BR Ground State (integral transhelical membrane protein): This graphic shows the seven transmembrane helices in white ribbon. The top represents the extracellular domain and the bottom represents the cytoplasmic space. The retinal cofactor is shown in green stick form with the Schiff base shown in red adjacent to a yellow Lys316 in a covalent attachment.

Initial proton transfer (extracellular domain): The photocycle begins with photoisomerization of retinal (shown here in all trans configuration) and the deprotonation of the Schiff base to light blue Asp85. A proton is subsequently released to the extracellular space from the proton releasing group (Glu204, Glu194, and Asp82), shown here in magenta

Cytoplasmic proton transfer and reprotonation of the Schiff base: The retinal reorients itself toward the cytoplasm following deprotonation, allowing for reprotonation via the cytoplasmic residue Asp96 and a complex network of hydrogen bonds between the helices and water molecules.