Native Electrospray Mass Spectrometry Reveals the Nature and Stoichiometry of Pigments in the FMO Photosynthetic Antenna Protein

摘要

The nature and stoichiometry of pigments in thenFennau0001Matthewsu0001Olson (FMO) photosynthetic antenna pro-ntein complex were determined by native electrospray massnspectrometry. The FMO antenna complex was the first chlor-nophyll-containing protein that was crystallized. Previous resultsnindicate that the FMO protein forms a trimer with sevennbacteriochlorophyll a in each monomer. This model has longnbeen a working basis to understand the molecularmechanismofnenergy transfer through pigment/pigment and pigment/proteinncoupling. Recent results have suggested, however, that an eighthnbacteriochlorophyll is present in some subunits. In this report, andirect mass spectrometry measurement of the molecular weightnof the intact FMO protein complex clearly indicates the ex-nistence of an eighth pigment, which is assigned as a bacterio-nchlorophyll a by mass analysis of the complex and HPLCnanalysis of the pigment. The eighth pigment is found to beneasily lost during purification, which results in its partialnoccupancy in the mass spectra of the intact complex prepared by different procedures. The results are consistent with thenrecent X-ray structural models. The existence of the eighth bacteriochlorophyll a in this model antenna protein gives newninsights into the functional role of the FMOprotein andmotivates the need for new theoretical and spectroscopic assignments ofnspectral features of the FMO protein.