Final answer:
In proteins, the ionizable groups include the acidic amino acids aspartic acid and glutamic acid, the neutral amino acid alanine, and the basic amino acids arginine, lysine, and histidine. These groups can gain or lose protons depending on the pH, influencing the protein's overall charge and function.
Step-by-step explanation:
Proteins contain ionizable groups that can carry a charge under certain pH conditions. These groups are commonly associated with the side chains of amino acids, and their propensity to gain or lose protons can influence the overall charge of a protein molecule.
Acidic Amino Acids
The acidic amino acids include aspartic acid (Asp) and glutamic acid (Glu). They contain carboxyl side chains that can donate a proton to become negatively charged at physiological pH.
Neutral Amino Acids
A neutral amino acid has side chains that do not ionize at physiological pH. An example is alanine (Ala), which has a nonpolar side chain.
Basic Amino Acids
The basic amino acids, namely arginine (Arg), lysine (Lys), and histidine (His), have side chains that can accept protons. Arginine and lysine are positively charged at physiological pH due to their primary and secondary amine groups, respectively. Although histidine has a pKa near neutral pH and is less likely to be protonated at pH 7.4, it can still accept protons under slightly acidic conditions.
All amino acids can act as zwitterions, having both positively and negatively charged groups within the same molecule, allowing them to act as both acids and bases. This amphoteric nature often plays a critical role in protein structure and function.