Interactions of fatty acids, nonsteroidal anti-inflammatory drugs, and coxibs with the catalytic and allosteric subunits of cyclooxygenases-1 and -2

Abstract

Edited by George M. CarmanProstaglandin endoperoxide H synthases-1 and -2, commonly called cyclooxygenases-1 and -2 (COX-1 and -2), catalyze the committed step in prostaglandin biosynthesis-the conversion of arachidonic acid to prostaglandin endoperoxide H 2 . Both COX isoforms are sequence homodimers that function as conformational heterodimers having allosteric (Eallo) and catalytic (Ecat) subunits. At least in the case of COX-2, the enzyme becomes folded into a stable Eallo/Ecat pair. Some COX inhibitors (i.e. nonsteroidal anti-inflammatory drugs and coxibs) and common fatty acids (FAs) modulate Ecat activity by binding Eallo. However, the interactions and outcomes often differ between isoforms. For example, naproxen directly and completely inhibits COX-1 by binding Ecat but indirectly and incompletely inhibits COX-2 by binding Eallo. Additionally, COX-1 is allosterically inhibited up to 50% by common FAs like palmitic acid, whereas COX-2 is allosterically activated 2-fold by palmitic acid. FA binding to Eallo also affects responses to COX inhibitors. Thus, COXs are physiologically and pharmacologically regulated by the FA tone of the milieu in which each operates-COX-1 in the endoplasmic reticulum and COX-2 in the Golgi apparatus. Cross-talk between Eallo and Ecat involves a loop in Eallo immediately downstream of Arg-120. Mutational studies suggest that allosteric modulation requires a direct interaction between the carboxyl group of allosteric effectors and Arg-120 of Eallo; however, structural studies show some allosterically active FAs positioned in COX-2 in a conformation lacking an interaction with Arg-120. Thus, many details about the biological consequences of COX allosterism and how ligand binding to Eallo modulates Ecat remain to be resolved. Figure 4. Model for allosteric interactions between monomers of the COX-2 homodimer caused by the binding of FAs.Ecat is the catalytic monomer to which heme is bound, whereas Eallo is the allosteric monomer that does not bind heme. A, binding of AA to Eallo and Ecat of huCOX-2 and subsequent oxygenation of AA. The K d value for Ecat is the K m for AA (8.4 M) in the absence of other FAs. The K d value for AA binding to Eallo was determined to be 0.26 M. 6 B, binding of AA to Eallo and Ecat in the presence of PA. We make the following assumptions: (a) that the K d value for binding of AA to the Ecat site is equal to the K m value in the presence of PA (i.e. Ͻ 3.5 M), and (b) that the K d value for PA binding to the Ecat site is high (Ն50 M) because PA does not inhibit AA oxygenation by huCOX-2 even at low concentrations of AA. We estimate that the K d values for the binding of PA and stearic acid to the Eallo site are both ϳ7.5 M. 5 Note that the K d value for PA binding to Eallo is 30-fold that for AA binding to Eallo so that at all relevant FA concentrations AA and PA will compete for binding to Eallo, and the effect of PA will be determined by the PA/AA ratio. This figure and the legend are modified from Ref. 51. JBC REVIEWS: Allosteric regulation of COX-2

COX-1 – lipid interactions: arachidonic acid, cholesterol, and phospholipid binding to the membrane binding domain of COX-1

“…stearic and oleic acid) have the same effect on COX-1. Measurements of binding kinetics for these fatty acids indicate that their inhibition of COX-1 may not be due to competition with arachidonic acid for Ecat (15). These studies reveal a complex and isoform-dependent interplay between fatty acids and cyclooxygenases.…”

“…These studies reveal a complex and isoform-dependent interplay between fatty acids and cyclooxygenases. In a recent review, Smith and Malkowski (15) conclude that cyclooxygenase activity is both pharmacologically and physiologically dependent on the fatty acid content and flavor of the membrane environment. MD simulation results presented here broaden our perspective on COX-1 binding to ER membranes.…”

“…The MBD is composed of four short helical segments (helices A-D) that anchor COX-1 into the upper (outer or luminal) leaflet (13) of the endoplasmic reticulum (ER) and inner membrane of the nuclear envelope (14). While cyclooxygenases are sequence homodimers, they function as heterodimers, with one subunit serving an allosteric (Eallo) and the other a catalytic (Ecat) role (15,16). This is referred to as half of site COX activity whereby only one subunit is functionally active at a given time and is regulated allosterically by the other subunit.…”

A seven-step plan for becoming a moderately rich and famous biochemist

“…Finally, we found that omega-3 fish oil FAs and particularly eicosapentaenoic acid effectively inhibit the oxygenation of AA by binding Ecat of COX-1 but that eicosapentaenoic acid binds Eallo of COX-2 and has little effect on AA oxygenation by Ecat of COX-2 (56). As my scientific career comes to its conclusion, I feel very fortunate to have been involved in the discovery that COXs function as conformational heterodimers (1).…”

Two Undervalued Functions of the Golgi Apparatus: Removal of Excess Ca2+ and Biosynthesis of Farnesol-Like Sesquiterpenoids, Possibly as Ca2+-Pump Agonists and Membrane “Fluidizers–Plasticizers”

“…It involves the high flexibility of farnesol/FLS. The self-closing issue of membranes is of course also essential during mitosis, a process that requires Golgi activity (Smith and Malkowski, 2019).…”