Electron transfer (ET) is significant in heme catalysis but usually difficult to be characterized. In the present DFT calculations, a transition state for ET has been optimized during the decomposition of peracetic acid (PM) catalyzed by chlorite dismutase (Cld), using an active-site model based on the X-ray crystal structure of Cld. The Cld-catalyzed PAA reaction is revealed to proceed via a homolytic O-O bond cleavage to transiently form compound II (Cpd II) and acetate radical (OAc), and a subsequent fast ET from Cpd II porphyrin to OAc' leading to compound I (Cpd I) and acetate anion. The second step of ET is rate limiting. The results highlight the importance of ET in heme chemistry and imply that the mechanisms of heme enzymes may be masked by fast ET even if a key intermediate has been detected (like Cpd I in this case). The comparison with the Cld-catalyzed chlorite reaction further indicates that a heme enzyme may employ different mechanisms for different substrates. (C) 2017 Elsevier Inc. All rights reserved.