Structural determination of the active site of a sweet protein A 1H NMR investigation of pMNEI

Abstract
PMNEI, a single chain sweet protein related to monellin, has been studied by means of 1H NMR at 500 MHz. A partial sequential assignment performed by means of the MCD method allowed the determination of the secondary structure of a large portion of the β‐sheet of pMNEI that contains a likely ‘sweet finger’: the loop connecting the β‐strands from residue 59 to residue 78, corresponding to segment 16–35 of the A chain of monellin. The detailed three‐dimensional structure of the loop (Tyr66‐Ala67‐Ser68‐Asp69), determined from several interresidue and intraresidue NOEs and subsequent energy minimization, shows that the side chains or Tyr66 and Asp69 fit our model of the sweet receptor in a manner very similar to that of the side chains of Phe and Asp or aspartame.