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Electron transfer flavoprotein (ETF) is a heterodimer that contains a single equivalent of FAD and accepts electrons from nine flavoprotein dehydrogenases in the mitochondrial matrix. Human ETF was expressed in Escherichia coli using the expression vector previously employed to express Paracoccus denitrificans ETF (Bedzyk, L. A., Escudero, K. W., Gill, R. E., Griffin, K. J., and Frerman, F. E. (1993) J. Biol. Chem. 268, 20211-20217). cDNAs encoding the beta and alpha subunits of the human protein were inserted into the vector, mimicking the arrangement of the P. denitrificans genes in which coding sequences are joined by overlapping termination and initiation codons. A human ETF containing 30% P. denitrificans sequence at the amino terminus of the beta subunit was also expressed and purified. This chimeric ETF has 64% sequence identity with the human sequence in the substituted region. Kinetic constants of medium chain and short chain acyl-CoA dehydrogenases for the chimeric ETFs were slightly changed from those of human ETF; but, there are marked differences in the kinetic constants of sarcosine dehydrogenase and electron transfer flavoprotein-ubiquinone oxidoreductase with the two ETFs. Absorption spectra of the three redox states of human, chimeric, and P. denitrificans ETF flavins are identical. However, the flavin circular dichroism spectra of the three ETFs are characteristic for each species. The spectrum of the chimeric ETF has both human and P. denitrificans ETF features. The amplitude of the 436 nm band is identical to that of the of the human ETF flavin, but the amplitude of the 375 nm band is identical to that of the P. denitrificans ETF flavin. Thus, flavin in the chimeric ETF appears to be exposed to dipoles in the protein framework provided by human and bacterial sequences. These spectral data indicate that the flavin is located in the vicinity of the amino-terminal region of the beta subunit. The kinetic data suggest that the amino-terminal region of the beta subunit comprises part of the docking site for some primary dehydrogenases and electron transfer flavoprotein-ubiquinone oxidoreductase. |
