Protein three-dimensional structure and molecular recognition: a story of soft locks and keys

Abstract

One hundred years ago Emil Fischer proposed a descriptive but provocative analogy for molecular recognition: the lock and key hypothesis. At a time when little was known of the molecular structures of even the relatively simple substrates of enzymes, let alone the complex structures of proteins, this gave an extraordinarily useful visual image of enzyme action. Similar recognition processes, such as antigen-antibody, hormone or growth factor-receptor, lectin-sugar, repressor-DNA and so on, have since been identified in other classes of proteins. Can the Fischer hypothesis be applied to these systems? Has the hypothesis stood the test of time? In this paper, we examine the crystal structures of proteins complexed with their ligand molecules: the pentraxins bound to carbohydrate, several aspartic proteinases complexed with inhibitors, the SH3 domains bound to proline-rich peptide motifs, the periplasmic binding proteins and growth factor systems bound to cell surface receptors. We discuss the modes of binding in terms of surface rigidity, charge and shape complementarity. Such recognition processes are often accompanied by distinct conformational changes at the binding site. The ligand selectivity demonstrated in these systems supports a “soft” lock-and-key hypothesis.