Final answer:
The highest level of structure for proteorhodopsin, with its single polypeptide chain, is tertiary. This three-dimensional folding is crucial for its function in light detection and signal transduction.
Step-by-step explanation:
The highest level of structure found in proteorhodopsin, which consists of a single polypeptide chain, is the tertiary structure. The protein structure helps to determine its function.
Proteins have four levels of structure. The primary structure is the sequence of amino acids. Secondary structure includes α-helix and β-pleated sheets formed by hydrogen bonding. The tertiary structure is the three-dimensional folding pattern of a protein due to side chain interactions. The quaternary structure is present in proteins with multiple polypeptide chains. Since proteorhodopsin has only one polypeptide chain, it does not form a quaternary structure. Instead, the folding of this single polypeptide chain into a three-dimensional shape represents the highest level of its structural organization. This folding is essential for the protein's biological activity, allowing it to interact with light and initiate a signal transduction pathway by changing shape and activating a G-protein called transducing.