The CcP:Cytochrome c Complex
Here is the interaction interface between CcP and yeast cytochrome c, as revealed by the crystal structure (Pelletier & Kraut, 1992).
The CcP molecule is in blue on the right, while yeast cytochrome c is in raspberry, on the left.
Important Lys residues at the surface of yCc are shown in orange, while
their complementary Asp and Glu residues on CcP are shown in yellow.
Carboxylates that are near the interfaace, but of lesser importance, are shown in light blue.
Also shown in yellow is Ala 193, the point of closest contact between the two proteins.
Hemes are shown in red.
Several groups have claimed that this complex is either irrelevant for catalysis
or that it is one of two sites involved in electron transfer.
To address this point, I replaced Ala 193 with Cys, and added a maleimidyl-biocytin derivative to the enzyme at this location. The effect on intracomplex electron transfer rates was dramatic - a 50-fold decrease in the unimolecular rate constant.
This decrease was independent of ionic strength, indicating that
there is NO
contribution from a second, low affinity binding site at low ionic strength.
Subsequent work has shown that turnover is limited by yCc dissociation at very
low ionic strength, and that binding at the second, low affinity site increases
enzyme turnover by increasing the the rate of product dissociation from the high
affinity site.
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