The prostaglandin endoperoxide H synthases-1 and 2 (PGHS-1 and PGHS-2; also cyclooxygenases-1 and 2, COX-1 and COX-2) catalyze the committed step in prostaglandin synthesis. PGHS-1 and 2 are of particular interest because they are the major targets of nonsteroidal anti-inflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2 inhibitors. Inhibition of the PGHSs with NSAIDs acutely reduces inflammation, pain, and fever, and long-term use of these drugs reduces fatal thrombotic events, as well as the development of colon cancer and Alzheimer's disease. In this review, we examine how the structures of these enzymes relate mechanistically to cyclooxygenase and peroxidase catalysis, and how differences in the structure of PGHS-2 confer on this isozyme differential sensitivity to COX-2 inhibitors. We further examine the evidence for independent signaling by PGHS-1 and PGHS-2, and the complex mechanisms for regulation of PGHS-2 gene expression.

No abstract

The three-dimensional structure of prostaglandin H2 synthase-1, an integral membrane protein, has been determined at 3.5 A resolution by X-ray crystallography. This bifunctional enzyme comprises three independent folding units: an epidermal growth factor domain, a membrane-binding motif and an enzymatic domain. Two adjacent but spatially distinct active sites were found for its haem-dependent peroxidase and cyclooxygenase activities. The cyclooxygenase active site is created by a long, hydrophobic channel that is the site of non-steroidal anti-inflammatory drug binding. The conformation of the membrane-binding motif strongly suggests that the enzyme integrates into only one leaflet of the lipid bilayer and is thus a monotopic membrane protein.

Detergents are invaluable tools for studying membrane proteins. However, these deceptively simple, amphipathic molecules exhibit complex behavior when they self-associate and interact with other molecules. The phase behavior and assembled structures of detergents are markedly influenced not only by their unique chemical and physical properties but also by concentration, ionic conditions, and the presence of other lipids and proteins. In this minireview, we discuss the various aggregate forms detergents assume and some misconceptions about their structure. The distinction between detergents and the membrane lipids that they may (or may not) replace is emphasized in the most recent high resolution structures of membrane proteins. Detergents are clearly friends and foes, but with the knowledge of how they work, we can use the increasing variety of detergents to our advantage.Over the past decade, our understanding of the structure and function of membrane proteins has advanced significantly as well as how their detailed characterization can be approached experimentally. Detergents have played significant roles in this effort. They serve as tools to isolate, solubilize, and manipulate membrane proteins for subsequent biochemical and physical characterization. Many of the successful methods for reconstituting (1) and crystallizing (2-4) membrane proteins rely on the unique behavior of detergents. Although many new detergents are now available for use with membrane proteins, their behavior in solution and in the presence of protein may limit their use with specific experimental techniques. Hence, the choice of detergent and experimental conditions will have an enormous impact on whether a technique can be successfully applied to a specific membrane protein. A clear understanding of basic detergent behavior and of the structure of micelles and protein-detergent complexes is thus crucial for membrane biochemists.

Well ordered reproducible crystals of cytochrome c oxidase (CcO) from Rhodobacter sphaeroides yield a previously unreported structure at 2.0 Å resolution that contains the two catalytic subunits and a number of alkyl chains of lipids and detergents. Comparison with crystal structures of other bacterial and mammalian CcOs reveals that the positions occupied by native membrane lipids and detergent substitutes are highly conserved, along with amino acid residues in their vicinity, suggesting a more prevalent and specific role of lipid in membrane protein structure than often envisioned. Well defined detergent head groups (maltose) are found associated with aromatic residues in a manner similar to phospholipid head groups, likely contributing to the success of alkyl glycoside detergents in supporting membrane protein activity and crystallizability. Other significant features of this structure include the following: finding of a previously unreported crystal contact mediated by cadmium and an engineered histidine tag; documentation of the unique His-Tyr covalent linkage close to the active site; remarkable conservation of a chain of waters in one proton pathway (D-path); and discovery of an inhibitory cadmium-binding site at the entrance to another proton path (K-path). These observations provide important insight into CcO structure and mechanism, as well as the significance of bound lipid in membrane proteins.cadmium binding ͉ membrane protein structure ͉ proton-conducting water chains C ytochrome c oxidase (CcO) is the terminal enzyme in the electron transfer chain in eukaryotes and many bacteria. It provides the final electron sink by accepting electrons from cytochrome c and reducing oxygen to water (1). The energy generated from this reaction is used to translocate protons across the membrane against the membrane potential and pH gradient (2). The proton gradient so formed is then used to make ATP, a universal energy source. CcO is an intrinsic membrane protein with varying numbers of subunits from 3 in some bacteria to 13 in mammalian mitochondria. However, only subunits I and II, which contain the redox active heme and metal centers and are highly conserved among different species, are required for electron transfer and proton pumping activities. Other subunits, particularly the highly conserved subunit III, likely play key roles in stabilizing and regulating the enzyme activity and energy metabolism in general.Over the past decade, several x-ray crystal structures of aa 3 -type CcOs from bovine heart mitochondria and bacteria have been determined (3-8). However, the mechanism of vectorial translocation of protons by CcO remains to be solved (9). It is now recognized that water molecules play a critical role in facilitating and controlling proton transfer (10). Thus, highresolution crystal structures of different redox states and key mutants with defined waters are necessary to elucidate the energy conservation process. However, achieving a reproducible high-resolution structure of membrane proteins remains a ...