| Abstract
| - The three-dimensional structure in aqueous solution of SFA-8, a 2S albumin 103-residue proteinfrom seeds of sunflower (Helianthus anuus L.), has been determined by NMR methods. An almost complete1H resonance assignment was accomplished from analysis of two-dimensional (2D) COSY and 2D TOCSYspectra, and the structure was computed by using restrained molecular dynamics on the basis of 1393upper limit distance constraints derived from NOE cross-correlation intensities measured in 2D NOESYspectra. In contrast with most other 2S albumins, SFA-8 consists of a single polypeptide chain withoutany cleavage in the segment of residues 30−46. The computed structures exhibited an rmsd radius of0.52 Å for the backbone structural core (residues 11−30 and 46−101) and 1.01 Å for the side chainheavy atoms. The resulting structure consists of five amphipathic helices arranged in a right-handedsuperhelix, a folding motif first observed in nonspecific lipid transfer (nsLTP) proteins, and common toother 2S albumins. In contrast to nsLTP proteins, neither SFA-8 nor RicC3 (a 2S albumin from castorbean) has an internal cavity that is able to host a lipid molecule, which results from an exchange in thepairing of disulfide bridges in the CXC segment. Both 2S albumins and nonspecific lipid transfer proteinsbelong to the prolamin superfamily, which includes a number of important food allergens. Differences inthe extension and solvent exposition of the so-called “hypervariable loop” (which connects helices IIIand IV) in SFA-8 and RicC3 may be responsible for the different allergenic properties of the two proteins.SFA-8 has been shown to form highly stable emulsions with oil/water mixtures. We propose that theseproperties may be determined partly by a hydrophobic patch at the surface of the protein which consistsof five methionines that partially hide the Trp76 residue. The flexibility of the loop which contains Trp76and the hydrophobicity of the whole environment may favor a conformational change, by which the Trp76side chain may become inserted into the oil phase.
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