Amino acids can be categorized based on the charge of their side chains (R-groups) at a physiological pH of around 7. This charge is crucial for protein structure, function, and interactions.
Here's a breakdown of the amino acid types based on charge:
1. Negatively Charged (Acidic) Amino Acids
These amino acids have acidic side chains with a net negative charge at pH 7. This is because they have a carboxyl group (-COOH) in their side chain that can donate a proton (H+).
- Aspartic Acid (Asp, D): Contains a carboxylic acid group in its side chain.
- Glutamic Acid (Glu, E): Also contains a carboxylic acid group in its side chain.
2. Positively Charged (Basic) Amino Acids
These amino acids have basic side chains with a net positive charge at pH 7. They typically have an amino group (-NH2) in their side chain that can accept a proton (H+).
- Lysine (Lys, K): Possesses an amino group at the end of its side chain.
- Arginine (Arg, R): Contains a guanidino group, which is positively charged at physiological pH.
- Histidine (His, H): Has an imidazole ring in its side chain. The pKa of histidine's side chain is close to physiological pH, meaning it can be either charged or uncharged depending on the precise environment. This makes it important in enzyme active sites.
3. Uncharged Polar Amino Acids
Although not charged at pH 7, these amino acids have polar side chains containing atoms (like oxygen or nitrogen) that create a partial charge. They can form hydrogen bonds, influencing protein structure and interactions. Examples include serine, threonine, cysteine, asparagine, and glutamine. These are NOT determined by charge.
4. Nonpolar Amino Acids
These amino acids have hydrophobic side chains, meaning they are repelled by water. They tend to cluster together in the interior of proteins, stabilizing the structure. These are NOT determined by charge. Examples include alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, and methionine. These are NOT determined by charge.
In summary, only aspartic acid, glutamic acid, lysine, arginine, and histidine have a definitive charge at a neutral pH and are grouped based on that charge.
