Proteins Practice Test

A lab compares two proteins with identical amino acid composition but different sequences. In water, Protein 1 folds into a compact globular shape with a specific active site; Protein 2 folds into a different shape and shows no catalytic activity. Both proteins have the same numbers of polar, nonpolar, acidic, and basic side chains, but their order differs. Which statement best explains why only Protein 1 is catalytically active?

A membrane channel protein spans the lipid bilayer and contains many nonpolar amino acids on its exterior surface where it contacts phospholipid tails. Polar and charged amino acids line the channel interior, allowing selective passage of ions. The protein’s primary structure determines where α-helices form, and helix arrangement contributes to the tertiary structure that creates the pore. A mutation replaces several exterior leucines with lysines. Channel protein is still produced but is less abundant in the membrane. Which feature best explains the reduced membrane abundance?