Final answer:
Changing valine or isoleucine to alanine decreases hydrophobicity due to alanine's smaller side chain, which also leads to a reduced molecular weight and steric hindrance, with no significant change in polarity.
Step-by-step explanation:
Going from valine or isoleucine to alanine decreases B) Hydrophobicity. Valine and isoleucine are amino acids with aliphatic side chains, which are nonpolar and hydrophobic. This hydrophobicity allows them to participate in hydrophobic interactions, contributing to the proteins' overall three-dimensional structure by fitting these amino acids within the protein's interior away from the water environment. Alanine, while also being a hydrophobic amino acid, has a smaller side chain compared to valine and isoleucine, resulting in less hydrophobic character. Therefore, when valine or isoleucine is replaced by alanine, the hydrophobicity of that portion of the protein is reduced. Along with this, a decrease in molecular weight and steric hindrance occurs because alanine's side chain is smaller. However, there is usually no significant change in polarity because all three amino acids have nonpolar side chains.