Insights into the binding behavior of native and non-native cytochromes to photosystem I from Thermosynechococcus elongatus

The binding of photosystem I (PS I) from Thermosynechococcus elongatus to the native cytochrome (cyt) c(6) and cyt c from horse heart (cyt c(HH)) was analyzed by oxygen consumption measurements, isothermal titration calorimetry (ITC), and rigid body docking combined with electrostatic computations of binding energies. Although PS I has a higher affinity for cyt c(HH) than for cyt c(6), the influence of ionic strength and pH on binding is different in the two cases. ITC and theoretical computations revealed the existence of unspecific binding sites for cyt c(HH) besides one specific binding site close to P(700). Binding to PS I was found to be the same for reduced and oxidized cyt c(HH). Based on this information, suitable conditions for cocrystallization of cyt c(HH) with PS I were found, resulting in crystals with a PS I:cyt c(HH) ratio of 1:1. A crystal structure at 3.4-Å resolution was obtained, but cyt c(HH) cannot be identified in the electron density map because of unspecific binding sites and/or high flexibility at the specific binding site. Modeling the binding of cyt c(6) to PS I revealed a specific binding site where the distance and orientation of cyt c(6) relative to P(700) are comparable with cyt c(2) from purple bacteria relative to P(870). This work provides new insights into the binding modes of different cytochromes to PS I, thus facilitating steps toward solving the PS I–cyt c costructure and a more detailed understanding of natural electron transport processes.