Electronic couplings for electron transfer reaction in myoglobin whose His-48 is ruthenated are calculated nonperturbatively by applying a recursion method. An extended Huckel method is used for electronic calculations. A new index that measures the importance of each amino acid for electron transfer pathways is introduced. It is found that a few amino acids play a key role, amino acids without which the electronic couplings is decreased drastically. It is also found that the electronic coupling is increased by some important amino acids around the key amino acids in this myoglobin derivative.