How electrostatic interactions can change the kinetic behaviour of a Michaelis-Menten enzyme. Application to the hyaluronane / hyaluronidase system.

International audienceWe show how ionic strength can induce changes in the apparent kinetic behaviour of a Michaelis-Menten type enzyme. The requirements are (i) the existence of a polyelectrolyte substrate able to form electrostatic complexes with the enzyme in a given pH domain, and (ii) the noncatalytic activity of the complexed enzyme. Under these conditions, a high ionic strength prevents complex formation, while a low ionic strength favours the complex formation with the enzyme, which then loses its catalytic activity. The higher the substrate concentration, the higher the proportion of noncatalytic enzyme molecules. The result is an apparent kinetic behaviour similar to an inhibition by excess of substrate, although the enzyme is purely of the Michaelis-Menten type. Hyaluronidase (HAase) belongs to this type of enzyme because its substrate, hyaluronan (HA), is an anionic polysaccharide able to form electrostatic complexes with HAase, complexed HAase being catalytically inactive. The two types of apparent kinetic behaviour of the HA/HAase system (Michaelis-Menten type and with inhibition by excess of substrate) have been demonstrated theoretically through a physicochemical model and experimentally at pH 4. Finally, the biological interest of HAase inhibition in cancer, and the possibility that other important enzyme systems may behave similarly, are discussed