H. Hernandez et al., Observation of the iron-sulfur cluster in Escherichia coli biotin synthaseby nanoflow electrospray mass spectrometry, ANALYT CHEM, 73(17), 2001, pp. 4154-4161
Biotin synthase from Escherichia coli was analyzed by nanoflow electrospray
ionization mass spectrometry. From solution conditions in which the protei
n is in its native state, a distribution of monomeric, dimeric, and tetrame
ric species was observed. The distribution of these species was sensitive t
o changes in ionic strength: in the positive ion spectrum, biotin synthase
at low ionic strength (pH 7.0-8.5) yielded less than 10% dimer. The masses
of the monomeric species were consistent with the presence of a [2Fe-2S] cl
uster with a mass difference of 175.3 Da from the apomonomer with one disul
fide bond. Despite the molecular mass of the noncovalent dimer (77 kDa), it
was possible to observe a dimeric species containing one iron-sulfur clust
er in both positive and negative ion spectra. Additionally, observation of
a series of charge states assigned to the apodimer indicated that binding o
f the iron-sulfur cluster was not required to maintain the dimer. Binding o
f Cu2+ to biotin synthase was also observed; in the presence of excess chel
ating agent, free metals were removed and the iron-sulfur cluster remained
intact. Evidence for the coordination of the iron-sulfur cluster in biotin
synthase was obtained in a tandem mass spectrometry experiment. A single ch
arge state containing the cluster at m/z 2416.9 was isolated, and collision
-induced dissociation resulted in sequential loss of sulfur and retention o
f Fe3+.