Binding kinetics of an antibody against HIV p24 core protein measured with real-time biomolecular interaction analysis suggest a slow conformational change in antigen p24.

The interaction between HIV core protein p24 and the murine monoclonal antibody CB-4/1 or its Fab fragment showed unusual kinetics. Recombinant p24 was immobilised in a hydrophilic carboxymethyldextran matrix. At high concentration of CB-4/1 Fab the association of the antigen-antibody complex proceeds in two phases, while dissociation is mono-exponential. The antigen has a 'memory', i.e. shortly after dissociation of Fab-antigen complex the fast association phase is enhanced. Biphasic association was also found in solution. Experiments suggest a reversible change of binding properties in the epitope region with an overall time constant of about 100 s at room temperature. Intermediate steps with faster time constants must be involved. Slow conformational changes of p24 seem to be the most probable explanation. A simple model that provides a quantitative description of this process could not be found. Real-time analysis of antibody binding by surface plasmon resonance is a powerful method for studying such changes in the time domain of a few seconds to a few minutes.