The endothelial receptor tyrosine kinase Tie2 plays an important role in vascular formation and maintenance. Mutations in Tie2 lead to vascular malformations, which are painful vascular lesions that cause disfigurement, bleeding, and thrombosis. R849W Tie2 is the most common mutation implicated in an inherited form of vascular malformations and has been shown to be activating, though little is known about the kinetic features of catalysis. Here we undertake a steady-state kinetic analysis of heterologously expressed and purified wild type (WT) and R849W Tie2. While the catalytic efficiencies of the two forms are not significantly different, the observed maximal rate of phosphorylation, k(cat,obs), is > 3-fold higher for R849W Tie2 compared to WT. Notably, steady-state catalysis by R849W Tie2 has more striking sigmoidal features compared to WT, suggesting enhanced positive cooperativity. We propose that activating catalytic features are one important consequence of the R849W mutation, though likely other factors such as increased protein binding affinity also contribute to the phenotypes observed in patients. (C) 2019 Elsevier Inc. All rights reserved.