Loop engineering reveals the importance of active-site-decorating loops and gating residue in substrate affinity modulation of arginine deiminase (an anti-tumor enzyme)
Highlights: • Loop engineering on the active-site-decorating loops yielded several PpADI variants with low S{sub 0.5} values. • Several amino acid positions in active-site-decorating loops essential for the substrate affinity of PpADI were identified. • High loop flexibility (especially the gating residue Arg400) is beneficial for the substrate affinity of PpADI. Protein engineering of enzyme loop regions is an effective strategy to improve enzymatic properties. Previous studies that aimed to boost the activity of PpADI (an arginine deiminase from Pseudomonas plecoglossicida) under physiological conditions yielded several significantly improved variants that harbor substitutions predominantly located in active-site-decorating loops. A multi-site saturation mutagenesis at four positions in loop 1 (37, 38, 42, and 43) and three positions in loop 4 (402, 403, and 404) was performed to elucidate the importance of these loops in modulating the substrate affinity of PpADI. The identified “best” variant (M6-L1-4) showed a decreased S{sub 0.5} (‘K{sub M}’) of 0.48 mM compared with the parent M6 (0.81 mM). Subsequently, a rational design to recombine beneficial substitutions within loops 1 and 4 yielded variant L6 with a substantially decreased S{sub 0.5} value (0.17 mM). A comprehensive simulation analysis resulted in a conclusion that high loop flexibility (especially the gating residue Arg400) is beneficial for substrate affinity due to less efficient blocking of the active site.