FhuA, the receptor for ferrichrome-iron in Escherichia coli , is a member of a family of integral outer membrane proteins, which, together with the energy-transducing protein TonB, mediate the active transport of ferric siderophores across the outer membrane of Gram-negative bacteria. The three-dimensional structure of FhuA is presented here in two conformations: with and without ferrichrome-iron at resolutions of 2.7 and 2.5 angstroms, respectively. FhuA is a β barrel composed of 22 antiparallel β strands. In contrast to the typical trimeric arrangement found in porins, FhuA is monomeric. Located within the β barrel is a structurally distinct domain, the “cork,” which mainly consists of a four-stranded β sheet and four short α helices. A single lipopolysaccharide molecule is noncovalently associated with the membrane-embedded region of the protein. Upon binding of ferrichrome-iron, conformational changes are transduced to the periplasmic pocket of FhuA, signaling the ligand-loaded status of the receptor. Sequence homologies and mutagenesis data are used to propose a structural mechanism for TonB-dependent siderophore-mediated transport across the outer membrane.

Peridinin-chlorophyll-protein, a water-soluble light-harvesting complex that has a blue-green absorbing carotenoid as its main pigment, is present in most photosynthetic dinoflagellates. Its high-resolution (2.0 angstrom) x-ray structure reveals a noncrystallographic trimer in which each polypeptide contains an unusual jellyroll fold of the α-helical amino- and carboxyl-terminal domains. These domains constitute a scaffold with pseudo-twofold symmetry surrounding a hydrophobic cavity filled by two lipid, eight peridinin, and two chlorophyll a molecules. The structural basis for efficient excitonic energy transfer from peridinin to chlorophyll is found in the clustering of peridinins around the chlorophylls at van der Waals distances.

The integral membrane sensor kinase CitA of Klebsiella pneumoniae is part of a two-component signal transduction system that regulates the transport and metabolism of citrate in response to its environmental concentration. Two-component systems are widely used by bacteria for such adaptive processes, but the stereochemistry of periplasmic ligand binding and the mechanism of signal transduction across the membrane remain poorly understood. The crystal structure of the CitAP periplasmic sensor domain in complex with citrate reveals a PAS fold, a versatile ligand-binding structural motif that has not previously been observed outside the cytoplasm or implicated in the transduction of conformational signals across the membrane. Citrate is bound in a pocket that is shared among many PAS domains but that shows structural variation according to the nature of the bound ligand. In CitAP, some of the citrate contact residues are located in the final strand of the central β-sheet, which is connected to the C-terminal transmembrane helix. These secondary structure elements thus provide a potential conformational link between the periplasmic ligand binding site and the cytoplasmic signaling domains of the receptor.

[FeFe]-hydrogenases are nature's fastest catalysts for the evolution or oxidation of hydrogen. Numerous synthetic model complexes for the [2Fe] subcluster (2Fe_H) of their active site are known, but so far none of these could compete with the enzymes. The complex Fe₂[μ-(SCH₂)₂X](CN)₂(CO)₄ ²⁻ with X = NH was shown to integrate into the apo-form of [FeFe]-hydrogenases to yield a fully active enzyme. Here we report the first crystal structures of the apo-form of the bacterial [FeFe]-hydrogenase CpI from Clostridium pasteurianum at 1.60 Å and the active semisynthetic enzyme, CpI^ADT, at 1.63 Å. The structures illustrate the significant changes in ligand coordination upon integration and activation of the [2Fe] complex. These changes are induced by a rigid 2Fe_H cavity as revealed by the structure of apoCpI, which is remarkably similar to CpI^ADT. Additionally we present the high resolution crystal structures of the semisynthetic bacterial [FeFe]-hydrogenases CpI^PDT (X = CH₂), CpI^ODT (X = O) and CpI^SDT (X = S) with changes in the headgroup of the dithiolate bridge in the 2Fe_H cofactor. The structures of these inactive enzymes demonstrate that the 2Fe_H-subcluster and its protein environment remain largely unchanged when compared to the active enzyme CpI^ADT. As the active site shows an open coordination site in all structures, the absence of catalytic activity is probably not caused by steric obstruction. This demonstrates that the chemical properties of the dithiolate bridge are essential for enzyme activity.