The structure of estrogen sulphotransferase has been solved in the presence of inactive cofactor PAP and substrate 17 beta-estradiol. This structure reveals structural similarities between cytosolic sulphotransferases and nucleotide kinases.

Heme oxygenase catalyzes the oxidative cleavage of protoheme to biliverdin, the first step of heme metabolism utilizing O 2 and NADPH. We determined the crystal structures of rat heme oxygenase-1 (HO-1)^heme and selenomethionyl HO-1^heme complexes. Heme is sandwiched between two helices with the N N-meso edge of the heme being exposed to the surface. Gly143N forms a hydrogen bond to the distal ligand of heme, OH 3 . The distance between Gly143N and the ligand is shorter than that in the human HO-1^heme complex. This difference may be related to a pH-dependent change of the distal ligand of heme. Flexibility of the distal helix may control the stability of the coordination of the distal ligand to heme iron. The possible role of Gly143 in the heme oxygenase reaction is discussed.z 2000 Federation of European Biochemical Societies.

Sulfotransferases (STs) catalyze the transfer reaction of the sulfate group from the ubiquitous donor 3 -phosphoadenosine 5 -phosphosulfate (PAPS) to an acceptor group of numerous substrates. This reaction, often referred to as sulfuryl transfer, sulfation, or sulfonation, is widely observed from bacteria to humans and plays a key role in various biological processes such as cell communication, growth and development, and defense. The cytosolic STs sulfate small molecules such as steroids, bioamines, and therapeutic drugs, while the Golgi-membrane counterparts sulfate large molecules including glucosaminylglycans and proteins. We have now solved the X-ray crystal structures of four cytosolic and one membrane ST. All five STs are globular proteins composed of a single ␣/␤ domain with the characteristic five-stranded ␤-sheet. The ␤-sheet constitutes the core of the Paps-binding and catalytic sites. Structural analysis of the PAPS-, PAP-, substrate-, and/or orthovanadate (VO 4 3؊ )-bound enzymes has also revealed the common molecular mechanism of the transfer reaction catalyzed by sulfotransferses. The X-ray crystal structures have opened a new era for the study of sulfotransferases.

The Escherichia coli chromosome encodes toxin-antitoxin pairs. The toxin RelE cleaves mRNA positioned at the A-site in ribosomes, whereas the antitoxin RelB relieves the effect of RelE. The hyperthermophilic archaeon Pyrococcus horikoshii OT3 has the archaeal homologs aRelE and aRelB. Here we report the crystal structure of aRelE in complex with aRelB determined at a resolution of 2.3 A. aRelE folds into an alpha/beta structure, whereas aRelB lacks a distinct hydrophobic core and extensively wraps around the molecular surface of aRelE. Neither component shows structural homology to known ribonucleases or their inhibitors. Site-directed mutagenesis suggests that Arg85, in the C-terminal region, is strongly involved in the functional activity of aRelE, whereas Arg40, Leu48, Arg58 and Arg65 play a modest role in the toxin's activity.