The last enzyme (LytB) of the methylerythritol phosphate pathway for isoprenoid biosynthesis catalyzes the reduction of (E)-4-hydroxy-3-methylbut-2-enyl diphosphate into isopentenyl diphosphate and dimethylallyl diphosphate. This enzyme possesses a dioxygen-sensitive [4Fe^4S] cluster. This prosthetic group was characterized in the Escherichia coli enzyme by UV/visible and electron paramagnetic resonance spectroscopy after reconstitution of the puri¢ed protein. Enzymatic activity required the presence of a reducing system such as £avodoxin/£avodoxin reductase/reduced nicotinamide adenine dinucleotide phosphate or the photoreduced deaza£avin radical.

Duox2 (and probablyDuox1Reactive oxygen species (ROS) 1 have emerged as important molecules involved in regulating essential cell functions, such as growth and differentiation (1). NAD(P)H oxidases are a major source of ROS. Phagocyte oxidase is the oxidase that has been investigated most thoroughly (2). It catalyzes the production of superoxide by the one-electron reduction of oxygen, using NADPH as the electron donor. The catalytic moiety of the phagocyte NADPH oxidase is gp91phox , a plasma membraneassociated flavohemoprotein. Recently, it was discovered that gp91 phox belongs to a family consisting of several very similar oxidases. Seven NOX (NADPH oxidase) and DUOX/ThOX (dual oxidase/thyroid oxidase) genes have been identified that encode different NADPH oxidases with differing mRNA tissue expression. The Nox family comprises gp91phox , now known as Nox2; Nox1, which is predominantly expressed in the colon (3); Nox3, cloned from fetal kidney (4); Nox4 found in the kidney cortex (5, 6); and Nox5, expressed in the testis, spleen, and lymph nodes (7). In addition to the basic structure of gp91 phox , Nox5 has a long intracellular N-terminal domain with four calcium binding sites implicated in its Ca 2ϩ -dependent activation (8).The biological functions of these Nox proteins are now under investigation. They are involved in signal transduction related to cell growth and cancer (9 -11) and to angiogenesis (12).Duox1 and Duox2 are large homologues of Nox2 with an N-terminal extension comprising two EF-hand motifs, an additional transmembrane helix, and a peroxidase homology ectodomain (see Fig. 4A) (13,14). DUOX genes have been identified in the thyroid gland, where they are strongly expressed (13,14). However, the DUOX are also expressed on the mucosal surfaces of the trachea and the bronchi (15) and in the airway epithelial cells (16,17), where it has been suggested that Duox1 is the isoform responsible for acid production and secretion in airways (16) and plays a critical role in mucin expression (18). DUOX2 was also expressed throughout the digestive tract, where it was found to be functional (19,20), in addition to the salivary gland and rectum (15).It has been suggested that Duox2, which was identified by purifying thyroid NADPH oxidase, may constitute the catalytic core of this enzyme and generate the H 2 O 2 used by Tpo to catalyze the biosynthesis of thyroid hormones at the apical surface of the thyrocytes (13). Although no functional Duoxbased H 2 O 2 -generating system has yet been reconstituted (21), this proposal is corroborated by a recent report of permanent and severe congenital hypothyroidism in a patient with a bial-* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.¶ Recipient of a fellowship from the Commissariat à l'Energie Atomique (Paris, France).ʈ Recipients of a fellowship from the National Education, Research, an...