The Structure of a Bacterial Ferredoxin

Abstract

The structure of the bacterial ferredoxin from Peptococcus aerogenes (54 amino acids) has been determined at 2.8 A resolution.The molecule is a prolate ellipsoid with approximate dimension of 22 x 27 A. The iron and sulfur atoms are in two complexes 12 A apart with 4 iron, 4 inorganic sulfur, and 4 cysteine sulfur atoms in each.Cys 8, 11, 14, and 45 are coordinated to iron atoms in one complex and Cys 18, 35, 38, and 41 are coordinated to iron in the other.The two tyrosine groups present in the molecule are oriented in a similar way with respect to the complexes: Tyr 2 is parallel to and apparently in contact with one face of one complex and Tyr 28 is oriented similarly with respect to the other.Both tyrosine rings have an edge exposed to the solvent.The structure was solved with three isomorphous derivatives, at first used independently but later combined, to determine phases for the native data.Refinement of the structure has been initiated with 2 A resolution data.The ferredoxins are low molecular weight iron-sulfur proteins that function as electron transfer agents in some important biological reactions.They can be divided into two main classes: the bacterial, usually with 8 iron atoms and 8 labile sulfur atoms and molecular weights usually about 6000, and the chloroplast with 2 iron atoms and 2 labile sulfur atoms and molecular weights approximately 12,000.The bacterial ferredoxins found in anaerobes play a key role in fermentative metabolism and in nitrogen fixation, and chloroplast ferredoxins appear to function as a primary electron acceptor from the photoreactive pigments.A preliminary report of the structure of the Fe-S complexes in ferredoxin from Peptococcus aerogenes (formerly Micrococcus aerogenes) has appeared (1).The present account describes important details of the work and provides a view of the molecule as deduced from an electron density map at 2.8 A resolution.Details of the iron-sulfur complexes that have emerged from the initial refinement of the iron and sulfur parameters by least squares based on 2,4 resolution data are also included. EXPERIMENTAL PROCEDUREPurification and Crystallization P. aerogenes was grown anaerobically at 37" for 18 hours in loo-liter batches.The medium contained Difco peptone, yeast