EVIDENCE FOR OXIDATION-STATE-DEPENDENT CONFORMATIONAL-CHANGES IN HUMAN FERREDOXIN FROM MULTINUCLEAR, MULTIDIMENSIONAL NMR-SPECTROSCOPY

Human ferredoxin belongs to the vertebrate ferredoxin family which inc ludes bovine adrenodoxin. It is a small (13.8 kDa) acidic protein with a [2Fe-2S] cluster. It functions as an electron shuttle in the choles terol side-chain cleavage reaction which is the first step of steroid hormone biosynthesis. The protein studied here was produced in Escheri chia coli and doubly labeled with C-13 and N-15. The diamagnetic N-15, C-13', Ca-13(alpha), C-13(beta), H-1(alpha), and H-1(N) resonances fr om about 70% of the 124 amino acid residues for oxidized human ferredo xin and 80% of those for the reduced protein have been assigned primar ily on the basis of results from three-dimensional, triple-resonance e xperiments. Secondary structure features for human ferredoxin in its o xidized and reduced states have been identified from a combination of chemical shift index and NOE data. Comparison of NMR results from the protein in its oxidized and reduced states indicates that structural c hanges accompany the change in the oxidation state of the [2Fe-2S] clu ster. Major differences are localized at two regions: residues 29-31 a nd residues 109-124; the latter stretch forms the C-terminal region of the protein. The possible functional significance of these oxidation- state-dependent structural changes is discussed.