PROPERTIES OF LIPOAMIDE DEHYDROGENASE ALTERED BY SITE-DIRECTED MUTAGENESIS AT A KEY RESIDUE (I184Y) IN THE PYRIDINE-NUCLEOTIDE BINDING DOMAIN

The binding of pyridine nucleotide to human erythrocyte glutathione reductase, an enzyme of known three-dimensional structure, requires some movement of the side chain of Tyr197. Moreover, this side chain lies very close to the isoalloxazine ring of the FAD cofactor. The analogous residue, Ile184, in the homologous enzyme Escherichia coli lipoamide dehydrogenase has been altered by site-directed mutagenesis to a tyrosine residue (I184Y) [Russell, G. C., Allison, N., Williams, C. H., Jr., & Guest, J. R. (1989) Ann. N.Y. Acad. Sci. 573, 429-431]. Characterization of the altered enzyme shows that the rate of the pyridine nucleotide half-reaction has been markedly reduced and that the spectral properties have been changed to mimic those of glutathione reductase. Therefore, Ile184 is shown to be an important residue in modulating the properties of the flavin in lipoamide dehydrogenase. Turnover in the dihydrolipoamide/NAD+ reaction is decreased by 10-fold and in the NADH/lipoamide reaction by 2-fold in I184Y lipoamide dehydrogenase. The oxidized form of I184Y shows remarkable changes in the fine structure of the visible absorption and circular dichroism spectra and also shows nearly complete quenching of FAD fluorescence. The spectral properties of the altered enzyme are thus similar to those of glutathione reductase and very different from those of wild-type lipoamide dehydrogenase. On the other hand, spectral evidence does not reveal any change in the amount of charge-transfer stabilization at the EH2 level. Stopped-flow data indicate that, in the reduction of I184Y by NADH, the first step, reduction of the flavin, is only slightly slowed but the subsequent two-electron transfer to the disulfide is markedly inhibited. This allows direct observation of the presumed reduced flavin species. Reduction of I184Y by dihydrolipoamide to the EH2 stage is not detectably altered, as expected given the site of the alteration.