In myoglobin and hemoglobin, the heme binding pocket is predominantly composed of which type of amino acid side chains?

Introduction:
Heme proteins such as myoglobin and hemoglobin bind a heme prosthetic group to carry and store oxygen. The microenvironment of the heme pocket determines ligand binding, stability, and prevention of heme oxidation or loss. This question targets the chemical nature of residues lining that pocket.

Given Data / Assumptions:

• Heme is a planar porphyrin ring with hydrophobic character.
• Iron in heme coordinates with a proximal histidine; a distal histidine modulates ligand binding.
• Protein environments are tailored to stabilize cofactors and exclude unfavorable interactions.

Concept / Approach:
Because heme is largely hydrophobic, burying it within a hydrophobic pocket helps retain the cofactor and protect it from solvent. Strategic polar residues (notably histidines) are present for coordination and ligand discrimination, but the overall lining is hydrophobic to prevent heme loss and reduce reactive oxygen side reactions.

Step-by-Step Solution:

Verification / Alternative check:
Structural studies show largely nonpolar residues surrounding heme with key histidines at the proximal and distal positions. This design balances stability with functional ligand binding.

Why Other Options Are Wrong:

• Negatively or positively charged residues: excessive charge would destabilize the hydrophobic porphyrin ring.
• Polar residues: too many would increase heme solvation and loss risk.
• Alternating charges: not a recognized motif for heme retention.

Common Pitfalls:
Overgeneralizing the presence of histidine to claim a polar pocket; the histidines are functional exceptions within a predominantly hydrophobic environment.

Final Answer:
hydrophobic residues