Probing the Effects of Multisite Mutations in the Lipoic Acid Region of the BCOADC-E2 Protein

Primary biliary cholangitis (PBC) is a chronic disease, the prevalence of which has been increasing in recent years. And the prevalence of patients who test negative with existing diagnostic techniques remains high. It was found that the antigenic BCOADC-E2 protein could detect patients with a negative original test. And experiments revealed that the lipoyl domain of BCOADC-E2 plays an important role. The present study was carried out to verify the necessity of maintaining the folding conformation of the lipoyl β-sheet of the protein in the lipoyl domain during the recognition of the BCOADC-E2 protein and the importance of the glutamic acid and isoleucine residues at position 4 and position 13, respectively. In order to search for a new pathway for the pre-detection of patients with PBC, firstly, the mutant proteins were subjected to an enzyme-linked immunosorbent assay (ELISA) with serum. Then, MTSSL spin tags were positioned at specific sites of the Cys mutant and reacted with serum samples from PBC patients and controls, and EPR spectroscopic data were measured. The multiple mutant proteins all reacted less specifically with the serum than the wild-type protein in the ELISA; the spectra measured for the pGEX-BCKD-E4A-I13A mutant were severely broadened, and the compactness at the conformational position of the lipoyl β-sheet structural conformation of the proteins of amino acids 4 and 13 remained unchanged. The EPR spectral data validate the importance of the glutamate and isoleucine residues at position 4 and position 13 and their necessity in the maintenance of the lipoyl β-sheet structural conformation of proteins in the lipoyl domain in anti-BCOADC-E2 recognition.