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Isothermal titration calorimetry of membrane protein interactions: FNR and the cytochrome bf complex. | LitMetric

Ferredoxin-NADP reductase (FNR) was previously inferred to bind to the cytochrome bf complex in the electron transport chain of oxygenic photosynthesis. In the present study, this inference has been examined through analysis of the thermodynamics of the interaction between FNR and the bf complex. Isothermal titration calorimetry (ITC) was used to characterize the physical interaction of FNR with bf complex derived from two plant sources (Spinacia oleracea and Zea maize). ITC did not detect a significant interaction of FNR with the bf complex in detergent solution nor with the complex reconstituted in liposomes. A previous inference of a small amplitude but defined FNR-bf interaction is explained by FNR interaction with micelles of the undecyl β-D maltoside (UDM) detergent micelles used to purify bf. Circular dichroism, employed to analyze the effect of detergent on the FNR structure, did not reveal significant changes in secondary or tertiary structures of FNR domains in the presence of UDM detergent. However, thermodynamic analysis implied a significant decrease in an interaction between the N-terminal FAD-binding and C-terminal NADP-binding domains of FNR caused by detergent. The enthalpy, ΔH, and the entropy, ΔS, associated with FNR unfolding decreased four-fold in the presence of 1 mM UDM at pH 6.5. In addition to the conclusion regarding the absence of a binding interaction of significant amplitude between FNR and the bf complex, these studies provide a precedent for consideration of significant background protein-detergent interactions in ITC analyses involving integral membrane proteins.

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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8790201PMC
http://dx.doi.org/10.1016/j.bpj.2021.12.014DOI Listing

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