Ee. Bullesbach et C. Schwabe, Tryptophan B27 in the relaxin-like factor (RLF) is crucial for RLF receptor-binding, BIOCHEM, 38(10), 1999, pp. 3073-3078
The relaxin-like factor (RLF) is a circulating hormone that is synthesized
in the gonads of mammals and released into the bloodstream. The distributio
n of its receptor and a trace of cross-reactivity to relaxin receptors impl
ied that this relatively new factor is more relaxin- than insulin-like. The
chemical synthesis of RLF analogues with specific modifications in positio
ns B27 and B25, or the truncated form des(B27-31)RLF, clearly indicate that
the intact indole ring in position B27 is crucial for high RLF receptor-bi
nding. Receptor-binding was reduced by 2 orders of magnitude for Leu(B27)RL
F (3%), Ala(B27)RLF (2.1%), and des(B27-31)RLF (0.4%), whereas slightly bet
ter binding was observed for His(B27)RLF (7.5%), Phe(B27)RLF (21%), D-Trp(B
27) (26%), and the oxindole(B27)RLF (41%). On the basis of these observatio
n it seems that an aromatic ring system in the beta- or gamma-position is r
equired for binding. Structure prediction of the C-terminal region of the B
chain indicated a possible type I or type III turn for residues C-G-G-P-R
(B22-26) preceding the tryptophan. Exchanging Pro(B25) for D-Pro within the
turn caused a severe structural rearrangement at the C terminus and a 96%
drop in activity. It appears that the steric effect of L-Pro(B25) is import
ant for the proper positioning of Trp(B27).