Antagonistic enzymes may generate alternate phase transitions leading to ephemeral gels

In some biological processes, two enzymes with antagonistic activities-the one creating a bond, the other destroying it-are involved in a reaction cycle. Several catalysts have the ability to modify the rheological properties of biological media participating in the production of a solid gel phase which later dissolves. Transglutaminase, catalyzing intermolecular protein cross- linking, is considered here as a reverse protease as far as the physical state of a proteic gel is concerned. A kinetic model including diffusion constraints and based on a protease/transglutaminase cycle interconverting insoluble gel and soluble proteolysis fragments showed that alternate sol/gel and gel/sol transitions could occur within such a system, generating transient gel phases. Then, ephemeral gels were obtained in vitro using an experimental system consisting of gelatin, transglutaminase, and thermolysin. Modulating the enzyme activity ratio allows us to "program'' the global behavior: polymerization/solubilization cycle of a mixture containing at least one protein and two enzymes without any change in temperature or medium composition.