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Constraints on the radical cation center of cytochrome c peroxidase for electron transfer from cytochrome c #MMPMID27499202
Payne TM; Yee EF; Dzikovski B; Crane BR
Biochemistry 2016[Aug]; 55 (34): 4807-22 PMID27499202show ga
The tryptophan 191 cation radical of cytochrome c peroxidase (CcP) compound I (Cpd I) mediates long-range electron transfer (ET) to cytochrome c (Cc). Here we test the effects of chemical substitution at the 191 position. CcP W191Y forms a stable tyrosyl radical on reaction with peroxide and produces spectral properties similar to that of Cpd I but has low reactivity toward reduced Cc. CcP W191G(or F) variants also have low activity, as do redox ligands that bind within the W191G cavity. Crystal structures of complexes between Cc and CcP W191X (X = Y, F, G), as well as W191G with four bound ligands, reveal similar 1:1 association modes and heme pocket conformations. The ligands display structural disorder in the pocket and do not hydrogen bond to Asp235, as does Trp191. Well-ordered Tyr191 directs its hydroxyl group toward the porphyrin ring, with no basic residue in range of interaction. CcP W191X (X = Y, F, G) variants substituted with zinc-porphyrin (ZnP) undergo photoinduced ET with Cc(III). Their slow charge recombination kinetics that results from loss of the radical center allow resolution of difference spectra for the charge-separated state (ZnP+, Cc(II)). The change from a phenyl moiety at position 191 in W191F to a water-filled cavity in W191G produces much smaller effects on ET rates than the change from Trp to Phe. Low net reactivity of W191Y toward Cc(II) either derives from the inability of ZnP+ or the Fe-CcP ferryl to oxidize Tyr or from a low potential of the resulting neutral Tyr radical.