What classification does the amino acid Threonine belong to?

Threonine is classified as a polar amino acid because of its side chain, which contains a hydroxyl (-OH) group. This functional group is hydrophilic, allowing threonine to interact favorably with water and other polar molecules. Polar amino acids generally have side chains that can form hydrogen bonds, contributing to their solubility in aqueous environments. The presence of the hydroxyl group also enables threonine to participate in essential biochemical processes, such as protein interactions and enzyme reactions.

In contrast, nonpolar amino acids lack such hydrophilic side chains and are typically hydrophobic, making them less soluble in water. Aromatic amino acids have unique ring structures that contain delocalized electrons, playing different roles in proteins, particularly in terms of stability and absorption of UV light. Positively-charged amino acids have side chains that can accept protons, giving them a positive charge at physiological pH, which is different from threonine's characteristics. Thus, threonine’s classification as polar reflects its inherent properties and interactions within biological systems.