Question

which functional group on the amino acid side chains would you expect to find in the active site of an enzyme that binds a hydrophobic, lipid - rich substrate?

Explanation:

1. Understand the Substrate: The substrate is hydrophobic and lipid - rich. Hydrophobic substances interact favorably with other hydrophobic groups.
2. Analyze Functional Groups:

• Hydroxyl ($\ce{R - OH}$), carboxyl ($\ce{R - COOH}$), amino ($\ce{R - NH_2}$), carbonyl ($\ce{R - CO - R'}$), phosphate, and sulfhydryl ($\ce{R - SH}$) groups are polar or charged. Polar/charged groups interact well with polar substances (like water) and would not be favorable for binding a hydrophobic substrate.
• The methyl group ($\ce{R - CH_3}$) is non - polar. Non - polar groups (hydrophobic groups) can interact with the hydrophobic, lipid - rich substrate through hydrophobic interactions (like van der Waals forces or London dispersion forces), which are important for binding such a substrate in the enzyme's active site.

Answer:

Methyl ($\boldsymbol{\ce{R - CH_3}}$)