Conversion of glycerol into lactic acid using glycerol dehydrogenase : evaluation of the feasability of the first step of a cascade reaction with an enzyme

A worldwide awareness of the issues resulting from the bad management of natural resources provided by our planet has been emerging in the last few years. To partially solve those environmental issues, one of the approaches is to develop newer and greener processes of fabrication and combustion, on top of imagining innovative courses of actions to manage all the waste rejected by the industries. One of many initiatives walking that path is the replacement of oil by biodiesel, which seems to be the future green alternative. Indeed, it is made from renewable biomass which only needs water, CO2 and sun to grow. Currently, the reaction used to make biodiesel at an industrial scale produces up to 10% of a byproduct, the glycerol. The latter is a molecule processed already in various applications. However, due to the development of biodiesel industries, the glycerol market has become saturated with crude glycerol sold at a ridiculously low price. As this molecule stays currently unexploited, a lot of studies have been conducted about the transformation of crude glycerol into another valuable product. One reaction particularly seems to be of great interest: the production of lactic acid. Lactic acid is a molecule which has the privilege to be extremely studied, especially since the discovery of polylactic acids, polymers used in special medical material or in 3D printing. However, up to now, the formation of lactic acid is mainly achieved through bacteria production. On the other side, new inorganic catalysts are developed to free oneself of the biological variation due to bacteria. The issue for these catalysts is that they are either often toxic for the environment, or they are made of expensive and rare compounds like noble metals. To overcome these problems, an innovative process has been designed to attempt producing lactic acid from glycerol. This new method will be based on a two-step cascade reaction, meaning that both reactions will immediately follow each other in a one pot reaction. The unconventional concept takes place during the first step. First, an enzyme, the glycerol dehydrogenase (GlyDh), will catalyze the reaction. Second, an inorganic catalyst will achieve the last step of the reaction and form lactic acid. To study the feasibility of this reaction, the GlyDh is studied in depth. Different parameters such as the optimal temperature and pH are investigated. Other characteristics like the inhibitions and the Michaelis-Menten constant, as well as the reverse reaction are also explored. Enzymes activity depends highly on their environmental conditions and are incidentally easily deactivated if the conditions are no more optimized. To stabilize the enzymes and give them a higher resistance to the exterior fluctuations, crosslinked enzyme aggregates (CLEAS) are formed. Different parameters are explored in order to discover a protocol able to form CLEAS from GlyDh. The solvents, their ratio and the crosslinker are examined to determine their optimal concentration. Centrifugation techniques and time reactions are also investigated. Finally, it is concluded that CLEAS can be achieved. However, further investigations are still needed.