acid: Has a carboxyl group (COOH) as its R group, giving it a negative charge at physiological pH (acidic).
- Lysine: Has an amino group (NH3+) as its R group, giving it a positive charge at physiological pH (basic).
The combination and arrangement of these different amino acids within a polypeptide chain ultimately determine the three-dimensional structure and function of the resulting protein.
