A biosensor study indicating that entropy, electrostatics, and receptor glycosylation drive the binding interaction between interleukin-7 and its receptor.
The interaction between interleukin-7 (IL-7) and its α-receptor, IL-7Rα, plays fundamental roles in the development, survival, and homeostasis of B- and T-cells. N-Linked glycosylation of human IL-7Rα enhances its binding affinity for human IL-7 300-fold versus that of the nonglycosylated receptor through an allosteric mechanism. The N-glycans of IL-7Rα do not participate directly in the binding interface with IL-7. This biophysical study involves dissection of the properties of binding of IL-7 to both nonglycosylated and glycosylated forms of the IL-7Rα extracellular domain (ECD) as functions of salt, pH, and temperature using surface plasmon resonance (SPR) spectroscopy. Interactions of IL-7 with both IL-7Rα variants display weaker binding affinities with increasing salt concentrations primarily reflected by changes in the first on rates of a two-step reaction pathway. The electrostatic parameter of the IL-7-IL-7Rα interaction is not driven by complementary charge interactions through residues at the binding interface or N-glycan composition of IL-7Rα, but presumably by favorable global charges of the two proteins. van't Hoff analysis indicates both IL-7-IL-7Rα interactions are driven by large favorable entropy changes and smaller unfavorable (nonglycosylated complex) and favorable (glycosylated complex) enthalpy changes. Eyring analysis of the IL-7-IL-7Rα interactions reveals different reaction pathways and barriers for the transition-state thermodynamics with the enthalpy and entropy changes of IL-7 binding to nonglycosylated and glycosylated IL-7Rα. There were no discernible heat capacity changes for the equilibrium or transition-state binding thermodynamics of the IL-7-IL-7Rα interactions. The results suggest that the unbound nonglycosylated IL-7Rα samples an extensive conformational landscape relative to the unbound glycosylated IL-7Rα, potentially explaining the switch from a "conformationally controlled" reaction (k(1) ∼ 10(2) M(-1) s(-1)) for the nonglycosylated interaction to a "diffusion-controlled" reaction (k(1) ∼ 10(6) M(-1) s(-1)) for the glycosylated interaction. Thus, a large favorable entropy change, a global favorable electrostatic component, and glycosylation of the receptor, albeit not at the interface, contribute significantly to the interaction between IL-7 and the IL-7Rα ECD.
Walsh
Scott T R
ST
Department of Cell Biology and Molecular Genetics, Institute for Bioscience and Biotechnology Research, W. M. Keck Laboratory for Structural Biology, University of Maryland, 9600 Gudelsky Drive, Rockville, Maryland 20850, USA. swalsh12@umd.edu
D015374:Biosensing Techniques; D019277:Entropy; D006031:Glycosylation; D006801:Humans; D015851:Interleukin-7 / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D020395:Receptors, Interleukin-7 / Q000737:chemistry / Q000378:metabolism*; D012965:Sodium Chloride / Q000378:metabolism; D055672:Static Electricity
D015851:Interleukin-7; D020395:Receptors, Interleukin-7; C415350:interleukin-7 receptor, alpha chain; D012965:Sodium Chloride
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