Different Fatty Acids Compete with Arachidonic Acid for Binding to the Allosteric or Catalytic Subunits of Cyclooxygenases to Regulate Prostanoid Synthesis.

Prostaglandin endoperoxide H synthases (PGHSs), also called cyclooxygenases (COXs), convert arachidonic acid (AA) to PGH2. PGHS-1 and PGHS-2 are conformational heterodimers, each composed of an (Eallo) and a catalytic (Ecat) monomer. Previous studies suggested that the binding to Eallo of saturated or monounsaturated fatty acids (FAs) that are not COX substrates differentially regulate PGHS-1 versus PGHS-2. Here, we substantiate and expand this concept to include polyunsaturated FAs known to modulate COX activities. Non-substrate FAs like palmitic acid bind Eallo of PGHSs stimulating human (hu) PGHS-2 but inhibiting huPGHS-1. We find the maximal effects of non-substrate FAs on both huPGHSs occurring at the same physiologically relevant FA/AA ratio of ∼20. This inverse allosteric regulation likely underlies the ability of PGHS-2 to operate at low AA concentrations, when PGHS-1 is effectively latent. Unlike FAs tested previously, we observe that C-22 FAs, including ω-3 fish oil FAs, have higher affinities for Ecat than Eallo subunits of PGHSs. Curiously, C-20 ω-3 eicosapentaenoate preferentially binds Ecat of huPGHS-1 but Eallo of huPGHS-2. PGE2 production decreases 50% when fish oil consumption produces tissue EPA/AA ratios of ≥0.2. However, 50% inhibition of huPGHS-1 itself is only seen with ω-3 FA/AA ratios of ≥5.0. This suggests that fish oil-enriched diets disfavor AA oxygenation by altering the composition of the FA pool in which PGHS-1 functions. The distinctive binding specificities of PGHS subunits permit different combinations of non-esterified FAs, which can be manipulated dietarily, to regulate AA binding to Eallo and/or Ecat thereby controlling COX activities.

Interactions of 2-O-arachidonylglycerol ether and ibuprofen with the allosteric and catalytic subunits of human COX-2.

Prostaglandin (PG) endoperoxide H synthase (PGHS)-2, also known as cyclooxygenase (COX)-2, can convert arachidonic acid (AA) to PGH2 in the committed step of PG synthesis. PGHS-2 functions as a conformational heterodimer composed of an allosteric (Eallo) and a catalytic (Ecat) monomer. Here we investigated the interplay between human (hu)PGHS-2 and an alternative COX substrate, the endocannabinoid, 2-arachidonoylglycerol (2-AG), as well as a stable analog, 2-O-arachidonylglycerol ether (2-AG ether). We also compared the inhibition of huPGHS-2-mediated oxygenation of AA, 2-AG, and 2-AG ether by the well-known COX inhibitor, ibuprofen. When tested with huPGHS-2, 2-AG and 2-AG ether exhibit very similar kinetic parameters, responses to stimulation by FAs that are not COX substrates, and modes of inhibition by ibuprofen. The 2-AG ether binds Ecat more tightly than Eallo and, thus, can be used as a stable Ecat-specific substrate to examine certain Eallo-dependent responses. Ibuprofen binding to Eallo of huPGHS-2 completely blocks 2-AG or 2-AG ether oxygenation; however, inhibition by ibuprofen of huPGHS-2-mediated oxygenation of AA engages a combination of both allosteric and competitive mechanisms.

Human cyclooxygenase-1 activity and its responses to COX inhibitors are allosterically regulated by nonsubstrate fatty acids.

Recombinant human prostaglandin endoperoxide H synthase-1 (huPGHS-1) was characterized. huPGHS-1 has a single high-affinity heme binding site per dimer and exhibits maximal cyclooxygenase (COX) activity with one heme per dimer. Thus, huPGHS-1 functions as a conformational heterodimer having a catalytic monomer (E(cat)) with a bound heme and an allosteric monomer (E(allo)) lacking heme. The enzyme is modestly inhibited by common FAs including palmitic, stearic, and oleic acids that are not COX substrates. Studies of arachidonic acid (AA) substrate turnover at high enzyme-to-substrate ratios indicate that nonsubstrate FAs bind the COX site of E(allo) to modulate the properties of E(cat). Nonsubstrate FAs slightly inhibit huPGHS-1 but stimulate huPGHS-2, thereby augmenting AA oxygenation by PGHS-2 relative to PGHS-1. Nonsubstrate FAs potentiate the inhibition of huPGHS-1 activity by time-dependent COX inhibitors, including aspirin, all of which bind E(cat). Surprisingly, preincubating huPGHS-1 with nonsubstrate FAs in combination with ibuprofen, which by itself is a time-independent inhibitor, causes a short-lived, time-dependent inhibition of huPGHS-1. Thus, in general, having a FA bound to E(allo) stabilizes time-dependently inhibited conformations of E(cat). We speculate that having an FA bound to E(allo) also stabilizes E(cat) conformers during catalysis, enabling half of sites of COX activity.

