A protein has been used by comparative molecular biologist to show the evolutionary path of how organisms did what things in regards to energy?

Cytochrome C; it provides evidences that there is similarities in the respiratory pathways for producing ATPs by  all living organisms especially mammals.

Explanation:

Cytochrome c is located in the intermembranes of mitochondria, and it functions in the transfer of  of one electron in electron transport chain,(ETC) needed for  generation of proton motive force;  for generation of energy in the synthesis of ATPs by ATPase synthase  during chemiosmosis. Its allows oxdation-reduction by the switching of its iron ii to iron iii. during electron transports.However its iron atoms does not undergo oxidation with  oxygen. This feature makes it stable and  an ideal carrier of electrons.

Its amino acid sequences is very similar in all living organisms especially  between mammals(e.g man and chimpanzees)with little variation in few amino acid residues due to mutation.This similarity in its amino acids sequences in all  living  organism together with small molecular size makes it ideal  molecular evidence  for studying comparative molecular evidence of  evolution.

This is because it can be used  to trace  the pathways of respiration to synthesize energy, and therefore to conclude that  most  organisms share common ancestry, since a very similar protein sequence in  a structure(Cytochrome c) participated in unique ETC mechanisms  in all, needed for energy synthesis .