Vertebrate-type and plant-type ferredoxins: Crystal structure comparison and electron transfer pathway modelling

Crystallographic analysis of a fully functional, truncated bovine adrenodox in, Adx(4-108), has revealed the structure of a vertebrate-type [2Fe-2S] fe rredoxin at high resolution. Adrenodoxin is involved in steroid hormone bio sythesis in adrenal gland mitochondria by transferring electrons from adren odoxin reductase to different cytochromes P450. Planttype [2Fe-2S] ferredox ins interact with photosystem I and a diverse set of reductases. A systematic structural comparison of Adx(4-108) with plant-type fer redoxi ns which share about 20% sequence identity yields these results. (1) The fe rredoxins of both types are partitioned into a large, strictly conserved co re domain bearing the [2Fe-2S] cluster and a smaller interaction domain whi ch is structurally different for both subfamilies. (2) In both types, resid ues involved in interactions with reductase are located at similar position s on the molecular surface and coupled to the [2Fe-2S] cluster via structur ally equivalent hydrogen bonds. (3) The accessibility of the [2Fe-2S] clust er differs between Adx(4-108) and the plant-type ferredoxins where a solven t funnel leads from the surface to the cluster. (4) All ferredoxins are neg ative monopoles with a clear charge separation into two compartments, and a ll resulting dipoles but one point into a narrow cone located in between th e interaction domain and the [2Fe-2S] cluster, possibly controlling predock ing movements during interactions with redox partners. (5) Model calculatio ns suggest that FE1 is the origin of electron transfer pathways to the surf ace in all analyzed [2Fe-2S] ferredoxins and that additional transfer proba bility for electrons tunneling from the more buried FE2 to the cysteine res idue in position 92 of Adx is present in some. (C) 1999 Academic Press.