Which component of ETC is not a protein?

Ubiquinone is a component of ETC and is not a protein.
Ubiquinone is a mobile component of the mitochondrial electron transport chain, where it acts as a pro-oxidant in its ubisemiquinone state.
Ubiquinone (Q) accepts the electrons from both complex I and complex II and delivers them to complex III. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes.