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Catalytic Site Atlas

CSA LITERATURE entry for 1bh2

SpeciesRattus norvegicus (Rat)
E.C. Number (IntEnz) -.-.-.-
CSA Homologues of 1bh2There are 64 Homologs
CSA Entries With UniProtID
CSA Entries With EC Number -.-.-.-
PDBe Entry 1bh2
PDBSum Entry 1bh2
MACiE Entry 1bh2

Literature Report

IntroductionThe alpha subunits of heterotrimeric G proteins are members of the Ras superfamily of GTP hydrolases and function in signal transduction. They are activated by replacement of bound GDP by GTP, a process promoted by activated 7-transmembrane receptors, and by subsequent dissociation of the alpha subunit from the beta and gamma subunits. The G-alpha subunit remains active until its GTPase activity converts the bound GTP back to GDP.
The alpha subunit of the G protein Gi1 is expressed in many different tissues and is activated by the alpha-2 adrenergic and M-2 muscarinic cholinergic receptors, amongst others. Its downstream effectors include particular isoforms of adenylate cyclase; types V and VI are particularly sensitive to inhibition by G-alpha-i1.
MechansimThe mechanism of enzymatic GTP hydrolysis (and similar phosphoryl transfer reactions) has been subject to significant debate and still remains to be firmly established. More recent models have proposed a dissociative-like process, in which the transition state shows significant cleavage of the gamma-beta phosphoanhydride bond and relatively little bond formation between the gamma phosphate and the nucleophilic water molecule. (See, for example, the reference pubmed ID 15236956 for a recent discussion).
Assuming a dissociative mechanism for G-alpha-i1, the key catalytic residues are Arg 278 (side chain) and Glu 43 (backbone NH), which stabilise accumulation of negative charge on the beta-gamma bridging oxygen as it departs from the gamma phosphate in the transition state. An Mg2+ ion may play a similar role in stabilising accumulation of negative charge on the beta phosphate, although whether such metal ions are catalytic has been disputed (see for example the reference pubmed ID 9383480). Gln 204 and Thr 181 may have a role in polarising and orientating the lytic water molecule, although in a dissociative mechanism activation of the water molecule would not make a large contribution to catalysis (see the reference pubmed ID 8710841). There is some evidence that conformational changes immediately prior to hydrolysis are required to properly position the catalytic residues.

Catalytic Sites for 1bh2

Annotated By Reference To The Literature - Site 1 (Perform Site Search)
ResidueChainNumberUniProtKB NumberFunctional PartFunctionTargetDescription
ThrA181181macie:mainChainCarbonylMay position and polarise the lytic water molecule.
GlnA204204macie:sideChainProposed to position and polarise the lytic water molecule.
GluA4343macie:mainChainAmideBackbone NH forms a hydrogen bond to the beta-gamma bridging oxygen to stabilise accumulation of negative charge on this atom as it departs from the gamma phosphate in the transition state.
ArgA178178macie:sideChainProposed to stabilise accumulation of negative charge on the beta-gamma bridging oxygen as this atom departs from the gamma phosphate in the transition state.

Literature References

Notes:In proposed mechanisms involving an associative-like transition state, Lys 46, Arg 178, Gly 203, Gln 204 and Mg2+ have all been proposed to stabilise negative charge around the gamma phosphate in the pentacoordinate transition state (see the reference pubmed ID 10507001).
Coleman DE
Structures of active conformations of Gi alpha 1 and the mechanism of GTP hydrolysis.
Science 1994 265 1405-1412
PubMed: 8073283
Coleman DE
Reaction dynamics of G-protein catalyzed hydrolysis of GTP as viewed by X-ray crystallographic snapshots of Gi alpha 1.
Methods Enzymol 1999 308 70-92
PubMed: 10507001
Maegley KA
Ras-catalyzed hydrolysis of GTP: a new perspective from model studies.
Proc Natl Acad Sci U S A 1996 93 8160-8166
PubMed: 8710841
Li G
GTP hydrolysis mechanism of Ras-like GTPases.
J Mol Biol 2004 340 921-932
PubMed: 15236956
Admiraal SJ
Mapping the transition state for ATP hydrolysis: implications for enzymatic catalysis.
Chem Biol 1995 2 729-739
PubMed: 9383480
Thomas CJ
Uncoupling conformational change from GTP hydrolysis in a heterotrimeric G protein alpha-subunit.
Proc Natl Acad Sci U S A 2004 101 7560-7565
PubMed: 15128951