GTPases
GTPases (singular GTPase) are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.
GTPases play an important role in:
The hydrolysis of the γ phosphate of GTP into guanosine diphosphate (GDP) and Pi, inorganic phosphate, occurs by the SN2 mechanism (see nucleophilic substitution) via a pentavalent intermediate state and is dependent on the magnesium ion Mg2+.
In most GTPases, the specificity for the base guanine is imparted by the base-recognition motif, which has the consensus sequence [N/T]KXD.
Regulatory GTPases, also called the GTPase superfamily, are GTPases used for regulation of other biochemical processes. Most prominent among the regulatory GTPases are the G proteins.
All regulatory GTPases have a common mechanism that enables them to switch a signal transduction chain on and off. Toggling the switch is performed by the unidirectional change of the GTPase from the active, GTP-bound form to the inactive, GDP-bound form by hydrolysis of the GTP through intrinsic GTPase-activity, effectively switching the GTPase off. This reaction is initiated by GTPase-activating proteins (GAPs), coming from another signal transduction pathway. It can be reversed (switching the GTPase on again) by Guanine nucleotide exchange factors (GEFs), which cause the GDP to dissociate from the GTPase, leading to its association with a new GTP. This closes the cycle to the active state of the GTPase; the irreversible hydrolysis of the GTP to GDP forces the cycle to run only in one direction. Only the active state of the GTPase can transduce a signal to a reaction chain.
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