Inactivation and unfolding of the hyperthermophilic inorganic pyrophosphatase from Thermus thermophilus by sodium dodecyl sulfate.

Inorganic pyrophosphatase (PPase, EC 3.6.1.1) is an essential constitutive enzyme for energy metabolism and clearance of excess pyrophosphate. In this research, we investigated the sodium dodecyl sulfate (SDS)-induced inactivation and unfolding of PPase from Thermus thermophilus (T-PPase), a hyperthermophilic enzyme. The results indicated that like many other mesophilic enzymes, T-PPase could be fully inactivated at a low SDS concentration of 2 mM. Using an enzyme activity assay, SDS was shown to act as a mixed type reversible inhibitor, suggesting T-PPase contained specific SDS binding sites. At high SDS concentrations, T-PPase was denatured via a two-state process without the accumulation of any intermediate, as revealed by far-UV CD and intrinsic fluorescence. A comparison of the inactivation and unfolding data suggested that the inhibition might be caused by the specific binding of the SDS molecules to the enzyme, while the unfolding might be caused by the cooperative non-specific binding of SDS to T-PPase. The possible molecular mechanisms underlying the mixed type inhibition by SDS was proposed to be caused by the local conformational changes or altered charge distributions.

Effects of acrylamide on the activity and structure of human brain creatine kinase.

Acrylamide is widely used worldwide in industry and it can also be produced by the cooking and processing of foods. It is harmful to human beings, and human brain CK (HBCK) has been proposed to be one of the important targets of acrylamide. In this research, we studied the effects of acrylamide on HBCK activity, structure and the potential binding sites. Compared to CKs from rabbit, HBCK was fully inactivated at several-fold lower concentrations of acrylamide, and exhibited distinct properties upon acrylamide-induced inactivation and structural changes. The binding sites of acrylamide were located at the cleft between the N- and C-terminal domains of CK, and Glu232 was one of the key binding residues. The effects of acrylamide on CK were proposed to be isoenzyme- and species-specific, and the underlying molecular mechanisms were discussed.

The inhibition kinetics and thermodynamic changes of tyrosinase via the zinc ion.

We found that Zn(2+) conspicuously inactivated tyrosinase in a mixed-type inhibition manner: the final level of residual activity was abolished at the equilibrium state with concentration of 0.25 mM Zn(2+). Changes of both K(m) and V(max) by various concentrations of Zn(2+) in Lineweaver-Burk plot were observed. To see whether Zn(2+) also induced conformational change of tyrosinase and how thermodynamical changes by ligand binding were occurred, the intrinsic fluorescence studies as well as calorimetric measurements were conducted. The results showed that the Zn(2+) binding to tyrosinase directly induced conformational change of tyrosinase, and the changes of thermodynamic parameters such as enthalpy (DeltaH), Gibbs free-energy (DeltaG), and entropy (DeltaS) were obtained as 60+/-7.0 kJ/mol, -14.54 kJ/mol and 248.53 J/(K mol), respectively. The inactivating effect of Zn(2+) on tyrosinase was completely prevented by incubation with bovine serum albumin, which has a Zn(2+) binding motif in its structure. We suggested that Zn(2+) ligand-binding affected the substrate's accessibility due to the conformational changes and thus, the complex type of inhibition has occurred with the calorimetric changes.

Monomeric creatine kinase aggregation and sodium dodecyl sulfate-cyclodextrin assisted refolding.

The monomeric state of creatine kinase (CK) was stably captured at the equilibrium state by employing cysteine residue modifications in the presence of a denaturant, and at a partially folded state. The partially folded monomeric CK (PF-CK) was aggregated with kinetic order, which was mainly caused by the hydrophobic surface interactions between the CK subunits. The artificial chaperone, described as a SDS-cyclodextrin, was applied to prevent aggregation as well as to refold the PF-CK: SDS treatment onto the monomeric CK can significantly block aggregation and can be successfully refolded in the solutions containing cyclodextrins and DTT. Three types of cyclodextrins such as alpha-, beta-, and gamma-cyclodextrins were applied to strip SDS from the enzyme molecule, and each kinetic course was measured. The intrinsic fluorescence changes showed that reactivation occurred and this accompanied the conformational changes. The size exclusion chromatography detected the variously trapped monomeric CKs such as the thiol residue modified PF-CK, the SDS-binding PF-CK, the cyclodextrin treated PF-CK, and the DTT treated SDS-binding PF-CK. Our study demonstrated monomer CK aggregation for the first time; we also demonstrated the complex reassociation of CK during refolding with the aid of the SDS-cyclodextrin, and these pathways followed first-order kinetics.

