an amino acid that doesn't manifest in proteins but is found in the muscle tissue of vertebrates equally inside the free variety and as phosphocreatine; provides Electricity for muscle contraction??? ????? ???? ?? ??? ????? ???? ??? ?? ????? ?? ??? ??????????? ?? ????? ????? ??? ???? ??? ?? ????? ??? ?? ?????? ?? ???? ??? ???????? ?????? ?? ??????