Energetics of the HIV gp120-CD4 binding reaction

HIV infection is initiated by the selective interaction between the cellula r receptor CD4 and gp120, the external envelope glycoprotein of the virus. We used analytical ultracentrifugation, titration calorimetry, and surface plasmon resonance biosensor analysis to characterize the assembly state, th ermodynamics, and kinetics of the CD4-gp120 interaction. The binding thermo dynamics were of unexpected magnitude; changes in enthalpy, entropy, and he at capacity greatly exceeded those described for typical protein-protein in teractions. These unusual thermodynamic properties were observed with both intact gp120 and a deglycosylated and truncated form of gp120 protein that lacked hypervariable loops V1, V2, and V3 and segments of its N and C termi ni. Together with previous crystallographic studies, the large changes in h eat capacity and entropy reveal that extensive structural rearrangements oc cur within the core of gp120 upon CD4 binding. CD spectral studies and slow kinetics of binding support this conclusion. These results indicate consid erable conformational flexibility within gp120, which may relate to viral m echanisms for triggering infection and disguising conserved receptor-bindin g sites from the immune system.