Towards a general model for protein-substrate stereoselectivity

Protein-substrate interactions in enzymatic, neurological, and immunological systems are typically characterized by a high degree of stereoselectivity towards complex substrates. We propose a novel stereocenter-recognition (SR) model for stereoselectivity of proteins (or receptors in general) towards substrates that have multiple stereocenters, based on the topology of substrate stereocenters. The model provides the minimum number of substrate locations that need to enter into binding, nonbinding, or repulsive interactions with receptor sites, for stereoselectivity to occur. According to this model, a substrate location may interact with multiple receptor sites, or multiple substrate locations may interact with a single receptor site, but a stereoselective receptor has to offer, in the correct geometry, at least as many interactions as the required minimum number of substrate locations. The SR model predicts that stereoselectivity towards an acyclic substrate with N stereocenters distributed along a single chain requires interactions involving a minimum of N + 2 substrate locations, distributed over all stereocenters in the substrate, such that effectively three locations exist per stereocenter. Thus, enantioselective recognition of molecules with one chiral center requires a protein to interact with a minimum of three substrate locations, while stereoselectivity towards substrates with two or three stereocenters requires interactions with a minimum of four or five substrate locations, respectively, and so on. We demonstrate the general applicability of this model to protein-substrate interactions by interpreting several previous experimental observations.