We report a catalytically promiscuous enzyme able to efficiently promote the hydrolysis of six different substrate classes. Originally assigned as a phosphonate monoester hydrolase (PMH) this enzyme exhibits substantial second-order rate accelerations (ðk cat ∕K M Þ∕ k w ), ranging from 10 7 to as high as 10 19 , for the hydrolyses of phosphate mono-, di-, and triesters, phosphonate monoesters, sulfate monoesters, and sulfonate monoesters. This substrate collection encompasses a range of substrate charges between 0 and −2, transition states of a different nature, and involves attack at two different reaction centers (P and S). Intrinsic reactivities (half-lives) range from 200 days to 10 5 years under near neutrality. The substantial rate accelerations for a set of relatively difficult reactions suggest that efficient catalysis is not necessarily limited to efficient stabilization of just one transition state. The crystal structure of PMH identifies it as a member of the alkaline phosphatase superfamily. PMH encompasses four of the native activities previously observed in this superfamily and extends its repertoire by two further activities, one of which, sulfonate monoesterase, has not been observed previously for a natural enzyme. PMH is thus one of the most promiscuous hydrolases described to date. The functional links between superfamily activities can be presumed to have played a role in functional evolution by gene duplication.catalytic promiscuity | evolution | mechanism | sulfatase | superfamily E nzymes are usually seen as highly specific catalysts following the classical rule "one enzyme, one activity." This view is challenged by an increasing number of enzymes with broad substrate specificities or side activities indicating that enzymes are catalytically more flexible than originally assumed. Some of these promiscuous enzymes can turn over substrates with different structures while catalyzing the same chemical reaction involving the same transition state (substrate promiscuity). Catalytic promiscuity, by contrast, is the ability of an enzyme to catalyze chemically distinct reactions by stabilization of different transition states (TSs) (1). Catalytic efficiencies (k cat ∕K M ) for the promiscuous substrates are often substantially lower (2 to 9 orders of magnitude) than for the native conversions (1-3). The growing number of examples of this phenomenon (1-4) has engendered excitement on a number of fronts. Catalytic promiscuity provides a functional basis for enzyme evolution by gene duplication. The initial head-start activity of the duplicated gene copy could support an immediate selective advantage (1, 2, 5, 6), to be followed by improvement of the initially weak activity (7). Even low k cat ∕K M values for a promiscuous function can support a significant selective advantage (8). Mimicking this evolutionary shortcut could also provide a more efficient route to changing the function of proteins by directed evolution (5).We describe multiple and efficient catalytic promiscuity in an enzyme from Burkh...