by Luis E. S. Netto
Peroxiredoxin (Prx) enzymes are becoming more and more popular among other reasons due to their high reactivity towards hydroperoxides and to their abundance. As a consequence, Prxs are proposed as biological sensors of hydrogen peroxide. It is interesting to observe that since their beginnings (in the end of the 60’s), one feature that called attention was their ability to form high molecular weight species, visible by electron microscopy . It was almost twenty years later that the thiol-dependent peroxidase activity of Prx enzymes was described.
Among Prx family of proteins, 2-Cys Prx enzymes (those belonging to the AhpC/Prx1 group) can adopt a wide array of quartenary structures. The one most often found in crystallographic studies is a decamer, composed of five dimers arranged as a doughnut. Decamers (or even higher molecular weight species) and dimers are in equilibrium, switching back and forth. The physiological meaning of these distinct quartenary structures is yet poorly understood. Initially it was proposed that dimers display peroxidase activity, whereas the decamers (or the higher molecular weight species) would be endowed with chaperone (holdase) activity. However, recent evidences indicated that decamers and high molecular weight species are more active as peroxidases than the dimeric counterparts. Therefore, the picture is clearly very complex and the factors governing the switch between decamers and dimers are elusive. Anyway, the reduced Prx forms exist primarily as decamers and disulfide bond formation in 2-Cys Prx favors decamer dissociation into dimeric units. In contrast to such redox-dependent pathways, pH-associated mechanisms underlying switch between dimers and decamers were unknown.
A study led by Dr. Mario Murakami (Laboratório Nacional de Biociências, Centro Nacional de Pesquisa em Energia e Materiais, Campinas/SP) in collaboration with Dr. Marcos Oliveira (UNESP São Vicente) and Dr. Luis Netto (Instituto de Biociências – USP) from CEPID redoxoma described a pH-dependent modulation of decamer and dimer inter-conversion for a mitochondrial tryparedoxin peroxidase from Leishmania braziliensis (LbPrx1m) . Previously, it was described that a 2-Cys Prx from Leishmania infantum confers thermotolerance to the parasite and displays in vitro chaperone activity, which appeared to be related to virulence . Two crystallographic structures were reported in a dimeric (pH = 8.5) and decameric (pH = 4.4) states and support the proposed model. Besides crystallography, site-directed mutagenesis and biophysical studies supported a model in which the histidine residue 113 (His113) acts as a pH sensor that may trigger LbPrx1m decamerization at acidic pHs. It is well known that the pKa of the histidine side chain lies in the physiological range (imidazole group has a pKa of approximately 6.0) and, therefore, upon protonation gains a positive charge. Therefore, in acidic pHs, His113 is protonated and can interact with the negative charge of the Asp76 side chain (with a negative charge) from other dimer. Remarkably, Asp76 side chain is located at the same loop that contain the reactive cysteine (so called peroxidatic cysteine = CysP). The inter-conversion between dimer and decamer occurs in narrow pH interval (between 7.0 – 8.0) and might have physiological consequences. Noteworthy, both His113 and Asp76 are highly conserved not only in protozoa like Leishmania braziliensis, but also in bacteria, archaea, mammals and plants. Accordingly, the pH-dependent dimer to decamer conversion in a narrow interval was also described for human Prx2 and the chloroplast 2-Cys Prx from Pisum sativum. Processes such as apoptosis in mammalian cells and CO2 fixation in plants are under modulation by pH variations. Further studies are required to fully understand the biological meanings of these distinct quartenary structures as well as the mechanisms that regulate switches among them, while these subjects appear to be relevant chapters for redox biology.
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Luis Eduardo Soares Netto, PhD.
Professor at Department of Genetics and Evolutionary Biology,
Institute of Biosciences, University of São Paulo, Brazil