Double site enzyme and squatting: where one regulatory ligand is also a substrate of the reaction.

A regulatory model termed "squatting" has already been proposed (Mazat et al., 1977). It involves competition between at least two ligands binding to two regulatory sites. In this paper we study the case where one of the ligands is a substrate or a product of the reaction, one of the sites where competition occurs being the catalytic one. Complex regulatory patterns are evidenced and some experimental examples are analysed according to this model. It is worth noting that this model takes into account competition between chemically related ligands as occurs in vivo.