CYTOCHROMES c 425
had been published on the interaction of cytochrome c with its respiratory
pathway partner cytochrome c oxidase (Section 7.8), so electron transfer to
cytochrome c peroxidase will substitute in this discussion.
In 1992, Pelletier and Kraut published their study of complexes between
yeast cytochrome c peroxidase (CcP) and yeast iso - 1 - cytochrome c (ccY),
PDB: 2PCC, resolution 2.3 Å , and CcP and horse heart cytochrome c (ccH),
PDB: 2PCB, resolution 2.8 Å.^130 Both structures shows ferricytochrome c
bound to CcP through electrostatic, hydrophobic, and van der Waals interac-
tions. The two cytochromes, ccY and ccH, bind somewhat differently to CcP;
most notable is a rotation and translation of the cytochromes c relative to CcP.
Positively charged ccY surface residues lys73, lys87, and asn70 form longish
hydrogen bonds ( > 3.0 Å ) with CcP negatively charged residues asp34 and
glu290. The ccY heme pyrrole ring C and especially its vinyl substituent (atoms
CAC and CBC, Figure 7.32B ) interact with side - chain and backbone atoms of
CcP ala193 and ala194. There are a number of van der Waals interactions
involving ccY ala81, conserved residue phe82, and gly83 with CcP residues
val197 and gln120. Most residues reside on ccY and CcP loops that face each
other at the proteins ’ interface. Lys73 and asn70 are residues in ccY helix IV.
The ccY heme iron ion distance to the CcP heme iron ion is 26.4 Å. Two dif-
ferent surface residues from ccH — lys8, lys72 — and one identical aa — lys87 —
form longish hydrogen bonds ( > 3.0 Å ) to two different CcP residues — glu35,
asn38 — and one identical aa — glu290 — at the ccH – CcP interface. Heme atom
CBC of ccH interacts with CcP ala 193 and ala194 in similar fashion but at
longer distances than for the ccY – CcP pair. Van der Waals interactions also
differ for the ccH – CcP pair. The ccH heme iron ion distance to the CcP heme
iron ion is 29.4 Å. The authors mention that the ccH – CcP crystals (PDB:
2PCB) were grown in a medium of essentially zero ionic strength, while the
ccY – CcP crystals (PDB: 2PCC) were grown in high ionic strength media.
Whether these differences are responsible for the different binding arrange-
ments or the differing electron transfer kinetic behavior of the two cannot be
determined solely from this work. The reference 130 authors assume that the
most stable complex between CcP and the cytochromes c in solution is the
one that crystallized. They show that the CcP – ccY structure does represent a
true electron transfer complex and that the CcP – ccH structure is not in an
optimal geometry for electron transfer (long distance between Fe centers
represents one problem). However, they do believe that a highly specifi c elec-
tron transfer complex must be formed between redox partners and that future
studies would show this to be the case. These structures did elucidate the
important concept that direct electrostatic interactions between the two pro-
teins are not critical in complex formation but that hydrophobic and van der
Waals interactions are equally, or perhaps more, important.
Recent structural studies and electron transfer kinetic experiments focus on
structures in which a site - specifi c covalent crosslink between cytochrome c and
cytochrome c peroxidase subunits exists. One of these used site - directed muta-
genesis to form a disulfi de bond between a V197C mutant CcP and an A81C