M-like state

Figure 33 Portions undergoing intermediate-fluctuations with frequency of 104-105 Hz in the M-like state of D85N are shaded. Residue numbers without alphabets are from Val, and Pro and Ala residues are indicated by the numbers initiated by the P and A, respectively (see Ref. 30).

Fig. 26. I3C CP-MAS NMR spectra of [l-l3C]Val-labeled wild type (A) and D85N (B and D) and D85N/D96N (C and E) recorded at pH 7 and 10. It is noteworthy that the arrowed peaks from the transmembrane helices are displaced to higher frequencies by protonation of Asp 85 at neutral pH and suppressed at pH 10 taking the M-like state.117 The peak at 177.81 ppm is ascribed to Val 213.88 Reproduced with permission from the American Chemical Society.

Fig. 24. Schematic representation of dynamic behavior of the B and C a-helices of D85N accompanied with protonation of Schiff base, as viewed from the NMR spectral behavior of Val 49 and Pro 91, described in the text. (A) Ground state at pH 7, and (B) M-like state at pH 10. Reproduced with permission from Springer.

Undoubtedly, acquisition of such fluctuation motion at the cytoplasmic side of the transmembrane B and C a-helices in the M-like state of D85N mutant (Fig. 24B) is responsible for a transient environmental change from the hydrophobic to hydrophilic conditions both at the Asp 96 and Schiff base (SB) as compared with the ground state (Fig. 24A), resulting in a reduced pAa value of Asp 96 in the M-like state, which makes proton uptake more efficient. Further, it is demonstrated that the presence of the van der Waals contact of Val 49 with Lys 216 at the Schiff base is essential to trigger this sort of dynamic change, as revealed from the NMR... 

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