Ribonucleotide reductases (RNRs) catalyze all new production in nature of deoxyribonucleotides for DNA synthesis by reducing the corresponding ribonucleotides. The reaction involves the action of a radical that is produced differently for different classes of the enzyme. Class I enzymes, which are present in eukaryotes and microorganisms, use an iron center to produce a stable tyrosyl radical that is stored in one of the subunits of the enzyme. The other classes are only present in microorganisms. Class II enzymes use cobalamin for radical generation and class III enzymes, which are found only in anaerobic organisms, use a glycyl radical. The reductase activity is in all three classes contained in enzyme subunits that have similar structures containing active site cysteines. The initiation of the reaction by removal of the 3'-hydrogen of the ribose by a transient cysteinyl radical is a common feature of the different classes of RNR. This cysteine is in all RNRs located on the tip of a finger loop inserted into the center of a special barrel structure. A wealth of structural and functional information on the class I and class III enzymes can now give detailed views on how these enzymes perform their task. The class I enzymes demonstrate a sophisticated pattern as to how the free radical is used in the reaction, in that it is only delivered to the active site at exactly the right moment. RNRs are also allosterically regulated, for which the structural molecular background is now starting to be revealed.

Here we describe the three-dimensional crystal structures of human glucocorticoid receptor ligand-binding domain (GR-LBD) in complex with the antagonist RU-486 at 2.3 Å resolution and with the agonist dexamethasone ligand together with a coactivator peptide at 2.8 Å. The RU-486 structure was solved in several different crystal forms, two with helix 12 intact (GR1 and GR3) and one with a protease-digested C terminus (GR2). In GR1, part of helix 12 is in a position that covers the co-activator pocket, whereas in the GR3, domain swapping is seen between the crystallographically identical subunits in the GR dimer. An arm consisting of the end of helix 11 and beyond stretches out from one molecule, and helix 12 binds to the other LBD, partly blocking the coactivator pocket of that molecule. This type of GR-LBD dimer has not been described before but might be an artifact from crystallization. Furthermore, the subunits of the GR3 dimers are covalently connected via a disulfide bond between the Cys-736 residues in the two molecules. All three RU-486 GR-LBD structures show that GR has a very flexible region between the end of helix 11 and the end of helix 12.The glucocorticoid receptor (GR) 1 is a foundational member of the nuclear receptor family. A large part of the basic knowledge of the mechanism of nuclear receptor function and action has been obtained by analysis of glucocorticoid receptor function. The three-domain structure of the nuclear receptor was originally described based on results from proteolytic cleavage of GR (1, 2). The first specific binding of a nuclear receptor to a defined DNA sequence, the glucocorticoid response element, was shown using purified GR (3, 4), and GR was the first steroid receptor to be cloned (partial clone) (5). The first threedimensional structure of a nuclear receptor was obtained when the structure of the GR DNA-binding domain was solved (6, 7). However, until recently, the structure of the GR ligand-binding domain (LBD) has proven elusive. The structures of numerous other nuclear receptor LBDs have been described, including those of most of the homologues of progesterone and androgen receptors (8, 9). Very recently, the first GR-LBD structure was described in complex with the agonist dexamethasone and a coactivator peptide (10).The four steroid receptors, GR, the progesterone (PR), androgen (AR), and mineralocorticoid receptors, are very closely related. They all bind to response elements with the same degenerate consensus sequence (11), and there is considerable overlap in ligand specificity and action (12)(13)(14). Progesterone is a glucocorticoid antagonist, and many synthetic progestins are also androgens. Glucocorticoids, and particularly the endogenous hormone cortisol, bind with similar affinities to both GR and the mineralocorticoid receptor, although aldosterone is a poor GR agonist. Thus, detailed structural and functional data will be needed to understand the specific function of these four steroid receptors. Despite the problems purifying and crystallizing GR-LBD...

The structures of the liver X receptor LXR␤ (NR1H2) have been determined in complexes with two synthetic ligands, T0901317 and GW3965, to 2.1 and 2.4 Å, respectively. Together with its isoform LXR␣ (NR1H3) it regulates target genes involved in metabolism and transport of cholesterol and fatty acids. The two LXR␤ structures reveal a flexible ligand-binding pocket that can adjust to accommodate fundamentally different ligands. The ligand-binding pocket is hydrophobic but with polar or charged residues at the two ends of the cavity. T0901317 takes advantage of this by binding to His-435 close to H12 while GW3965 orients itself with its charged group in the opposite direction. Both ligands induce a fixed "agonist conformation" of helix H12 (also called the AF-2 domain), resulting in a transcriptionally active receptor.Liver X receptors (LXR) 1 are members of the superfamily of nuclear receptors. These transcription factors regulate target genes through a dynamic series of interactions with specific DNA response elements as well as transcriptional coregulators. The binding of ligand has profound effects on these interactions and has the potential to trigger both gene activation and, in some cases, gene silencing. There are 48 sequence-related nuclear receptors in humans and the family comprises receptors that recognize hormones, both steroidal and non-steroidal, but also receptors responding to metabolic intermediates and to xenobiotics. There are also a number of so-called orphan receptors where the natural ligand is unknown. Some of the receptors show a very specific and high affinity ligand binding, like the thyroid hormone receptors, whereas others have a substantially lower affinity for their ligands and are less discriminating in their ligand selectivity. Like many of the other nonsteroid hormone receptors, LXR functions as a heterodimer with the retinoid X receptor (RXR) to regulate gene expression (1, 2). Together with peroxisome proliferator-activated receptor (PPAR) and farnesoid X receptor (FXR), LXRs represent a subclass of so-called permissive RXR heterodimers. In this subclass, the RXR heterodimers can be activated independently by either the RXR ligand, the partner's ligand, or synergistically by both (3).LXRs consist of two closely related receptor isoforms encoded by separate genes, LXR␣ (NR1H3) and LXR␤ (NR1H2). LXR␣ shows tissue-restricted expression with the highest mRNA levels in the liver and somewhat lower levels in the kidney, small intestine, spleen, and adrenal gland (4, 5). In contrast, LXR␤ is ubiquitously expressed (6, 7). Both LXR isoforms can be activated by specific oxysterols that are formed in vivo (2,8,9). In view of the high degree of homology between the LXR isoforms (75% identity in the ligand-binding domain (LBD), 54% identity overall), it is perhaps not surprising that few subtypespecific biological responses have been described and that information on subtype selective ligands is limited. LXRs have been shown to regulate several genes involved in cholesterol and lipid homeos...

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