Methionine oxidation and its effect on the stability of a reconstituted subunit of the light-harvesting complex from Rhodospirillum rubrum

An additional component in the purified core light-harvesting complex (LH1) from wild-type purple photosynthetic bacterium Rhodospirillum rubrum has b een identified as an oxidized species of alpha -polypeptide by MALDI-TOF ma ss spectrometry. This component appears as a slightly earlier-eluting peak in the RP-HPLC chromatogram compared with the authentic alpha -polypeptide. The oxidation site has been determined to be the N-terminal methionine res idue by high-resolution NMR spectroscopy, where the methionine is oxidized to methionine sulfoxide in a diastereoisomeric form. Interconversion betwee n the oxidized and authentic alpha -polypeptides has been confirmed by sele ctive oxidation and reduction. The oxidative modification of methionine is shown to have discernible effects on the ability to form B820 subunit with beta -polypeptide and bacteriochlorophyll a, and on the stability of the re constituted B820 subunit. Both the ability and the stability for the sample s using the oxidized alpha -polypeptide are moderately reduced, indicating that the oxidation-induced conformational change in the N-terminal domain o f alpha -polypeptide may affect the pigment-binding environment through a l ong-range interaction. The MALDI-TOF mass results also reveal that the N-te rminus of alpha -polypeptide is formylated and no phosphorylation has occur red in this polypeptide.