A protein–ligand complex is a complex of a protein bound with a ligand that is formed following molecular recognition between proteins that interact with each other or with various other molecules. Formation of a protein-ligand complex is based on molecular recognition between biological macromolecules and ligands, where ligand means any molecule that binds the protein with high affinity and specificity. Molecular recognition is not a process by itself since it is part of a functionally important mechanism involving the essential elements of life like in self-replication, metabolism, and information processing. For example DNA-replication depends on recognition and binding of DNA double helix by helicase, DNA single strand by DNA-polymerase and DNA segments by ligase. Molecular recognition depends on affinity and specificity. Specificity means that proteins distinguish the highly specific binding partner from less specific partners and affinity allows the specific partner with high affinity to remain bound even if there are high concentrations of less specific partners with lower affinity. Interactions The protein-ligand complex is a reversible non-covalent interaction between two biological (macro)molecules. In non-covalent interactions there is no sharing of electrons like in covalent interactions or bonds. Non-covalent binding may depend on hydrogen bonds, hydrophobic forces, van der Waals forces, π-π interactions, electrostatic interactions in which no electrons are shared between the two or more involved molecules. The molecules (protein and ligand) recognize each other also by stereospecificity i.e. by the form of the two molecules. Because of this real discriminative if not 'cognitive' property, Werner Loewenstein uses the term 'cognitive demon' or molecular demon referring to Maxwell's demon, the famous thought experiment. In fact, the proteins that form complexes are able to pick a substrate out of a myriad of different molecules. Jacques Monod attributed a teleonomic performance or function to these biological complexes. Teleonomy implies the idea of an oriented, coherent and constructive activity. Proteins therefore must be considered essential molecular agents in the teleonomic performances of all living beings. Affinity The highest possible affinity from a protein towards the ligand, or target molecule, can be observed when the protein has a perfect mirror image of the shape of the target surface together with a charge distribution that complements perfectly the target surface. The affinity between protein and ligand is given by the equilibrium dissociation constant Kd or the inverse of the association constant 1/Ka (or binding constant 1/Kb) that relates the concentrations of the complexed and