How a protein domain motion is coupled to the catalytic cycle is a current subject in enzymology. We render down a complicated domain motion in the 5'-deoxyadenosylcobalamin and pyridoxal-5'-phosphate codependent radical enzyme, lysine 5,6-aminomutase, into dominant contributions from Lys370α and Asp298α to the critical Co-C bond cleavage trigger and open-closed cycle transitions.