Stick around and pass the electron!

Photosynthesis is a fundamental process that uses light to synthesize molecular fuel, driven by large protein complexes at internal membranes of vegetal cells. The protein plastocyanin (Pc) is responsible for shuttling electrons between two of these protein complexes in the photosynthetic electron transport chain. For an efficient electron flow, Pc must transiently bind its partner complex (Photosystem I) keeping a balance between specificity and binding strength (too weak will bind slowly and too strong will get stuck upon binding). Researchers at the Institute for Bioengineering of Catalonia (IBEC) studied the binding frequency and the unbinding force between suitably oriented plant PSI and Pc under electrochemical control using single molecule force spectroscopy. The observation of individual binding-unbinding events between PSI and Pc depends on their electrochemical states (that is, whether they carry an electron or are ready to receive one). The frequency of PSI-Pc interaction is higher when at least one of the partners is in a state ready for electron transfer, and it is lower once the electron is transferred. This appears to facilitate Pc unbinding and leave PSI ready to bind the next electron-carrying Pc.