Towards Artificial Enzymes - Engineering a better-than-nature catalyst based on enzyme mimetics

Enzymes are excellent biological catalysts as they exhibit high selectivity and specificity. They can facilitate specific reactions to take place under "green" conditions, potentially reducing the negative impact played by the industrial chemical synthesis on the environment. The diverse range of enzyme catalysed reactions is continuously increasing, but despite their excellent catalytic activity, enzymes have reasonable efficiency only in conditions that mimic the biological world: moderate temperatures under pH neutral aqueous conditions.

Artificial enzymes have emerged as promising candidates as they show improved stability over the natural ones under a wider variety of conditions. They are based on the "minimal catalytic unit" from a known enzyme, and, despite their small size, they provide the basic advantages of natural enzymes. An alternative to the systems based on enzyme scaffolds, is other small molecules as they could provide similar engineered physio-chemical environments. Such systems are not restricted to amino acid functional group chemistry, but instead can be programmed with a more complex array of functional moieties.

Dynamic combinatorial chemistry (DCC) has been successfully used to obtain efficient catalysts, e.g. for the Diels-Alder reaction. In DCC, a mixture of compounds is generated via a reversible reaction under thermodynamic control (e.g., disulphide or imine exchange) by mixing simple building blocks, resulting in a dynamic combinatorial library (DCL). Upon exposure of a DCL to a molecular target, those library members that bind to the target are stabilised. This principle allows the generation of artificial catalysts. In this approach, the assembly arising from the DCL is formed around a transition state analogue (TSA), that is subsequently removed to generate catalytically active molecular-sized cavities.

The project aims to develop a proof-of-principle system based on a DCL (macrocycle imine exchange) that stabilises an isoalloxazine-nicotinamide binary complex - redox couple commonly found in a range of enzyme catalysed reactions. These systems catalyse hydride transfer from the nicotinamide to the flavin, and can be easily tracked by transient absorption kinetics. We subsequently aim to take this proof-of-principle system and modify the DCL optimised architecture to be more "enzyme-like". That is, tuning the hydrophobicity of the DCL components to achieve improved entropic stabilisation of the TSA. By incorporating electrostatic stabilisation will further enhance the redox potential of the isoalloxazine / nicotinamide redox couple. This will be investigated by molecular dynamics simulations and structure based calculations.

The first idea thus focuses on the generation of the initial DCL and identify an assembly architecture that forms a stable host-guest complex with the isoalloxazine-nicotinamide binary complex. The project will also involve computational simulations of this architecture to enable rationale tuning of the artificial catalyst through subsequent chemical functionalisation. We aim to demonstrate the potential of this approach for providing a novel solution to many of the challenges faced in industrial enzyme biotechnology. We also aim to suggest architectures that can be further functionalised for different types of chemistry or substrates.