Amino acids are important substances for biological research and interesting building blocks for the production of chemicals and medicines. Researchers from the University of Groningen have adapted the enzyme methylaspartate ammonia-lyase by means of ‘targeted evolution’ in the laboratory. They were thus able to develop biocatalysts for the synthesis of new amino acids – a method that offers important advantages for the environment. The results were published online last weekend in the prominent scientific journal Nature Chemistry.
In the adapted form, the enzyme can be used to synthesize a wide range of new substituted aspartate molecules. These are amino acids that can be useful in research on brain diseases such as Alzheimer and Parkinson’s, research that – the researchers hope – will lead to the development of drugs to prevent these diseases. However, the food industry is also interested in aspartate derivatives that can be used as the building blocks for the production of new sweeteners (as an alternative to aspartame).
In its natural form, the methylaspartate ammonia-lyase enzyme is very selective. It can only be induced to any meaningful activity if it is offered its natural substrates ammonia and dimethyl fumarate. The reaction leads then to only one amino acid product: 3-methyl aspartate.
With other, non-natural substrates, such as substituted amines and fumaric acid molecules, the enzyme displays no activity whatsoever. And that’s a shame, because that makes it unsuitable for biotechnological applications. A number of years ago, various groups at the University of Groningen took up the gauntlet and started research that was intended to lead to expanding the substrate spectrum of this enzyme, and thus increase the number of amino acids that could be produced.
Adapting enzymes to convert non-natural substrates is never easy. The usual method is to create large numbers of mutants, which are then tested for activity for a specific substrate, a costly and time-consuming process. Luckily the Groningen researchers could make use of a previously published structure of the enzyme in the state in which it forms a complex with its natural substrate (see figure). Analysing this structure provided insight into the precise interactions between enzyme and natural substrate.
On the basis of that knowledge, the researchers were able to create more space in the active centre for larger substrate molecules. Two variants were developed that can each bind with and convert a large number of non-natural substrates.
Researcher Gerrit Poelarends (pharmaceutical biology) is particularly pleased with the new enzyme variants because they provide him and his colleagues with the opportunity to create new substances that they can use to study glutamate-dependent channel proteins and transport proteins in nerve cells. If these proteins do not function properly, this can lead to nerve cells dying off, and this is possibly related to the emergence of brain diseases such as Alzheimer’s and Parkinson’s. Substances that have an inhibitory or activating effect on these channel and transport proteins are useful in the research and could provide a basis for a preventive drug.
However, the food industry may also become involved with new biocatalysts in the future. New aspartate derivatives are important building blocks for dietary supplements. In addition, the biotechnology also has advantages for the environment because the enzymes work in water, thus making chemical solvents superfluous. Poelarends: ‘There are new challenges for us there, too. We are going to work on increasing the activity and stability of these enzymes so that they can also be applied industrially.’
The joint research that the scientists are reporting on in Nature Chemistry was conducted over several years by groups from the Groningen Research Institute of Pharmacy, the Groningen Biomolecular Sciences and Biotechnology Institute, the Center for Systems Chemistry and DSM Pharmaceutical Products. Research funding was awarded by NWO.
More information: Dr Gerrit J. Poelarends
Reference: Engineering methylaspartate ammonia lyase for the asymmetric synthesis of unnatural amino acids, Nature Chemistry, 29 April 2012. Authors: Hans Raj, Wiktor Szymański, Jandré de Villiers, Henriëtte J. Rozeboom, Vinod Puthan Veetil, Carlos R. Reis, Marianne de Villiers, Frank J. Dekker, Stefaan de Wildeman, Wim J. Quax, Andy-Mark W.H. Thunnissen, Ben L. Feringa, Dick B. Janssen and Gerrit J. PoelarendsSee also: http://dx.doi.org/10.1038/NCHEM.1338
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