The Beta-2 Adrenergic Receptor(B2AR) is a single poly-peptide chain which weaves inside and outside of the cellular membrane. Its secondary structure is constituted by 8 α helices, 7 of which are trans-membrane and one which runs parallel to the intracellular face of the cell membrane. It also contains three extra cellular loops, three intracellular loops, an extracellular N-terminus and intracellular C-terminus. The agonist binding site lies within the trans-membrane section of the protein. The Gs subunit is a heterotrimic G-protein, with the subunits, Gαs, Gβ and Gγ. The Gαs, which binds to GDP and GTP, is the primary functional subunit of the Gs protein which binds to and activates the adenyl-cyclase channel, continuing signal transduction. It is further divided into two sub-units , GαsRas and GαsAH which clamp onto the guanine-nuceloside when it is bound.
The agonist binding site in the B2AR protein consists of the residues D130, V131, F288, F289, and N312. The primary interactions of this binding site are Hydrogen bonding between the D130 and N312 hydrogen acceptors and the agonist's proton donors, primarily in the form of hydroxyl groups. Additionally, the F288 and F289 groups help to stabilize the agonist in the binding site by providing non-polar-phenyl stacking interactions which hold the agonist in place.
The binding of the agonist to this site results in a conformational shift in the trans-membrane helix(TM) 3 and TM 6 of B2AR , which is hypothesised to activate the Gs subunit. However, this conformational shift also results in the weakening of the association of between these helices, which makes them vulnerable to outside attack. This is thought to be one of the primary reasons for the instability of B2AR in detergent solution. The reason for Gs activation is not certain because the B2AR-Gs complex does not include a nucleotide and thus activation can not be observed.
Additional
structural changes in B2AR include an eight residue shift in the TM5
helix and a change in in
secondary structure of the second
intracellular loop(2ICL) into
an α-helix
structure.
A major conformational shift in the Gs sub-unit allows GαsRas and GαsAH to bind to the Guanine nucleotide . GαsAH subunit displaces, rotating 127 degrees, from a non-nucleotide bound state to a nucleotide bound state. The lack of the stability in the bond with guanine in between the GαsRas and GαsAH subunits is responsible for the flexibility of the GαsAH subunit when in the non-nucleotide-bound state.
The
interactions between GαsRas
and Gβγ
appear to
be unaffected by the binding of guanine nucleotides. However, because a
crystal structure of GDP-bound Gs heterotrimer has not yet been
reported and the Nb35 binds to the junction of GαsRas
and Gβ
and could be responsible for the relative stability of the interface,
the veracity of this phenomenon in biological systems can't be certain.
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