MoBio G Proteins Chapter 6

The full name of "G protein" is GTP-binding protein because in the active state it binds to GTP (guanosine triphosphate). There are two types of G proteins: heterotrimeric G proteins and monomeric G proteins (or small G proteins). G-protein-coupled receptors are coupled to heterotrimeric G proteins.

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Figure 6-D-2. The structure of heterotrimeric G protein, consisting of three subunits: α, β and γ. Note that in the inactive state, the α subunit binds to GDP (guanosine diphosphate).

The heterotrimeric G protein consists of three subunits: α, β and γ. Based on the differences in their genes, 20 α, 6 β and 12 γ subunits have been identified. Their molecular weights are in the following ranges:

  • α subunit: 39 - 46 kD
  • β subunit: 35 - 39 kD
  • γ subunit: ~ 8 kD

In the inactive state, the α subunit binds to GDP and the three subunits are attached together (see above figure). When the α subunit binds to GTP, its affinity to the βγ subunits is decreased, resulting in their dissociation. The separated α and/or βγ subunits can then interact with their effectors.

Cycling of G protein between active and inactive states

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Figure 6-D-3. Cycling of G protein between active and inactive states.
(a) In the inactive state, Gα binds to GDP (guanosine diphosphate).
(b) Interaction between Gα and the agonist-stimulated receptor causes the release of GDP. GTP then binds to the empty site because its concentration in the cell is higher than GDP.
(c) The GTP-bound Gα has low affinity to βγ subunits, resulting in their dissociation.
(d) GTP is hydrolyzed to GDP because Gα has GTPase activity.