The significance of the free energy of hydrolysis of GTP for signal-transducing and regulatory GTPases.

A large number of GTP/GDP binding proteins, which in general have intrinsic and/or stimulatable GTPase activity, have been identified in recent years and are involved in a wide range of cellular regulatory and signal transducing processes. A common property of these proteins is that they exist in what is generally described as an active form when GTP is bound and an inactive (resting) form when GDP is present. Thus, the intrinsic or stimulated GTPase activity of these 'enzymes' serves to turn off a signal or to terminate a regulated process. It has been suggested that these proteins, together with ATPases whose prime function is to convert the free energy of ATP hydrolysis into another form of energy or into energy-requiring chemical reactions should be grouped together under the heading of 'energyases'. In this article, this suggestion is examined from the point of view of identifying the role of the free-energy of hydrolysis of GTP in the signal-transducing or regulatory process of the GTPases. It is concluded that there is a qualitative difference between ATPases and classical GTPases, in the sense that a quantitative relationship between the free-energy of GTP hydrolysis and the appearance of this energy in a different form cannot be directly defined. The significance of the high free energy of hydrolysis is that it allows efficient transition from the active to the inactive state of GTPases in spite of the tendency of the strong interaction of the GTP-bound form with a partner molecule ('effector'), an essential feature of their mode of action, to stabilize the GTP-bound form.