Synthetic protein scaffolds for the colocalisation of co-acting enzymes

Nature relies on complexes of colocated enzymes to efficiently perform multiple catalytic steps. Such enzyme colocalisation promotes substrate channelling, enhances the activity of multiple synergistically acting enzymes and avoids the loss of potentially toxic intermediates. The industrial biotechnology field develops sophisticated methods to mimic natural colocalisation mechanisms to produce increasingly complex bio-based chemicals. Synthetic protein scaffolds are an advanced way to achieve colocalisation of multiple enzymes in one protein complex. The backbone scaffold is composed of multiple domains that are either separated by linkers or fused to self-assembling proteins. Enzymes are recruited to this scaffold by fusing them to domains that bind to orthogonal domains in the scaffold. A particular feature of synthetic protein scaffolds is the control over spatial organisation and enzyme stoichiometry. Several successful examples of synthetic protein scaffolds have been reported, yet the optimisation of such multi-enzyme complexes is a multiparametric, and therefore often empirical process. This review focusses on pioneering scaffolding examples and elaborates on each parameter influencing the activity of these multi-enzyme complexes. Advances in this field are expected to result in a growing catalogue of chemicals that can be produced starting from cheap and widely available renewable materials.