Autotaxin is a secreted glycoprotein and the only member of the ectonucleotide pyrophosphatase/phosphodiesterase family that converts lysophosphatidylcholine (LPC) into lysophosphatidic acid (LPA). LPA controls key responses such as cell migration, proliferation, and survival, implicating ATX-LPA signalling in various (patho)physiological processes and establishing it as a drug target. ATX structural and functional studies have revealed an orthosteric and an allosteric site, called the "pocket" and the "tunnel," respectively. However, the mechanisms in allosteric modulation of ATX's activity as a lysophospholipase D are unclear. Here, using the physiological LPC substrate, a new fluorescent substrate, and diverse ATX inhibitors, we revisited the kinetics and allosteric regulation of the ATX catalytic cycle, dissecting the different steps and pathways leading to LPC hydrolysis. We found that ATX activity is stimulated by LPA and that LPA activates ATX lysophospholipase D activity by binding to the ATX tunnel. A consolidation of all experimental kinetics data yielded a comprehensive catalytic model supported by molecular modelling simulations, and suggested a positive feedback mechanism that is regulated by the abundance of the LPA products activating hydrolysis of different LPC species. Our results complement and extend the current understanding of ATX hydrolysis in light of the allosteric regulation by ATX-produced LPA species, and have implications for the design and application of both orthosteric and allosteric ATX inhibitors.Autotaxin (ATX or ENPP2) is a secreted glycoprotein and a unique member of the ectonucleotide pyrophosphatase / phosphordiesterase (ENPP) family (1). It is the only ENPP family member with lysophospholipase D (lysoPLD) activity (EC 3.1.4.39), and it is the main enzyme responsible for the hydrolysis of lysophosphatidylcholine (2-acyl-sn-glycero-3-phosphocholine or LPC) to produce the bioactive lipid lysophosphatidic acid (monoacyl-snglycerol-3-phosphate or LPA) (2-4). LPA acts as a ligand for several LPA receptors (LPARs) showing overlapping activities. The ATX-LPA signalling axis is vital for embryonic development and has been implicated in many (patho)physiological processes, which include vascular development (5), cancer metastasis (6), and other human diseases, such as fibrosis (7) and cholestatic pruritus (8). ATX is translated as a preproenzyme that is secreted to plasma upon its proteolytic processing, resulting in its native structural domains (9, 10). Close to the Nterminus, ATX presents two somatomedin B (SMB)-like domains, which are followed by the central catalytic phosphodiesterase (PDE) domain, and an inactive nuclease-like domain. Catalysis occurs in a bimetallic active site presenting two Zn 2+ atoms, and resembles that of other members of the alkaline phosphatase family (11). The catalytic site of ATX is organized in a tripartite binding site (Fig.1), where the active site is followed by a shallow hydrophilic groove that (11)(12)(13), and a tunnel, also called ...