Structure of Proteins Quiz

See Classification of amino acids. Option B is correct. When amino acids join in a chain, each one is left as an amino acid residue, the part remaining after the peptide bond has formed. A protein's acid-base behaviour is determined by the R groups (the side chains) of its residues. Some R groups are acidic, such as glutamate and aspartate, and give up a proton to become negatively charged, while others are basic, such as lysine, arginine and histidine, and accept a proton to become positively charged. Each ionisable group has a characteristic pKa (the pH at which it is half protonated, half not), so its charge depends on whether the surrounding pH is above or below that value. Below the pKa the group is protonated, and above it the group is deprotonated. Because a protein contains many such groups, its overall charge changes with the pH, which affects its folding, its solubility and how it works. Non-ionisable R groups, such as aliphatic ones, do not contribute to this.

• Deciding feature: the ionisable R groups (side chains) of the amino acid residues.
• Acidic and basic groups: acidic ones such as glutamate and aspartate lose a proton, and basic ones such as lysine, arginine and histidine gain one.
• Ionisation: each group's charge depends on whether the pH is above or below its pKa.