Some hints to avoid pitfalls in quantitative determination of glutathione peroxidase (EC 1.11.1.9)

Improving Catalytic Activity, Acid-Tolerance, and Thermal Stability of Glutathione Peroxidase by Systematic Site-Directed Selenocysteine Incorporation

Glutathione peroxidase (GPx) is an important antioxidant enzyme. Selenocysteine (Sec)-containing GPxs (Sec-GPxs) are usually superior to their conventional cysteine-containing counterparts (Cys-GPxs), which make up the majority of the natural GPxs but display unsuitable activity and stability for industrial applications. This study first heterologously expressed and characterized a Cys-GPx from Lactococcus lactis (LlGPx), systematically exchanged all the three Cys to Sec and introduced an extra Sec. The results showed that the insertion of Sec at the active site could effectively increase the enzyme activity and confer a lower optimal pH value on the mutants. The double mutant C36U/L157U increased by 2.65 times (5.12 U/mg). The thermal stability of the C81U mutant was significantly improved. These results suggest that site-directed Sec incorporation can effectively improve the enzymatic properties of LlGPx, which may be also used for the protein engineering of other industrial enzymes containing catalytic or other functional cysteine residues.

Structural and functional characterization of the glutathione peroxidase-like thioredoxin peroxidase from the fungus Trichoderma reesei

Glutathione peroxidases (GPx) are a family of enzymes with the ability to reduce organic and inorganic hydroperoxides to the corresponding alcohols using glutathione or thioredoxin as an electron donor. Here, we report the functional and structural characterization of a GPx identified in Trichoderma reesei (TrGPx). TrGPx was recombinantly expressed in a bacterial host and purified using affinity. Using a thioredoxin coupled assay, TrGPx exhibited activity of 28 U and 12.5 U in the presence of the substrates H₂O₂ and t-BOOH, respectively, and no activity was observed when glutathione was used. These results indicated that TrGPx is a thioredoxin peroxidase and hydrolyses H₂O₂ better than t-BOOH. TrGPx kinetic parameters using a pyrogallol assay resulted at K_mapp = 11.7 mM_, V_maxapp = 10.9 IU/μg TrGPx, k_cat = 19 s^-1 and a catalytic efficiency of 1.6 mM^-1 s^-1 to H₂O₂ as substrate. Besides that, TrGPx demonstrated an optimum pH ranging from 9.0-12.0 and a half-life of 36 min at 80 °C. TrGPx 3D-structure was obtained in a reduced state and non-catalytic conformation. The overall fold is similar to the other phospholipid-hydroperoxide glutathione peroxidases. These data contribute to understand the antioxidant mechanism in fungi and provide information for using antioxidant enzymes in biotechnological applications.