With a good grip on AfG3PS and its substrate scope, we continued by interrogating its alkene selectivity. The groups of Poulter and Walker 42 independently demonstrated that two relatives of AfG3PS are (E)-retentive with 1, 2 and 3i.e. (E)-3 yielded the (E)-configured ether product (see the SI for a summary of their methods). Like its relatives, AfG3PS also proved (E)-retentive with all tested substrates and we were surprised to find that it is also (Z)-retentive. For instance, AfG3PS retained the (E)/(Z)-ratio of 15:1 (E)/(Z)-2 during quantitative transformation to the ether (Fig. , left). Similarly, predominantly (Z)-configured pyrophosphates yielded equally (Z)-configured ethers, as apparent from transformation of 1:4.4 (E)/(Z)-3 (see the SI for the full product scope). In all these cases, the (E)-and (Z)-isomers can be differentiated by the characteristic chemical shifts of their allylic protons (Fig. ) and we additionally confirmed these assignments by 2D NMR. We also corroborated the retention of the total (E)/(Z)-ratio by careful analysis of the isomer distribution during the transformation of 20 (Fig. ). It is worth noting that AfG3PS is not agnostic of configuration and does prefer (E)-pyrophosphates. This is apparent, for instance, from the (E)-enriched product arising from incomplete conversion of 2.2:1 (E)/(Z)-6 (Fig. ). However, the degree of kinetic differentiation between (Z)-and (E)pyrophosphates is highly substrate-dependent. For instance, the rate difference between the (E)-and (Z)-isomers is approximately a factor of ten for 3 (as assessed with kinetic experiments by EPUB and 31 P NMR, Figs. and) and largely absent for 20 (estimated by 1 H NMR). We only observed pronounced selectivity for the minimally functionalized substrates 18 and 19 where AfG3PS furnished almost exclusively (E)-ether, likely due to low affinity for the (Z)-pyrophosphates. This pronounced degree of promiscuity -both regarding the substrate's tail and the configuration at the alkene -makes AfG3PS an intriguing general catalyst for synthetic applications. If and to what extent this (E)/(Z)selectivity can be tuned by protein engineering is currently unclear.