New aspects of electron transfer revealed by the crystal structure of a truncated bovine adrenodoxin, Adx(4-108).
BACKGROUND
Adrenodoxin (Adx) is a [2Fe-2S] ferredoxin involved in steroid hormone biosynthesis in the adrenal gland mitochondrial matrix of mammals. Adx is a small soluble protein that transfers electrons from adrenodoxin reductase (AR) to different cytochrome P450 isoforms where they are consumed in hydroxylation reactions. A crystallographic study of Adx is expected to reveal the structural basis for an important electron transfer reaction mediated by a vertebrate [2Fe-2S] ferredoxin.


RESULTS
The crystal structure of a truncated bovine adrenodoxin, Adx(4-108), was determined at 1.85 A resolution and refined to a crystallographic R value of 0.195. The structure was determined using multiple wavelength anomalous dispersion phasing techniques, making use of the iron atoms in the [2Fe-2S] cluster of the protein. The protein displays the compact (alpha + beta) fold typical for [2Fe-2S] ferredoxins. The polypeptide chain is organized into a large core domain and a smaller interaction domain which comprises 35 residues, including all those previously determined to be involved in binding to AR and cytochrome P450. A small interdomain motion is observed as a structural difference between the two independent molecules in the asymmetric unit of the crystal. Charged residues of Adx(4-108) are clustered to yield a strikingly asymmetric electric potential of the protein molecule.


CONCLUSIONS
The crystal structure of Adx(4-108) provides the first detailed description of a vertebrate [2Fe-2S] ferredoxin and serves to explain a large body of biochemical studies in terms of a three-dimensional structure. The structure suggests how a change in the redox state of the [2Fe-2S] cluster may be coupled to a domain motion of the protein. It seems likely that the clearly asymmetric charge distribution on the surface of Adx(4-108) and the resulting strong molecular dipole are involved in electrostatic steering of the interactions with AR and cytochrome P450.
Müller
A
A
Forschungsgruppe Kristallographie, Max-Delbrück-Centrum für Molekulare Medizin, Berlin, Germany.
Müller
J J
JJ
Muller
Y A
YA
Uhlmann
H
H
Bernhardt
R
R
Heinemann
U
U
D000325:Adrenodoxin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D018360:Crystallography, X-Ray; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004579:Electron Transport; D005287:Ferredoxin-NADP Reductase / Q000737:chemistry / Q000378:metabolism; D005288:Ferredoxins / Q000737:chemistry / Q000378:metabolism; D007501:Iron / Q000737:chemistry / Q000378:metabolism; D009154:Mutation; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012997:Solvents; D013455:Sulfur / Q000737:chemistry / Q000378:metabolism
D005288:Ferredoxins; D011994:Recombinant Proteins; D012997:Solvents; D000325:Adrenodoxin; C100137:putidaredoxin; D013455:Sulfur; D003577:Cytochrome P-450 Enzyme System; D007501:Iron; D005287:Ferredoxin-NADP Reductase