bImproving enzyme thermostability is of importance for widening the spectrum of application of enzymes. In this study, a structure-based rational design approach was used to improve the thermostability of a highly active, wide-pH-range-adaptable, and stable endopolygalacturonase (PG8fn) from Achaetomium sp. strain Xz8 via the optimization of charge-charge interactions. By using the enzyme thermal stability system (ETSS), two residues-D244 and D299 -were inferred to be crucial contributors to thermostability. Single (D244A and D299R) and double (D244A/D299R) mutants were then generated and compared with the wild type. All mutants showed improved thermal properties, in the order D244A < D299R < D244A/D299R. In comparison with PG8fn, D244A/D299R showed the most pronounced shifts in temperature of maximum enzymatic activity (T max ), temperature at which 50% of the maximal activity of an enzyme is retained (T 50 ), and melting temperature (T m ), of about 10, 17, and 10.2°C upward, respectively, with the half-life (t 1/2 ) extended by 8.4 h at 50°C and 45 min at 55°C. Another distinguishing characteristic of the D244A/D299R mutant was its catalytic activity, which was comparable to that of the wild type (23,000 ؎ 130 U/mg versus 28,000 ؎ 293 U/mg); on the other hand, it showed more residual activity (8,400 ؎ 83 U/mg versus 1,400 ؎ 57 U/mg) after the feed pelleting process (80°C and 30 min). Molecular dynamics (MD) simulation studies indicated that mutations at sites D244 and D299 lowered the overall root mean square deviation (RMSD) and consequently increased the protein rigidity. This study reveals the importance of charge-charge interactions in protein conformation and provides a viable strategy for enhancing protein stability. P ectin, the third most abundant polysaccharide of the plant cell wall, is the most structurally complex polysaccharide consisting of covalently linked galacturonic acid (1, 2). Pectinases-microbial enzymes which are capable of depolymerizing pectinplay an important role in wide biotechnological applications, including the fruit juice, vinification, paper, and textile industries, as well as the extraction of oils (3). Polygalacturonase (PG) is the most widely studied pectinase due to its high activity, mesophilic and acidic properties, and satisfactory performance in juice clarification, extraction, viscosity reduction, and yield improvement (4). Based on sequence similarity and action mode (5), PGs are classified into family 28 of glycoside hydrolase (GH) and endoand exo-types. Most endo-PGs are highly active and stable at 30 to 50°C (6; http://www.brenda-enzymes.org/), which are far from the requirements of thermophilic processing in the feed industry (7).To obtain a thermostable, highly active PG, either mining new genetic resources of thermophiles (8) or engineering the protein and application environment (9, 10) is the most popular practice. Thermophilic fungi Thielavia arenaria XZ7 (60°C) (7), Thermoascus aurantiacus CBMAI-756 (60 to 65°C) (11), Penicillium sp. strain SPC-F 20 (60°C) ...