Esterase-like “Superactivity” of Apo-Human Serum Transferrin: Specific Role of Hydrophobic Tail Chain Length and Charge of Headgroup of Surfactants

The enhanced catalytic activity (superactivity) of iron-depleted apo-human serum transferrin (apo-hTF) in the presence of cationic surfactants with varying chain lengths has been investigated in this work. The progress of ester hydrolysis of two different esterase substrates, para-nitrophenylacetate (PNPA) and 4-methylumbelliferylacetate (4-MUA), was monitored spectroscopically. Catalytic activity of apo-hTF gets enhanced with increasing concentrations of cationic surfactants, up to the micellar concentration, followed by a gradual decrease at postmicellar concentrations. However, the catalytic performance of the protein remained silent in its native form, in the presence of anionic and neutral surfactants, guanidinium hydrochloride-denatured conformation, temperature-induced aggregated form, and liquid-liquid phase-separated (LLPS) form of the protein. This work sheds light on the importance of the location and alignment of amino acids in the catalytic hub and the approachability of the substrate at the active site in micellar catalysis systems. These results provide new insights into enzyme-substrate interactions in the domain of micellar catalysis, potentially aiding the design of surfactant-based catalytic systems.