Biological Catalysis Regulated by Cucurbit[7]uril Molecular Containers

We report the synthesis of two-faced inhibitors 1-5 that contain both enzyme inhibitor and cucurbit[n]uril binding domains. The enzyme binding domains of 1-5 bind to the active sites of bovine carbonic anhydrase (BCA) or acetylcholinesterase (AChE) and inhibit their catalytic activities. Addition of CB[7] to BCA center dot 1 and BCA center dot 2 results in the transient formation of the BCA center dot 1 center dot CB[7] and BCA center dot 2 center dot CB[7] ternary complexes that undergo rapid dissociation to form free catalytically active BCA along with CB[7]center dot 1 and CB[7]center dot 2. The on-off cycle can be performed repetitively by the sequential addition of competitive guest 8 and CB[7]. The detailed origins of this on-off switching of the catalytic activity of BCA is delineated by the combined inference of UV/vis catalytic assays, fluorescence displacement assays, H-1 NMR, along with measurement of the fundamental values of K-a, k(on), and k(off) for the various complexes involved. In contrast, addition of CB[7] to AChE center dot 4(4) and AChE center dot 5(4) results in the formation of thermodynamically stable ternary complexes AChE center dot 4(4)center dot CB[7](4) and AChE center dot 5(4)center dot CB[7](4) that are catalytically inactive. We highlight some of the advantages and disadvantages of the strategy, based on the direct competition between two receptors (e.g., enzyme and CB[7]) for a common inhibitor, used in this paper to control enzyme catalytic activity compared to the strategy employed by Nature involving the binding of an allosteric small molecule remote from the enzyme active site.