Peroxiredoxin-6: A Guardian of Lung Pathophysiologies.

The moonlighting protein, Prdx-6, exhibits peroxidase activity, phospholipase activity, and lysophosphatidylcholine acyltransferase (LPCAT) activity. Although it is ubiquitous in expression, its level is prominently high in the lung. Prdx-6 has been known to be an important enzyme for the maintenance of normal lung physiologies including, anti-oxidant defense, lung surfactant homeostasis, and cell signaling. Studies further unveiled that the altered activity (peroxidase or ai- PLA2) of this enzyme is linked with various lung pathologies or diseases. In the present article, we attempted to address the various pathophysiologies or disease conditions (like lung ischemia, hyperoxia, lung cancer, emphysema, and acute lung injury) wherein Prdx-6 is involved. The study implicates that Prdx-6 could be used as a common drug target for multiple lung diseases. Important future insights have also been incorporated.

The anti-oxidant enzyme, Prdx6 might have cis-acting regulatory sequence(s).

Peroxiredoxin 6 (Prdx6) is a ubiquitously expressed 1-cysteine Peroxiredoxin found throughout all phyla. In mammals, under different physiological conditions, it has evolved from a peroxidase to a multifunctional enzyme. Among the mammalian Prdx6's, human and rat Prdx6's are the most extensively studied. Our study revealed that human and rat Prdx6's exhibit differences in their peroxidase activity. These two Prdx6's have only 8% difference in their primary sequence (with 19 amino acids) with no apparent modification at any of the key conserved residues. In the present communication, we have investigated the roles of thermodynamics, structure and internal flexibility of Prdx6 to account for the difference in their peroxidase activity. We discovered that these amino acid variations have led to structural alterations in human Prdx6 so that it shows enhanced intrinsic dynamics (or flexibility) than the rat protein. We could also identify the gain of intrinsic dynamics of the catalytic site in human Prdx6 due to relocation of an important active site residue (R132) to the loop region as the most plausible reason for high catalytic activity in the human protein as compared to rat variant. Since it is the thioredoxin fold that upholds the peroxidase function, certain structural alteration in the Prdx6 structure might help to regulate the efficiency of thioredoxin folds. Our results hint that Prdx6 might have a cis-acting regulatory sequence(s).

Hyperoxidation of Peroxiredoxin 6 Induces Alteration from Dimeric to Oligomeric State.

Peroxiredoxins(Prdx), the family of non-selenium glutathione peroxidases, are important antioxidant enzymes that defend our system from the toxic reactive oxygen species (ROS). They are thiol-based peroxidases that utilize self-oxidation of their peroxidatic cysteine (Cp) group to reduce peroxides and peroxidized biomolecules. However, because of its high affinity for hydrogen peroxide this peroxidatic cysteine moiety is extremely susceptible to hyperoxidation, forming peroxidase inactive sulfinic acid (Cys-SO2H) and sulfonic acid (Cys-SO3H) derivatives. With the exception of peroxiredoxin 6 (Prdx6), hyperoxidized sulfinic forms of Prdx can be reversed to restore peroxidase activity by the ATP-dependent enzyme sulfiredoxin. Interestingly, hyperoxidized Prdx6 protein seems to have physiological significance as hyperoxidation has been reported to dramatically upregulate its calcium independent phospholipase A2 activity. Using biochemical studies and molecular dynamic (MD) simulation, we investigated the roles of thermodynamic, structural and internal flexibility of Prdx6 to comprehend the structural alteration of the protein in the oxidized state. We observed the loosening of the hydrophobic core of the enzyme in its secondary and tertiary structures. These changes do not affect the internal dynamics of the protein (as indicated by root-mean-square deviation, RMSD and root mean square fluctuation, RMSF plots). Native-PAGE and dynamic light scattering experiments revealed the formation of higher oligomers of Prdx6 under hyperoxidation. Our study demonstrates that post translational modification (like hyperoxidation) in Prdx6 can result in major alterations of its multimeric status.