Diamond Annual Review 2023/24

17 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 3 / 2 4 A carnivorous plant offers hope of a therapy for coeliac disease Coeliac disease (CoD) is an autoimmune condition triggered by the gluten found in wheat, barley and rye - three cereals common in theWestern diet. To avoid severe symptoms that reduce their life expectancy, patients must stick to a strict gluten-free diet throughout their lives. ​ “Gluten” is an umbrella term referring to the combination of glutelin and prolamin proteins that store nutrients in plants. The problem lies in the way the human digestive system deals with gluten, breaking it down into smaller peptides that can be toxic. The 33-mer peptide, for example, is a fragment of alpha gliadin, a prolamin in wheat. In people with coeliac disease, 33-mer strongly binds to a receptor in the immune system, triggering an autoimmune response.​ Researchers are investigating if it is possible to develop a so-called ‘glutenase’ that could break down toxic peptides. Previous research has identified potential candidates in insects, fungi, bacteria and germinating cereal grains. However, even the best of these are insufficiently active at low pH, or require very high doses or protective modifications.​ In this research, scientists have shown that an enzyme (neprosin) found in the digestive juices of a pitcher plant (Nepenthes x ventrata) can dismantle the toxic peptides.​ The team sent neprosin crystals to Diamond, using remote-access X-ray diffraction on beamline I04-1 to resolve the 3D structure of the enzyme. The synchrotron data revealed the overall and active-site architecture of neprosin and its catalytic mechanism, and showed it to be a glutamate peptidase, a rare class of peptidases. Moreover, the results show that neprosin is a zymogen, an inactive substance converted into an active enzyme in the low pH conditions found in the stomach. ​ While sticking to a gluten-free diet remains the only option for CoD sufferers, this work shows that a pitcher plant has provided a promising candidate for a bona fide glutenase, and some hope of an effective CoD therapy in the future.​ ​del Amo-Maestro, L. et al. DOI: 10.1038/s41467-022-32215-1 ​ New studies towards lignin valorisation Plants rely on rigid cell walls that separate each plant cell. These cell walls are composed of cellulose, pectin and lignin, making these molecules among the most abundant on earth. Lignin is a hydrophobic compound and plays a crucial role in vascular tissue, making them impermeable and allowing the transport of water in the plant efficiently. Lignin is a huge and complex molecule composed of different precursors called monolignols. The composition of lignin varies among plants.​ From an industrial perspective, lignin is well known in the paper industry because it represents a third of the mass of the paper precursor. Lignin is a coloured component that yellows in the air and needs to be removed to have white paper. Currently there is only limited use for lignin and it is burned as low value fuel in these industries.​ Researchers performed biochemical, structural, and mechanistic characterisation of two bacterial enzymes, LdpA from N. aromaticivorans and a homolog protein, Sphingobium sp. SYK-6. Both bacteria were known for their capacity to open aromatic compounds. These enzymes perform an important degradation step, allowing the breakage of lignin that leads to the production of individual components, which can be further harvested. Structural characterisation of the protein in complex with its ligand allowed them to clearly determine where the interactions take place between the protein and the substrate.​ These findings provide a platform for future enzyme engineering and microbial engineering to expand the amount of lignin that is available for conversion into high-value biochemicals.​ Kuatsjah, E. et al. DOI: 10.1073/pnas.2212246120​ Figure: Structures of pro-neprosin and neprosin Figure: Structural architecture of LdpA and substrate interactions. (A) Superposition of SpLdpA (magenta) with NaLdpA (teal). (B) Side view of the SpLdpA trimer. Two protein chains are shown as surfaces (yellow and green) and one protein chain is shown in cartoon mode (red) with bound substrate erythro-DGPD (light blue). (C) Top view of the SpLdpA trimer. (D) Pseudo-stereoscopic view of the interaction of SpLdpA with the erythro-DGPD enantiomers (αS, βR) (Left) and (αR, βS) (Right). (E) Omit electron density map for the (αS, βR)- and (αR, βS)-erythro-DGPD enantiomers bound to SpLdpA at 2.5 σ level.