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Catalytic Site Atlas Version 2.2.12
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CSA entry for 1bh2
Original Entry
Title:
Signal transduction protein
Compound:
Guanine nucleotide-binding protein
Mutant:
Yes
UniProt/Swiss-Prot:
P10824-GBI1_RAT
EC Class:
No EC number available
Other CSA Entries:
Overview of all sites for 1bh2
Homologues of 1bh2
Entries for UniProt/Swiss-Prot: P10824
Other Databases:
PDB entry: 1bh2
PDBsum entry: 1bh2
UniProt/Swiss-Prot: P10824
Literature Report:
Introduction:
The 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.
Mechanism:
The 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.
Sites:

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Found by:
Literature reference 

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
357 0
ElectrostaticTransition state
May stabilise accumulation of negative charge on the beta phosphate in the transition state.
Evidence from paper Evidence concerns Evidence type
PubMed ID 10507001 Current protein Ligand is essential for catalysis
PubMed ID 10507001 Current protein Residue is positioned appropriately (ligand position known)

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
GLUA 43 43Backbone amide
ElectrostaticTransition state
Backbone 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.
Evidence from paper Evidence concerns Evidence type
PubMed ID 8710841 Current protein Structural similarity to homologue of known mechanism
PubMed ID 10507001 Current protein Residue is positioned appropriately (ligand position known)

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
ARGA 178 178Sidechain
ElectrostaticTransition state
Proposed to stabilise accumulation of negative charge on the beta-gamma bridging oxygen as this atom departs from the gamma phosphate in the transition state.
Evidence from paper Evidence concerns Evidence type
PubMed ID 10507001 Current protein Residue is positioned appropriately (ligand position known)
PubMed ID 8073283 Current protein Mutagenesis of residue
PubMed ID 8073283 Current protein Conservation of residue

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
THRA 181 181Backbone carbonyl
ElectrostaticWater
May position and polarise the lytic water molecule.
Evidence from paper Evidence concerns Evidence type
PubMed ID 10507001 Current protein Residue is positioned appropriately (ligand position known)

ResidueChainNumberUniProt numberFunctional part FunctionTargetDescription
GLNA 204 204Sidechain
ElectrostaticWater
Proposed to position and polarise the lytic water molecule.
Evidence from paper Evidence concerns Evidence type
PubMed ID 10507001 Current protein Residue is positioned appropriately (ligand position known)
PubMed ID 8073283 Current protein Mutagenesis of residue
PubMed ID 8073283 Current protein Conservation of residue
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).
References:
1
Structures of active conformations of Gi alpha 1 and the mechanism of GTP hydrolysis.
D. E. Coleman and A. M. Berghuis and E. Lee and M. E. Linder and A. G. Gilman and S. R. Sprang
Science 265, (5177) 1405-12, (1994).
8073283
2
Reaction dynamics of G-protein catalyzed hydrolysis of GTP as viewed by X-ray crystallographic snapshots of Gi alpha 1.
D. E. Coleman and S. R. Sprang
Methods Enzymol 308, () 70-92, (1999).
10507001
3
Ras-catalyzed hydrolysis of GTP: a new perspective from model studies.
K. A. Maegley and S. J. Admiraal and D. Herschlag
Proc Natl Acad Sci U S A 93, (16) 8160-6, (1996).
8710841
4
GTP hydrolysis mechanism of Ras-like GTPases.
G. Li and X. C. Zhang
J Mol Biol 340, (5) 921-32, (2004).
15236956
5
Mapping the transition state for ATP hydrolysis: implications for enzymatic catalysis.
S. J. Admiraal and D. Herschlag
Chem Biol 2, (11) 729-39, (1995).
9383480
6
Uncoupling conformational change from GTP hydrolysis in a heterotrimeric G protein alpha-subunit.
C. J. Thomas and X. Du and P. Li and Y. Wang and E. M. Ross and S. R. Sprang
Proc Natl Acad Sci U S A 101, (20) 7560-5, (2004).
15128951
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