Analysis of mobile residues of E. coli citrate synthase using nuclear magnetic resonance spectroscopy
E. coli Citrate Synthase (CS) is a large hexameric protein with a molecular weight of 280kDa and belonging to the type II class of citrate synthases. The crystal structure of E. coli CS in the T (inactive) state is known. The structure shows that the backbone atoms of 42 residues (1-5, 266-297 and 330-335) have temperature factors about 9 times greater than the average as compared to the rest of the molecule. Residues 266-297, also referred to as the “Mobile loop”, are particularly of interest since they form a part of the active site and any rearrangement in the mobile loop can provide useful information about the R(active) state of the protein. In this study, Nuclear Magnetic Resonance (NMR) Spectroscopy has been used to study the flexible regions of E. coli CS in solution. The flexible residues have been assigned based on the amino acid type by 15N-specific amino acid labeling, while the residue type has been assigned by site-directed mutagenesis. Changes in the dynamics of the flexible residues, in response to the substrate binding, have been studied using both NMR and Fluorescence Spectroscopy. Also a method to use Mass Spectrometry for accessing the isotopic incorporation in the samples prepared for NMR spectroscopy has been described. The initial hypothesis in this study was that only the mobile loop residues which show significant high B-factors will contribute to the NMR spectrum. However in the NMR spectrum, in addition to the mobile loop, some uncharacterized flexible regions were also observed. We also found that some of the residues show signs of slow conformational exchange resulting in multiple signals in the NMR spectrum. In addition we see that the environment of some flexible residues is changed in the presence of substrates, a few residues were immobilized, but most remained mobile.
citrate synthase, NMR