A Spatially Propagating Biochemical Reaction**

Unlike reactions performed in the laboratory—which use homogeneous solutions having spatially uniform concentration profiles of each reactant—reactions that occur in biological systems often involve reactants that are present in concentration gradients and can display unexpected kinetic properties. Such biological reactions can generate spatiotemporal structures that derive from the inclusion of feedback, autocatalysis, and other non-linear influences on rate. While these examples are prevalent in biology, it is still challenging to engineer such systems in the laboratory. Here, we demonstrate a reaction wherein a soluble kinase enzyme phosphorylates a peptide that is immobilized to a self-assembled monolayer (SAM) and that proceeds with a spatially organized propagation of the product. This spatial control over the reaction derives from an autocatalytic feature that operates at the boundary between the substrate and product. Our system is based on Abelson tyrosine kinase (Abl) which has both a catalytic domain and a Src homology 2 (SH2) domain that binds to the phosphopeptide product of the phosphorylation reaction. For example, Abl phosphorylates the substrate peptide AIYENPFARKC to give AIpYENPFARKC (which we abbreviate as Y and pY, respectively), and the kinase can then bind to pY. Previously, we used SAMs of alkanetiolates on gold to demonstrate that Abl can operate autocatalytically on an immobilized substrate (Figure 1a). The SAMs presented a peptide substrate for Abl kinase against a background of tri(ethylene glycol) groups, which render the monolayers inert to protein adsorption, and the peptides could be characterized with matrix-assisted laser desorption–ionization mass spectrometry (i.e., the SAMDI method). The autocatalytic kinetic property arises due to the binding of the phosphopeptide product to the SH2 domain of the kinase, which then recruits the kinase to the surface, giving an approximately 30-fold increase in the rate for phosphorylation of nearby peptides. Hence, the phosphorylation reaction is fastest in those regions of the monolayer that are directly adjacent to the regions that present product. Here, we explicitly characterize the spatial propagation of the reaction product and we also show that patterned monolayers display initial rates that depend on the