The proteorhodopsin protein consists of a single polypeptide chain. what is the highest level of structure possible in this protein?

Tertiary  structure

The highest level of structure possible in this protein is tertiary structure.

Tertiary structure is the final three dimensional shape of a protein. The protein tertiary structure is made up of a single polypeptide chain which serves as the backbone, with one or more protein secondary structures. The tertiary structure is mainly held together by interactions between the R groups of the amino acids that make up the protein.