| Popis: |
Continuing a previous investigation (Kintanar, A., Metzler, C. M., Metzler, D. E., and Scott, R. D. (1991) J. Biol. Chem. 266, 17222-17229), we have recorded 1H NMR spectra at 500 MHz in the 10-18-ppm range for the 93-kDa porcine cytosolic aspartate aminotransferase and for four specific mutant forms of the enzyme in which histidine 68 has been replaced by lysine or histidine 143, 189, or 193 has been replaced by glutamine. We have correlated resonances for apoenzyme, pyridoxamine and pyridoxal phosphate forms, and dicarboxylate complexes and have assigned imidazole NH resonances of active site histidines. The chemical shifts of several resonances undergo pH-dependent changes around the pKa of the Schiff base proton at the active site. Other resonances shift upon binding of dicarboxylates or other ligands. Phosphate or carboxylate ions, which can also occupy the site of the substrate's alpha-carboxylate, cause rapid exchange of the Schiff base proton. Although most resonances in the 10-18-ppm range disappear rapidly in D2O, a few are retained for months in the presence of the dicarboxylate inhibitor glutarate. We demonstrate that changes in chemical shifts and in exchange rates are sensitive indicators of electronic interactions of the enzyme with ligands and of conformational change. Nuclear Overhauser effects from NH protons have allowed us to identify resonances of CH protons of the imidazole rings of histidines 143, 189, and 193. Observed and predicted chemical shifts have been compared. We conclude that the net charge on this histidine cluster is zero but that some negative charge from the aspartate 222 carboxylate is donated inductively into the histidine 143 ring. Studies of the related enzyme from Escherichia coli are provided in an accompanying paper (Metzler, D. E., Metzler, C. M., Scott, R. D., Mollova, E. T., Kagamiyama, H., Yano, T., Kuramitsu, S., Hayashi, H., Hirotsu, K., and Miyahara, I. (1994) J. Biol. Chem. 269, 28027-28033). Our approach should be applicable to the study of active sites of a broad range of relatively large proteins. |
