Coexistence effect of titanate nanosheets (TNS) with nanometric lateral dimensions (ca. 3 nm), which were prepared through a hydrolysis reaction of titanium tetraisopropoxide, on catalytic activity of horseradish peroxidase (HRP) was investigated as a function of solution pH. Especially in diluted HRP solutions with a pH range of 7 ~ 8, enzymatic reaction rate, i.e., maximum velocity (Vmax) in the cconventional Michaelis-Menten equation, was significantly enhanced more than 2 times in the presence of TNS. In contrast, the zincrease in Vmax was not very large in acidic (pH = 4.0) and basic solutions (pH = 9.0). It was demonstrated that the TNS brought about peptization of aggregates composed of several HRP molecules in a diluted solution, causing increase in an apparent HRP concentration participating in the enzymatic reaction. Moreover, the coexistent TNS activated superoxide dismutase (SOD) with O2 -scavenging performance.

The present study reports the effects of binding of lipase, which is an inexpensive digestive enzyme (candida antarctica lipase) that catalyzes the hydrolysis reaction and is frequently utilized for artificial synthesis of a variety of organic molecules, to titanate nanosheets (TNSs) on their biocatalytic activities and stabilities under several lipase concentrations. TNSs were prepared through a hydrolysis reaction of titanium tetraisopropoxide (TTIP) with tetrabutylammonium hydroxide (TBAOH), resulting in formation of a colorless and transparent colloidal solution including TNSs with nanometric dimensions (hydrodynamic diameter: ca. 5.6 nm). TNSs were bound to lipase molecules through electrostatic interaction in an aqueous phase at an appropriate pH, forming inorganic-bio nanohybrids (lipase-TNSs). The enzymatic reaction rate for hydrolysis of p-nitrophenyl acetate (pNPA) catalyzed by the lipase-TNSs, especially in diluted lipase concentrations, was significantly improved more than 8 times as compared with free lipase. On the other hand, it was confirmed that heat tolerance of lipase was also improved by binding to TNSs. These results suggest that the novel lipase-TNSs proposed here have combined enhancements of the catalytic activity and the anti-denaturation stability of lipase.

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