TABLE 1.

Masses of protein species observed in Na⁺-NQR after labeling with 3-N₃-Q₂

Calculated m/z values are compared with the observed values. Peaks a–l denote species observed in the MALDI-MS spectra of the Na⁺-NQR, which was incubated with Q₁ in the control reaction (see supplemental Fig. S4).

Peak	Measured mass		Predicted protein species	Calculated massa
	+Q1	+ 3-N3-Q2
	kDa	kDa		kDa
a	21.52	21.51	NqrE	21.47
b	22.73	22.73	NqrDb	22.71
c	22.97	23.02	NAc
d	23.20	23.28	NAc
e	28.00	27.96	[NqrC + FMN]+	27.93
f	28.24	28.26	NAc
g	28.44	28.52	NAc
h	45.22	45.21	[NqrF − 2Fe-2S]+	45.10
i	45.41	45.44	[NqrB − FMN]+	45.36
j	50.79		NqrA	50.66
k	50.90	51.02	NAc
l	53.24	53.16	NAc

^a Calculated mass of the predicted protein species. Met-1 is omitted for NqrA, NqrB, NqrC, and NqrD. NqrE and NqrF are assumed to be N-formylated. Modification by FMN and N-formyl increase the calculated mass by 439 Da and 28 Da, respectively.

^b The m/z values of peak b would also match with the M²⁺ species of [NqrB-FMN], with a difference of 0.05 kDa.

^c NA, not assigned. The m/z ratio corresponding to these peaks cannot be assigned to any Nqr subunit or combinations of different Nqr subunits.