Properties of bacteriorhodopsin from Halobacterium: Which statements correctly describe this remarkable membrane protein?

Introduction / Context:
Bacteriorhodopsin is a photoreceptor protein found in the purple membrane of certain halophilic archaea. It is a classic model for studying proton pumps and membrane protein structure.

Given Data / Assumptions:

• Bacteriorhodopsin uses light energy to move protons across the membrane.
• Its structure and location are well characterized.

Concept / Approach:
Bacteriorhodopsin binds retinal as a chromophore. Upon photon absorption, retinal isomerization drives conformational changes that pump protons from cytoplasm to extracellular side, generating a proton motive force used by ATP synthase.

Step-by-Step Solution:
Check function → light-driven proton pump (true). Check localization → integral membrane protein in purple membrane (true). Check structure → seven transmembrane alpha-helices (true). Therefore, choose “All of the above”.

Verification / Alternative check:
Crystallographic and spectroscopic studies confirm the 7-helix bundle and photocycle driving proton translocation.

Why Other Options Are Wrong:
Option e is incorrect: it is not soluble and does not hydrolyze ATP; it creates the gradient used by ATP synthase.

Common Pitfalls:
Confusing bacteriorhodopsin (archaeal) with rhodopsin in animal vision; both bind retinal but differ in function and context.

Final Answer:
All of the above.