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2009/12/17

System whereby proteins 'resist' salt discovered

A team at the CIC bioGUNE Basque research centre has come up with the answer to one of the enigmas circulating in the scientific community to date – the curious capacity of certain proteins to adapt to hostile and extreme environments, in concrete those of high salinity. In other words, this research team has discovered the fundamental basis for explaining how and why these proteins manage to survive and adapt to salty environments, salt lakes, etc. This research work earned front coverage in the December issue of PLoS Biology.

Life on our planet has an enormous capacity for adaptation to the environment and living beings inhabit even the most inhospitable locations. Halophilic archeas are a group of unicellular organisms that live in salt mines and lakes (where the concentration of salt would cause cells to burst). To avoid this osmotic rupture, these archeas balance the concentration of salt in the interior of the cell with that of the surroundings.

Due to the high salinity of the cellular cytoplasm, the proteins making up these organisms have adapted to this and remain folded and functional in these conditions of high ionic force. As a result of evolution, the composition of the amino-acids of the proteins in these organisms is characteristic: there is a great abundance of residues with negative charge and a low frequency of lysines. However, the mechanism by which these amino-acids cause resistance of the protein to salt is unknown.

In order to establish the structural and thermodynamic bases and the mechanism of adaptation to different environments of high salinity, researchers used high-resolution techniques at the CIC bioGUNE’s Structural Biology Unit - such as nuclear magnetic resonance (NMR) and circular dichroism - with a series of proteins (a halophile, its mesophile homologue and another, unrelated mesophile).

In this way, an understanding of the relation between the composition of the amino-acids and adaptation to salt was achieved. In environments of high salinity the concentration of water is reduced and these amino-acids accumulate on the surface, thus minimising interactions with the water.

The research team expressed great satisfaction on evaluating the scientific import of this discovery. They realised they had found the key to a difficult question which had remained unanswered for 15-20 years.

This resolved enigma had been the target of a quest for some years by many research teams in the USA and Israel, in the latter country because of its proximity to the Dead Sea, given that the immense majority of these organisms are extracted from salt lakes.

CIC bioGUNE started the basic research work at their laboratory four years ago – investigating the effect on stability with various salts, sodium chloride, potassium chloride, etc. With these initial studies of the various mutants that they had obtained from a protein and having consulted the current literature, they realised that there was this unresolved problem. They proposed a hypothesis and thus the project began.

Using genomic data they identified this 'characteristic' composition of amino-acids but did not know how it functioned. The proteins of the halophilic organisms are the most acidic of all proteins that exist, i.e. they have a very particular composition. This gave rise to a small enigma.

The key was to understand that what is involved is the interaction with the solvent, i.e. there is a reduction in the interaction with the aqueous medium. Water is a solvent, it dissolves. If you have a protein in an aqueous medium, the water dissolves the protein. But if salt is present, the water has to divide its function: it has to dissolve the salt and the protein. This results in the protein having to lose contacts - this particular composition of amino-acids enables losing contacts without greatly prejudicing the conformation and stability of the protein. This was the quid of the question.

The enzymatic engineering of enzymes

The principal application that this scientific finding could have is in the enzymatic engineering of enzymes, because these bio-reactors operate under conditions of water scarcity – similar to saline environments.

In effect, their use would be viable in the field of biotechnology, i.e. the industrial use of biological substances that enable compliance with ‘green chemistry’ - the reduction of toxic emissions - and which could even reduce greenhouse gas emissions to the atmosphere in as much as the techniques employed contribute to reducing the energy necessary to carry out industrial processes.

The use of enzymes instead of chemical catalysts can greatly reduce the conditions of temperature and pressure in industrial reactors, which means that the waste generated can be reduced. Enzymes are biological catalysts, cutting down on the energy required for reactions and the products to be obtained.

The application of enzymes in biotechnology is limited due to the durability of enzymes – normally being transitory substances which, after a brief period of time, lose their activity. One of the reasons for this is that they are in an environment that is not very hydrated. If some of the modifications equivalent to those of the halophilic enzymes can be introduced, perhaps they would not lose their activity so much – given that the modifications are on the surface where the active centre is not located. Thus, the properties of adaptability to this environment can be enhanced.

There would also be a second application, in the future - the biotechnology of plants to adapt to environments affected by water shortages and droughts, etc. If a plant genome can be converted to a genome of a halophilically-adapted plant, the plant, in principle, would have less water requirements or could be watered with salty water. At present the plant does not have this ability to modify an entire genome but, if and when the possibility eventually arises, it could be embarked on. The technology to be able to apply this to plants does not exist as yet.