Carboxymethylation of equine heart cytochrome c (cytc) changes its tertiary structure by disrupting the heme-Fe-Met80 distal bond, such that carboxymethylated cytc (CM-cytc) displays myoglobin-like properties. Here, the effect of cardiolipin (CL) on peroxynitrite isomerization by ferric CM-cytc (CM-cytc-Fe(III)) is reported. Unlike native ferric cytc (cytc-Fe(III)), CM-cytc-Fe(III) catalyzes peroxynitrite isomerization, the value of the second order rate constant (k(on)) is 6.8 x 10(4) M(-1) s(-1). However, CM-cytc-Fe(III) is less effective in peroxynitrite isomerization than CL-bound cytc-Fe(III) (CL-cytc-Fe(III); k(on) = 3.2 x 10(5) M(-1) s(-1)). Moreover, CL binding to CM-cytc-Fe(III) facilitates peroxynitrite isomerization (k(on) = 5.3 x 10(5) M(-1) s(-1)). Furthermore, the value of the dissociation equilibrium constant for CL binding to CM-cytc-Fe(III) (K = 1.8 x 10(-5) M) is lower than that reported for CL-cytc-Fe(III) complex formation (K = 5.1 x 10(-5) M). Although CM-cytc-Fe(III) and CL-cytc-Fe(III) display a different heme distal geometry and heme-Fe(III) reactivity, the heme pocket and the CL cleft are allosterically linked. (C) 2011 Elsevier Inc. All rights reserved.