Intercalation of the disulfide bond between the A2 and A4 loop of cellobiohydrolase (Cel7A) of Aspergillus fumigatus enhances catalytic activity and thermostability.

Disulfide bonds maintain the proper structural conformation and stability of the protein. Introduction of new disulfide bonds or disulfide bonds engineering is a promising strategy of rational protein designing that has been applied to improve the stability of various proteins. In this work, we have applied this approach to an industrially important enzyme Glycoside Hydrolase family GH 7 cellobiohydrolase (GH7 CBHs) or Cel7A of our locally isolated strain of thermophilic fungus Aspergillus fumigatus ( Af Cel7A). Disulfide by Design 2.0 (DbD2), a web-based tool for mutation site detection in proteins, used and created four mutations (T416C-I432C, G460C-G465C, D276C-G279C, and D322C-G327C) in the peripheral loops but outside of the catalytic region. The disulfide bond (T416C-I432C) formed between the A2 and A4 loop showed higher thermostability (70% activity at 70 0 C), higher substrate affinity (K m = 0.081mM) and higher catalytic activity (Kcat =9.75 min -1 ; Kcat/Km = 120.37 mM min -1 ) than wild type Af Cel7A (50% activity at 70 0 C; K m = 0.128mM; Kcat = 4.833 min -1 ; Kcat/Km = 37.75 mM min -1 ). Whereas the other three mutants with high B factor showed the loss of thermostability and loss of catalytic activity compared to the wild type. This is the first report of the gain of function of both thermostability and enzyme activity of cellobiohydrolase Cel7A by disulfide bond engineering. Further, comparative molecular dynamics simulations revealed that the variant T416C-I432C is comparatively less flexible (RMSD) than both wild type and other variants at 300K, 325K. However, increased flexibility at tunnel entrance (RMSF at 300K and 325K) may responsible for the gain of catalytic activity and the formation of more hydrogen bonds while binding with cellobiose may responsible for the increase of substrate affinity.