Fig. 5
The crystal structure of the NADH-binding site of NuoEF S96MF in the reduced (a) and oxidized (b) state. Electron densities are given as 2Fo–Fc maps contoured at 1.0 σ. a One protomer in the asymmetric unit flips the peptide carbonyl of Glu95F to Arg135F. b In the second protomer, the carbonyl predominantly points toward FMN. The according backbone position of the first protomer shown in (a) is given as reference (cyan), demonstrating that this conformation does not fit well into the electron density. The crystal structure of the oxidized A. aeolicus NuoEF variant S96M that mimics the E. coli architecture of the binding pocket shows that here the peptide bond adopts both previously observed positions in each protomer of the asymmetric unit: One corresponds to the position in the oxidized NuoEF, the other to that in the reduced enzyme, although the electron density is not clearly enough defined to assign full occupancies for either conformer due to the relatively low resolution of 3.22 Å (PDB ID 6R7P). Intriguingly, the limited quality of the electron density map unequivocally allows for observing differences between the protomers in preferring a conformer: The first protomer predominantly flips the carbonyl toward Arg135F (a), the other one shows a higher occupancy for the carbonyl flipped toward the FMN (b). Thus, the E. coli complex I is capable of flipping the peptide bond
