Stereospecificity of multiple receptor sites in a labellar sugar receptor of the fleshfly for amino acids and small peptides.

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Abstract
N-Formylation and N-methylation of the .alpha.-amino group of L-phenylalanine result in extremely decreased responses of the labellar sugar receptor of the fleshfly; the same structural alteration of L-valine hardly affects the response. Methyl esterification of the .alpha.-carboxyl group of phenylalanine maintains the response to some extent, but similar treatment of valine completely diminishes the response. The aromatic structure in phenylalanine is not essential for stimulation. These results suggest a difference in the stereospecificities and functional group specificities of the furnase (F) and aliphatic carboxylate (T) sites in the sugar receptor. The effect of small peptides on the sugar receptor was examined systematically. Their effectiveness depends mainly on the place of the constituent amino acids rather than on their composition; this indicates the decisive role that certain aliphatic amino acids in the C-terminal position play in stimulation. Regularities in the stimulating effectiveness of small peptides exactly correspond to the stereospecificity of each receptor site. Two hypothetical models were proposed for the F and T sites, which involve 3 and 2 subsites, respectively, capable of H bond formation. The F and T sites also have a hydrophobic subsite that discriminates the R groups of the stimulants and a few spatial barriers.