Thermodynamics of binding of a low-molecular-weight CD4 mimetic to HIV-1 gp120.

The entry of human immunodeficiency virus (HIV-1) into target cells is mediated by the gp120 exterior envelope glycoprotein and the gp41 transmembrane envelope glycoprotein, which assemble into trimers on the virion surface (1, 2). Upon engaging the receptor, CD4, the gp120 glycoprotein undergoes extensive structural ordering; the resulting conformation of gp120 can bind the second HIV-1 receptor, CCR5 or CXCR4 (3-7). Receptor binding induces further conformational changes in the HIV-1 envelope glycoproteins that allow the gp41 glycoproteins to mediate the fusion of the viral and cell membranes. Thermodynamically, the binding of CD4 is characterized by large favorable enthalpy and large unfavorable entropy changes that reflect the structuring of gp120, a protein with large unstructured regions in its unligated form (8). CD4 mimetic miniproteins based on scyllatoxin containing the gp120-binding epitope (9) also elicit conformational changes in gp120 resembling those triggered by CD4 binding, and consequently are characterized by similar thermodynamic signatures (10, 11). Recently, two low-molecular weight compounds that presumably interfere with viral entry of HIV-1 into cells were reported (12) (Figure 1). The studies presented here show that these compounds are competitive inhibitors of CD4 and that they induce conformational changes in gp120 similar to those induced by CD4. These compounds activate coreceptor binding and, in our studies, enhance HIV-1 entry into CD4-negative cells expressing CCR5. This behavior is in contrast to that of the potent viral entry inhibitor BMS-378806, which binds gp120 with a small favorable enthalpy change in an entropically driven process (13). BMS-378806 does not compete with CD4 and does not induce any major structural ordering of the gp120 molecule exerting its antiviral action in an allosteric fashion. In this paper, we demonstrate experimentally that the binding mode and antiviral properties of these compounds are reflected in their binding thermodynamics and that the thermodynamic signature of a compound can be used in the design of more potent and effective viral entry inhibitors. Figure 1 General structure of the two compounds, NBD-556 (X = Cl) and NBD-557 (X = Br).