uncomplexed species in solution. The dissociation constant is defined as Kd = where [L], [P] and [LP] represent molar concentrations of the protein, ligand and complex, respectively. The lower the Kd value the higher the affinity of the protein for the ligand and vice versa. The Kd value is equivalent to the concentration of the ligand at which one-half of the proteins contain bound ligand. Affinity is influenced also by the properties of the solution, like pH, temperature and salt concentration, that may affect the stable state of the proteins and ligands and hence also their interaction and by the presence of other macromolecules that causes macromolecular crowding. Functions Protein–ligand complexes can be found in almost any cellular process. Binding of a ligand causes a conformational change in the protein and often also in the ligand. This change initiates a sequence of events leading to different cellular functions. The complexes are formed by different molecules like macromolecules as in protein complexes, protein DNA or protein RNA complexes as well as by proteins that bind smaller molecules like peptides, lipids, carbohydrates, small nucleic acids. They may have various functions within the cell: catalysis of chemical reactions (enzyme-substrate), defense of the organism through the immune system (antibodies antigen complexes), signal transduction (receptor-ligand complexes) that consists of a transmembrane receptor that upon binding the ligand activates an intracellular cascade. Lipophilic hormonal receptor complexes can pass the nuclear membrane where transcription may be regulated. Example Protein-Ligand complex is essential in many of the cellular processes that occur within organisms. One of these examples is the Glucagon receptor (GCGR). Glucagon receptor (GCGR) is a family of G-protein coupled receptors (GPCRs) in humans that plays an important role in maintaining glucose concentration within the blood during periods of low energy state. Glucagon binding to GPCRcauses a conformational change in the intracellular domain, allowing interaction with the heterotrimeric Gs protein. The alpha Subunit of the Gs protein releases bound GDP and binds GTP. The alpha subunit-GTP complex dissociates from the beta and gamma dimer and interacts with adenylate cyclase. Binding of glucagon molecule activates many of the alpha subunit, which amplifies the hormonal signal. Then, the alpha subunit activates the adenylate cyclase, which converts ATP to cAMP. The alpha subunit deactivates itself within minutes by hydrolyzing GTP to GDP (GTPase activity). The alpha subunit reassociates with beta-gamma dimer to form an inactive complex. A better understanding of