NEW ASPECTS OF ELECTRON-TRANSFER REVEALED BY THE CRYSTAL-STRUCTURE OFA TRUNCATED BOVINE ADRENODOXIN, ADX(4-108)

Background: Adrenodoxin (Adx) is a [2Fe-2S] ferredoxin involved in ste roid 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 wher e they are consumed in hydroxylation reactions, A crystallographic stu dy of Adx is expected to reveal the structural basis for an important electron transfer reaction mediated by a vertebrate [2Fe-2S] ferredoxi n. Results: The crystal structure of a truncated bovine adrenodoxin, A dx(4-108), was determined at 1.85 Angstrom resolution and refined to a crystallographic R value of 0.195, The structure was determined using multiple wavelength anomalous dispersion phasing techniques, making u se of the iron atoms in the [2Fe-2S] cluster of the protein, The prote in displays the compact (alpha+beta) fold typical for [2Fe-2S] ferredo xins. The polypeptide chain is organized into a large core domain and a smaller interaction domain which comprises 35 residues, including al l those previously determined to be involved in binding to AR and cyto chrome P450. A small interdomain motion is observed as a structural di fference 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. Co nclusions: The crystal structure of Adx(4-108) provides the first deta iled description of a vertebrate [2Fe-2S] ferredoxin and serves to exp lain a large body of biochemical studies in terms of a three-dimension al structure. The structure suggests how a change in the redox state o f the [2Fe-2S] cluster may be coupled to a domain motion of the protei n. It seems likely that the clearly asymmetric charge distribution on the surface of Adx(4-108) and the resulting strong molecular dipole ar e involved in electrostatic steering of the interactions with AR and c ytochrome P450.