The tertiary structure of full-length bovine adrenodoxin suggests functional dimers

The three-dimensional X-ray crystal structure of full-length oxidized bovin e adrenodoxin (Adx) has been determined at 2.5 Angstrom resolution by molec ular replacement using a structure of a truncated form as a starting model. Crystals of Adx belong to a primitive monoclinic space group P2(1) with fo ur Adx molecules in an asymmetric unit. The unit cell dimensions are a = 59 .44 Angstrom, b = 77.03 Angstrom, c = 59.68 Angstrom, and beta = 94,83 degr ees. The structure has been refined to an R factor of 23.5%. Structures of the four molecules of full-length Adx (127 amino acids) in the asymmetric u nit were compared with each other and also with that of the truncated Adx ( 4-108). The overall topology of full-length Adx remains the same as describ ed earlier for the truncated protein, Differences that do occur are almost wholly confined to alternate side-chain conformations that reflect differin g lattice contacts made by two proteins, Extensive interactions found betwe en molecules 1 and 2 in the full-length Adx asymmetric unit may reflect the ability of Adx to form dimers in vivo and are consistent with hydrodynamic measurements which show that in solution there is an equilibrium between m onomeric and dimeric forms of Adx. Dimerization of Adx could explain why th e truncated form has greater affinity for the P450 redox partner than the f ull-length form. From these results it can be considered that the mechanism of electron transfer is not necessarily the same in different mitochondria l P450 systems. (C) 2000 Academic Press.