Effects of inherited mutations on catalytic activity and structural stability of human glucose-6-phosphate isomerase expressed in Escherichia coli

Glucose-6-phosphate isomerase (GPI), a homodimeric enzyme, catalyzes the interconversion between glucose-6-phosphate and fructose-6-phosphate. In mammals, it can also act as an autocrine motility factor, neuroleukin, and maturation factor. Deficiency of the enzymatic activity in red blood cells causes nonspherocytic hemolytic anemia in human. To gain a more complete understanding of the molecular basis for the hemolytic anemia due to the GPI-deficiency, the wild-type enzyme and sixteen genetic variants were expressed in Escherichia coli and functionally characterized. Conclusions are as follows: (1) mutations usually have negative influences on catalytic parameters, particularly kcat, as well as structure stability; (2) mutations at or close to the active site, including R273H, H389R, and S278L, cause great damage to the catalytic function, yet those at distance can still reduce the magnitude of kcat, despite lesser extents; (3) mutations decrease the enzyme tolerance to heat or SDS by mechanisms of decreasing packing efficiency (V101M, T195I, S278L, L487F, L339P, T375R, I525T), weakening network bonding (R75G, R347C, R347H, R472H, E495K), increasing water-accessible hydrophobic surface (R83W), and destabilizing the ternary structure (T195I, R347C, R347H, and I525T); (4) A300P, L339P, and E495K mutations may also negatively affect the protein folding efficiency.