CellulosomePlus builds around two fundamental concepts: enzymes as extremely efficient nanocatalysts and protein scaffolds as spatio-temporal regulators of enzyme activity, both available in nature. It is also based on a bio-inspired engineering approach that integrates two different scientific and technological pillars: experimental and theoretical approaches for the redesign of natural enzyme systems specifically tailored for degradation of recalcitrant biomass residues for subsequent production of value-added chemicals like biofuels.
Plant cell wall polysaccharides as biomass source for biofuels: European law (Directive 2009/28/EC art. 3.1) imposes that by the year 2020, 20% of the fuel consumption in each of the member states should be obtained from renewable sources (including biofuels, which are potential major contributors). Increasing bioethanol production using standard technology would imply a massive investment and would have a strong impact on both food resources and environment. Currently, all the bioethanol produced in EU and in the whole world is obtained from plant storage polysaccharides. However, these are a major food source and its potential stored energy represents less than 10% of that stored in cellulose (a plant structural polysaccharide). Thus, the potential to produce biofuels (of so-called second generation) from plant lignocellulosic biomass is enormous.
Hence, transformation of lignocellulosic biomass to fermentable sugars represents a viable alternative to produce renewable biofuels. Nevertheless, hydrolysis of structural polysaccharides remains the key bottleneck in converting biomass into biofuels and thus, the development of more efficient enzyme systems is needed.
Enzymes as nanocatalysts: Classical catalysis has been accused of relying mainly on trial-and-error methods. The nascent field of nanocatalysis aims to control chemical reactions by changing the size, shape, chemical composition and morphology of the catalyst, for the purpose of driving the kinetics towards the desired reaction product. This approach opens up new avenues for atom-by-atom design of nanocatalysts with distinct and tunable chemical activity, specificity and selectivity. However, this discipline is highly focused on inorganic catalysts (typically expensive and pollutant rare-earths or precious metals) still with an emphasis on trial-and-error iterations.
By contrast, enzymes (a specific type of proteins) are nanometer-sized bioorganic catalysts refined by millions of years of evolution (i.e. a massive number of trial-and-error iterations inherent in natural selection), such that some of them are considered “catalytically perfect” (i.e. their specificity constant, kcat/KM, approaches the limit of diffusion: 108-109 M-1s-1). Thus, enzymes are ready-to-go nanomachines that provide efficient nanocatalysis and serve as the ideal starting point from which to set out on rational design for industrial applications.
Advantages of bio-inspired nanocatalysts: There are many advantages in using enzymes as nanocatalysts in industry. First, they are ideal catalysts with extremely high activity, selectivity and specificity. Second, the control of the particle size is extremely precise and reproducible, resulting in a very homogeneous population of nanocatalysts. Achieving homogeneity in particle size is one of the challenges in nanocatalysis. Third, they can be produced and re-engineered very precisely, easily and cheaply by established biotechnological processes (including site-directed mutagenesis, protein engineering and directed evolution) to make them more suitable for industrial applications. Fourth, they can be inactivated (by denaturation, cross-linking or hydrolysis) and are biodegradable, which further reduces their already minor environmental impact and makes them extremely safe.
The cellulosome as an efficient nanocatalyst: Life is based on the nanoworld, mainly the world of protein molecules (made from information encoded in DNA) that recognize and bind other molecules. Many complex biological processes are catalyzed by self-assembled cascades of multiple enzymes in a coordinated manner. To this end, organisms have developed protein scaffolds that anchor the corresponding enzymes in solid-phase so that their activities can be spatio-temporally coordinated. Not surprisingly, these scaffolding proteins are increasingly attracting the attention from scientists. In particular, “scaffoldin” coordinates a variety of polysaccharide-degrading enzymes into a highly polymorphic complex called the cellulosome. This multi-enzyme complex has been shown to be much more efficient in degrading plant cell wall derived polysaccharides than the sum of the component enzymes alone.
A natural cellulosome design. One of the many architectures of the cellulosome, a self-assembled enzymatic complex evolved for efficient lignocellulose degradation. CBM: carbohydrate-binding module.