Received for publication June 28, 2007.
Revised August 22, 2007.
Accepted for publication August 22, 2007.

Assessment of the roles of Serines 5.43(239) and 5.46(242) for binding and potency of agonist ligands at the human serotonin 5 HT_2A receptor

Abstract

We assessed the relative importance of two serine residues located near the top of transmembrane helix 5 of the human 5 HT_2A receptor comparing wild type to S5.43(239)A or S5.46(242)A mutations. Using the ergoline LSD, and a series of substituted tryptamine and phenethylamine 5 HT_2A receptor agonists, we found that S5.43(239) is more critical for agonist binding and function than S5.46(242). S5.43(239) appears to engage oxygen substituents at either the 4 or 5 position of tryptamine ligands and the 5 position of phenylalkylamine ligands. Even when a direct binding interaction cannot occur, our data suggest that S5.43(239) is still important for receptor activation. Polar ring-substituted tryptamine ligands also appear to engage S5.46(242), but tryptamines lacking such a substituent may adopt an alternate binding orientation that does not engage this residue. Our results are consistent with the role of S5.43(239) as a hydrogen bond donor, whereas S5.46(242) appears to serve as a hydrogen bond acceptor. These results are consistent with the functional topography and utility of our in silico activated homology model of the h5 HT_2A receptor. In addition, being more distal from the absolutely conserved P5.50, a strong interaction with S5.43(239) may be more effective in straightening the kink in helix 5, a feature that is possibly common to all Type A GPCRs with a polar residue at position 5.43.

Key words: Serotonin, Gq/11 family, IP3/DAG, Molecular dynamics, Structure-activity relationships and modeling, Mutagenesis/Chimeric approaches, Receptor binding studies