The project “Bio-based polyamides through fermentation”, undertaken by members of the Biopolymers/Biomaterials Cluster, started in early 2009 and has the goal of using biotechnological methods to produce materials as the basis for polyamides with new properties.
Tights, kite ropes and anchor bolts have one thing in common – they are produced from polyamides. The technical and quite prosaic word “polyamide” not only stands for success stories such as nylon, but also represents the material dreams of many product developers. The plastics are chemically synthesised and can be varied to such a degree that their properties can be adapted to a broad range of applications. It is therefore no surprise to learn that polyamides are not only turned into mass products such as T-shirts and tights, but are also used for the production of cogwheels, sliding bearings, casings or implants.
Although plastics producers have almost 75 years experience in the production and processing of polyamides, they are now approaching the limits of their possibilities. Progress requires polyamides with better properties, but it is unfortunately impossible to bend, draw, press or heat the long molecule chains in any order whatsoever. New developments are required in order to broaden the application spectrum of polyamides. The nylon of the future must combine material properties such as impact strength, tensile strength and resistance to heat in a completely new way. The polyamides of the future must be more malleable than today, whilst remaining mechanically robust at the same time as having the ability to withstand environmental influences and still feel good.
The secret of polyamide properties is to a large degree found in the basic components of which polyamides are composed. Polyamide developers use either amino acids or mixtures of diamines and dicarbon acids, all of which have the functional groups that are essential for polymerisation reactions, i.e. the chemical synthesis of polyamides, to take place.
In the "Bio-based polyamides through fermentation" project, the project partners, led by BASF SE, are dealing with the biological synthesis of diamines. There are many highly interesting technical variants that, up until now, can only be chemically synthesised with great difficulty. One of the most promising candidates is diaminopentane, of which the inventor of nylon, Wallace Carother, said in the mid 1930s that it had excellent properties. However, diaminopentane has so far not achieved a breakthrough as the basis for polymers due to its high price.
However, progress is leading to change. Diaminopentane has once again aroused the interest of the plastics industry since nowadays it is possible to biotechnologically produce more than 100,000 t per year of a substance that is closely related chemically to diaminopentane. This substance is lysine, an amino acid that is essential for both humans and animals. The Cluster partners are therefore focusing on two questions: 1) how can the biotechnological production of diaminopentane be realised in an economically feasible way using lysine as intermediary product, and 2) which new polyamides can be produced from diaminopentane?
For microorganisms or cells to be efficiently used for the production of compounds such as diaminopentane, two disciplines need to play an important role in this process: systems biology and metabolic engineering. Systems biology analyses metabolic pathways and creates mathematical models of material flows and metabolic rates, producing something a bit like a metabolic business card of a cell. The basis for such models is data on the genome of an organism and the mechanisms of gene regulation as well as data on the kinetics of enzymatic reactions and compound concentrations.
For example, the lysine synthesis pathway of Corynebacterium glutamicum has been analysed in great detail. The most important parameters that control the lysine metabolism are now known. Key to the lysine metabolism is the transition from glycolysis to the pentose phosphate pathway and the production of oxaloacetate and aspartate.
The natural metabolic pathways are very often not suitable for industrial production methods. There are bottlenecks, detours, side reactions and dead ends – factors that reduce the product yield. This flexibility is important for life under natural conditions, because it gives organisms the necessary scope for adapting to changing environmental conditions. In bioreactors, such flexibility is an undesired luxury; the only thing that counts here is a high production rate. High production rates can only be achieved by modifying the metabolism of a cell at specific sites and by putting the focus on a desired product. This is the focus of metabolic engineering, which develops made-to-measure metabolisms.The following two examples show the effect metabolic engineering has on the production behaviour of an organism. Corynebacterium glutamicum can increase the production of lysine by 50 percent when the pyruvate carboxylase gene is overexpressed. Pyruvate carboxylase converts pyruvate (final glycolysis product) into oxaloacetate, which in turn is a major precursor of lysine synthesis. Lysine synthesis comes to a halt when the pyruvate carboxylase gene is removed.In addition, scientists have found out that it is possible to increase lysine production by 40 percent when the second glycolysis reaction step is blocked. The scientists blocked the enzyme phosphohexose isomerase and forced the cell to use an alternative sugar metabolism pathway – the pentose phosphate pathway. This led to a reduction in side reactions and to an increase in NADP production. NADP is indispensable for the synthesis of lysine.
The increase in the lysine yield is very important, but it is only one of two major prerequisites on the road to developing new polyamides. The biotechnological process does not end with lysine, but has to be continued in order to give rise to diaminopentane. The comparison of lysine and diaminopentane shows that lysine has an extra carboxyl group. The modification of the Corynebacterium glutamicum metabolism leads to the removal of the carboxyl group in the cell, and thus brings the biotechnologists a big step closer to the fermentative production of diaminopentane.
Despite the numerous successes achieved so far, there are still a few questions that need to be solved in the “Bio-based polyamides through fermentation” project. The scientists hope to increase the diaminopentane yield by further developing already existing systems biology and metabolic engineering methods. Once this is achieved, biotechnologically produced diaminopentane will then be converted into applicable polyamides. Project partners from the plastics industry, for example Fischerwerke GmbH, but also end users such as Robert Bosch GmbH or Daimler AG, will comprehensively test the new polyamides. The project partners are thus taking into account all steps in the value creation chain – from the bio-based production of the basic substance, to the development of new raw polymers, composites and intermediary products up until the final product used in vehicles, structural elements or toys.