Structural analysis and inhibitory kinetics of brain type creatine kinase by sodium dodecyl sulfate.

The studies regarding the effect of sodium dodecyl sulfate (SDS) on enzyme activities and structures can provide a valuable insight into public health. We have predicted the 3D structure of the brain creatine kinase (CK-BB) with a high resolution and simulated the docking between CK-BB and SDS. The predicted structure had a root mean square deviation of 0.51 A. The docking between CK-BB and SDS was successful with significant scores (-4.67 kcal/mol, AutoDock4 and -48.32 kcal/mol, DOCK6). We have also investigated the inactivation by using SDS to study CK-BB's folding behaviors. The two-phase rate constants as a first-order reaction were measured during inactivation. SDS strongly inhibited the CK-BB activity in a noncompetitive inhibition manner (K (i) = 1.22 mM). The tertiary structural change was induced by SDS binding with the exposure of hydrophobic surface. The methyl-beta-cyclodextrin was used to strip SDS from the enzyme molecule to reactivate. The changes of thermodynamic parameters for the SDS ligand binding such as enthalpy, Gibbs free energy, and entropy were obtained as -13 + or - 7.0 MJ/mol, 8.39 kJ/mol, and -42.754 kJ/(K mol), respectively. Our study provides important structural information for CK-BB and its interaction with SDS with an insight on its folding and inhibition kinetics.

Kinetics of Zn(2+)-induced brain type creatine kinase unfolding and aggregation.

We studied the effect of Zn(2+) on the folding and aggregation of brain creatine kinase (CK-BB). We developed a method to purify CK-BB from rabbit brain and conducted inhibition kinetics and unfolding studies of CK-BB. Zn(2+) conspicuously aggregated and osmolytes, such as glycine and proline, were able to suppress the formation of aggregates and protect the enzymatic activity against Zn(2+). These results suggest that Zn(2+) might act as a risk factor for CK-BB in the brain under certain conditions, and some osmolytes may help CK-BB to sustain the active state when Zn(2+) is present. Our study provides useful information regarding the effect of Zn(2+) on brain-derived metabolic enzymes, especially those that are putatively related to brain disease. Furthermore, our study suggests that although Zn(2+) may induce CK-BB inactivation and misfolding, the ability of some abundant proteins and osmolytes to chelate Zn(2+) nonspecifically may protect CK-BB and allow it to exist in the active form.

Effect of cysteine modification on creatine kinase aggregation.

We studied the effect of cysteine modification on creatine kinase (CK) aggregation as well as the kinetics of the process. We found that CK aggregation was modulated by different pH conditions in the presence of Zn2+, which is a CK aggregation trigger. The CK aggregation followed first-order kinetics, and this was effectively suppressed in acidic conditions. Even under the acidic condition, cysteine modification at the active site with using 5,5'-dithiobis-2-nitrobenzoic acid (DTNB) induced conspicuous aggregation in a dose-dependent manner. This aggregation process is directly related with decreasing the change of transition free-energy (DeltaDeltaG(AG)). When dithiothreitol (DTT) was applied to the reaction system, the aggregates were significantly reduced: DTT treatment can fully reactivate (higher than 80%) the inactive CK that was separated from CK aggregates, whereas CK was completely inactivated by Zn2+ and DTNB. Some added osmolytes such as glycine and proline were able to successfully block CK aggregation by increasing the DeltaDeltaG(AG) as well as by suppressing the hydrophobic CK surface. Our study suggests the effect of cysteine modification on the unfavorable aggregation of CK and on the aggregation process that followed first-order kinetics with the accompanying changes of transitional free energy and disruptions of the hydrophobic surface. We also demonstrate the successful protocol to block the aggregation.

The effects of acrylamide on brain creatine kinase: inhibition kinetics and computational docking simulation.

The occurrence of acrylamide is frequently observed in processed foods. Therefore, the harmful effects of acrylamide on metabolic enzymes are important to understand. We studied the inhibitory effects of acrylamide on the brain creatine kinase (CK-BB). We found that CK-BB was kinetically inactivated by acrylamide accompanied by the disruption of the hydrophobic surface. Acrylamide mainly interacted with the thiol (-SH) residue of CK-BB and resulted in alkylation. A computational docking simulation supported that acrylamide directly bound to the active site of CK-BB where cysteine and glycine residues interacted mainly. The inhibition kinetics combined with computational prediction can be useful in order to have insights into the mechanisms regarding environmentally hazardous factors at the molecular level.