Antarctic marine ciliates under stress: superoxide dismutases from the psychrophilic Euplotes focardii are cold-active yet heat tolerant enzymes

Abstract

Oxidative stress is a particularly severe threat to Antarctic marine polar organisms because they are exposed to high dissolved oxygen and to intense UV radiation. This paper reports the features of three superoxide dismutases from the Antarctic psychrophilic ciliate Euplotes focardii that faces two environmental challenges, oxidative stress and low temperature. Two out of these are Cu,Zn superoxide dismutases (named Ef-SOD1a and Ef-SOD1b) and one belongs to the Mn-containing group (Ef-SOD2). Ef-SOD1s and Ef-SOD2 differ in their evolutionary history, expression and overall structural features. Ef-SOD1 genes are expressed at different levels, with Ef-SOD1b mRNA 20-fold higher at the ciliate optimal temperature of growth (4 °C). All Ef-SOD enzymes are active at 4 °C, consistent with the definition of cold-adapted enzymes. At the same time, they display temperatures of melting in the range 50–70 °C and retain residual activity after incubation at 65–75 °C. Supported by data of molecular dynamics simulation, we conclude that the E. focardii SODs combine cold activity, local molecular flexibility and thermo tolerance.

Effects of the substituted amino acid residues on the thermal properties of monomeric isocitrate dehydrogenases from a psychrophilic bacterium, Psychromonas marina, and a mesophilic bacterium, Azotobacter vinelandii

“…As reported previously (Watanabe et al 2005), the mesophilic AvIDH showed high specific activities at low temperatures such as 10˚C rather than the coldadapted PmIDH, implying that the thermolability of the enzyme proteins is not necessary for high catalytic activity at low temperatures. On the other hand, in cold-active and thermostable superoxide dismutases from psychrophilic Pseudoalteromonas haloplanktis and Euplotes focardii, their cold activities were suggested to be achieved by the increased local flexibility of their active sites (Merllino et al 2010;Pischedda et al 2018). This mechanism might be explaining the activity of AvIDH at low temperatures.…”

Identification and Molecular Characterization of Superoxide Dismutases Isolated From A Scuticociliate Parasite: Physiological Role in Oxidative Stress

“…In the marine ciliate E . focardii and the amoeba Acanthamoeba castellani , the enzymes Cu/ZnSODs are homodimeric 26,58 . The low homology between the AA sequences of the Cu/Zn-SOD in P .…”

“…The Fe- and Mn-SOD enzymes are the oldest SOD group and have probably evolved from orthologous genes 20 . Fe-SOD enzymes are found in prokaryotes and in chloroplasts, while Mn-SODs occur both in prokaryotes and in the mitochondrial matrix of eukaryotes 26 . It has been suggested that iron may have been the first metal used as a cofactor associated with the active site of the first SOD due to its abundance at that time in the form of soluble Fe 2+ 65 .…”

“…In protozoa, the Mn-SOD isoenzyme has been detected in Euglena gracilis 66 and in the ciliates Euplotes focardii and E . crassus 25,26 . Fe-SOD was originally considered a bacterial cytosolic enzyme; however, it has also been detected in archaea, in plant chloroplasts, as well as in the cytosol, glycosomes and mitochondria of protists 67 .…”

“…Many eukaryotes, including several microaerobic or microaerophilic protists, have an extracellular SOD (EC-SOD or SOD3) 21,22 . More specifically, in ciliates such as Euplotes , Tetrahymena and Strombidium , several types of SODs in the Cu/Zn-SOD family have been identified and characterized, and the presence of Mn-SODs and Fe/Mn-SODs has been demonstrated 23–26 .…”