Cross-linking and 1H n.m.r. spectroscopy of the pyruvate dehydrogenase complex of Escherichia coli

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Abstract
The pyruvate dehydrogenase complex of Escherichia coli was treated with o-phenylene bismaleimide in the presence of the substrate pyruvate, producing almost complete cross-linking of the lipoate acetyltransferase polypeptide chains as judged by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. This took place without effect on the catalytic activities of the other two component enzymes and with little evidence of cross-links being formed with other types of protein subunit. Limited proteolysis with trypsin indicated that the cross-links were largely confined to the lipoyl domains of the lipoate acetyltransferase component of the same enzyme particle. This intramolecular cross-linking had no effect on the very sharp resonances observed in the 1H n.m.r. spectrum of the enzyme complex, which derive from regions of highly mobile polypeptide chain in the lipoyl domains. Comparison of the spin–spin relaxation times, T2, with the measured linewidths supported the idea that the highly mobile region is best characterized as a random coil. Intensity measurements in spin-echo spectra showed that it comprises a significant proportion (probably not less than one-third) of a lipoyl domain and is thus much more than a small hinge region, but there was insufficient intensity in the resonances to account for the whole lipoyl domain. On the other hand, no evidence was found in the 1H n.m.r. spectrum for a substantial structured region around the lipoyl-lysine residues that was free to move on the end of this highly flexible connection. If such a structured region were bound to other parts of the enzyme complex for a major part of its time, its resonances might be broadened sufficiently to evade detection by 1H n.m.r. spectroscopy.