Editing GTPase (section)

== Mechanism ==
Hydrolysis of GTP bound to an (active) G domain-GTPase leads to deactivation of the signaling/timer function of the enzyme.<ref name="pmid3113327"/><ref name="pmid7579038"/> The hydrolysis of the third (γ) [[phosphate]] of GTP to create [[guanosine diphosphate]] (GDP) and P<sub>i</sub>, [[inorganic phosphate]], occurs by the S<sub>N</sub>2 mechanism (see [[nucleophilic substitution]]) via a pentacoordinate transition state and is dependent on the presence of a [[magnesium]] [[ion]] Mg<sup>2+</sup>.<ref name="pmid3113327"/><ref name="pmid7579038"/>

GTPase activity serves as the shutoff mechanism for the signaling roles of GTPases by returning the active, GTP-bound protein to the inactive, GDP-bound state.<ref name="pmid3113327"/><ref name="pmid7579038"/> Most "GTPases" have functional GTPase activity, allowing them to remain active (that is, bound to GTP) only for a short time before deactivating themselves by converting bound GTP to bound GDP.<ref name="pmid3113327"/><ref name="pmid7579038"/> However, many GTPases also use accessory proteins named [[GTPase-activating proteins]] or GAPs to accelerate their GTPase activity. This further limits the active lifetime of signaling GTPases.<ref name="pmid9430654">{{cite journal |last1=Berman |first1=DM |last2=Gilman |first2=AG |date=1998 |title=Mammalian RGS proteins: barbarians at the gate |journal=Journal of Biological Chemistry |volume=273 |issue=3 |pages=1269–1272 |doi=10.1074/jbc.273.3.1269 |pmid=9430654 |doi-access=free }}</ref> Some GTPases have little to no intrinsic GTPase activity, and are entirely dependent on GAP proteins for deactivation (such as the [[ADP-ribosylation factor]] or ARF family of small GTP-binding proteins that are involved in vesicle-mediated transport within cells).<ref name= "pmid3086320">{{cite journal |last1=Kahn |first1=RA |last2=Gilman |first2=AG |date=1986 |title=The protein cofactor necessary for ADP-ribosylation of Gs by cholera toxin is itself a GTP binding protein |pmid=3086320 |journal=Journal of Biological Chemistry |volume=261 |issue=17 |pages=7906–7911 |doi=10.1016/S0021-9258(19)57489-0 |doi-access=free }}</ref>

To become activated, GTPases must bind to GTP. Since mechanisms to convert bound GDP directly into GTP are unknown, the inactive GTPases are induced to release bound GDP by the action of distinct regulatory proteins called [[guanine nucleotide exchange factor]]s or GEFs.<ref name="pmid3113327"/><ref name="pmid7579038"/> The nucleotide-free GTPase protein quickly rebinds GTP, which is in far excess in healthy cells over GDP, allowing the GTPase to enter the active conformation state and promote its effects on the cell.<ref name="pmid3113327"/><ref name="pmid7579038"/> For many GTPases, activation of GEFs is the primary control mechanism in the stimulation of the GTPase signaling functions, although GAPs also play an important role. For heterotrimeric G proteins and many small GTP-binding proteins, GEF activity is stimulated by cell surface receptors in response to signals outside the cell (for heterotrimeric G proteins, the [[G protein-coupled receptors]] are themselves GEFs, while for receptor-activated small GTPases their GEFs are distinct from cell surface receptors).

Some GTPases also bind to accessory proteins called [[Guanosine nucleotide dissociation inhibitor|guanine nucleotide dissociation inhibitors]] or GDIs that stabilize the inactive, GDP-bound state.<ref name="pmid9588168">{{cite journal |last1=Sasaki |first1=T |last2=Takai |first2=Y |date=1998 |title=The Rho Small G Protein Family-Rho GDI System as a Temporal and Spatial Determinant for Cytoskeletal Control |journal=Biochemical and Biophysical Research Communications |volume=245 |issue=3 |pages=641–645 |doi=10.1006/bbrc.1998.8253 |pmid=9588168 }}</ref>

The amount of active GTPase can be changed in several ways: 
# Acceleration of GDP dissociation by GEFs speeds up the accumulation of active GTPase. 
# Inhibition of GDP dissociation by guanine nucleotide dissociation inhibitors (GDIs) slows down accumulation of active GTPase. 
# Acceleration of GTP hydrolysis by GAPs reduces the amount of active GTPase. 
# Artificial ''GTP analogues'' like ''GTP-γ-S'', ''β,γ-methylene-GTP'', and ''β,γ-imino-GTP'' that cannot be hydrolyzed can lock the GTPase in its active state.
# Mutations (such as those that reduce the intrinsic GTP hydrolysis rate) can lock the GTPase in the active state, and such mutations in the small GTPase Ras are particularly common in some forms of cancer.<ref name="pmid31255772">{{cite journal |last1=Murugan |first1=AK |last2=Grieco |first2=M |last3=Tsuchida |first3=N |date=2019 |title=RAS Mutations in Human Cancers: Roles in Precision Medicine |journal=Seminars in Cancer Biology |volume= 59|pages= 23–35|doi=10.1016/j.semcancer.2019.06.007 |pmid=31255772 |s2cid=195761467 }}</ref>