5.5 Summary of Section 5
Proteins are dynamic molecular machines. All proteins bind to other molecules, whether ions, small molecules or macromolecules, and these interactions are critical to the protein's function. The activity of proteins is regulated by changes in conformation.
In allosterically regulated proteins, binding of one ligand affects the conformation of a remote part of the protein, thereby regulating interaction with a second ligand. Cooperative binding is a type of allosteric regulation in which conformational changes are communicated between subunits (e.g. in O2 binding in haemoglobin).
A common mechanism for regulating protein conformation and activity is through cycles of phosphorylation (by kinases) and dephosphorylation (by phosphatases).
G proteins are regulated by binding and hydrolysis of GTP. Generally, the GTP-bound form of the protein is active and the GDP-bound form is inactive. Many G proteins require other proteins to enhance their GTPase activity (GAPs) or to accelerate exchange of GDP for GTP (GEFs). Two of the largest families of G proteins are small G proteins (exemplified by Ras) and trimeric G proteins.
Trimeric G proteins contain α, β and γ subunits and the α subunit has the GTP binding site and GTPase activity. These G proteins transduce signals from 7-helix transmembrane receptors.