Enzymes, a class of highly efficient and specific catalysts in Nature, dictate a myriad of reactions that constitute various cascades in biological systems. Self-assembly, a process prevalent in Nature, also plays important roles in biology, from maintaining the integrity of cells to performing cellular functions and inducing abnormalities that cause disease. To explore enzyme-regulated molecular self-assembly in an aqueous medium will help to understand and control those important biological processes. On the other hand, certain small organic molecules self-assemble in water to form molecular nanofibers and result in a hydrogel, which is referred to as a "supramolecular hydrogel" (and the small molecules are referred to as "supramolecular hydrogelators"). Supramolecular hydrogelators share common features, such as amphiphilicity and supramolecular interactions (pi-pi interactions, hydrogen bonding, and charge interactions among the molecules, among others) that result in nanostructures and form the three-dimensional networks as the matrices of hydrogels. In this Account, we discuss the use of enzymes to trigger and control the self-assembly of small molecules for hydrogelation, which takes place in vitro or in vivo, extra- or intracellularly. Using phosphatase, thermolysin, beta-lactamase, and phosphatase/kinase as examples, we illustrate the design and application of enzyme-catalyzed or -regulated formation of supramolecular hydrogels that offer a new strategy for detecting the activity of enzymes, screening for enzyme inhibitors, typing bacteria, drug delivery systems, and controlling the fate of cells. Since the expression and distribution of enzymes differ by the types and states of cells, tissues, and organs, using an enzymatic reaction to convert precursors into hydrogelators that self-assemble into nanofibers as the matrices of the hydrogel, one can control the delivery, function, and response of a hydrogel according to a specific biological condition or environment, thus providing an accessible route to create sophisticated materials for biomedicine. Particularly, intracellular enzymatic hydrogelation of small molecules offers a unique means for scientists to integrate molecular self-assembly with inherent enzymatic reactions inside cells for developing new biomaterials and therapeutics at the supramolecular level and improving the basic understanding of dynamic molecular self-assembly in water.

We have designed and synthesized a new hydrogelator Nap-FFGEY (1), which forms a supramolecular hydrogel. A kinase/phosphatase switch is used to control the phosphorylation and dephosphorylation of the hydrogelator and to regulate the formation of supramolecular hydrogels. Adding a kinase to the hydrogel induces a gel-sol phase transition in the presence of adenosine triphosphates (ATP) because the tyrosine residue is converted into tyrosine phosphate by the kinase to give a more hydrophilic molecule of Nap-FFGEY-P(O)(OH)(2) (2); treating the resulting solution with a phosphatase transforms 2 back to 1 and restores the hydrogel. Electron micrographs of the hydrogels indicate that 1 self-assembles into nanofibers. Subcutaneous injection of 2 in mice shows that 80.5 +/- 1.2% of 2 turns into 1 and results in the formation of the supramolecular hydrogel of 1 in vivo. This simple biomimetic approach for regulating the states of supramolecular hydrogels promises a new way to design and construct biomaterials.