Fig. 2.

Loops exhibit different levels of conformational variability. In some cases, they fluctuate around a main conformational state. For example, some CDR loops in antibodies adopt a single conformation suited to the recognition of a specific antigen. In other cases, a loop can adopt two main conformational states. For instance, this may correspond to a loop opening and closing the access to the active site of an enzyme. In general, the energy landscape of a loop is more complex, involving several basins corresponding to different meta-stable states. These multiple states can also be functionally important, for example in the case of multi-specific antibodies.