Alcoholysis catalysed by Candida antarctica lipase B in a gas / solid system

KEY WORDS Lipase B from Candida antarctica – Kinetics – transesterification – competitive inhibition – Ping Pong Bi Bi mechanism ABSTRACT The kinetics of alcoholysis of methylpropionate and n-propanol catalyzed by Candida antarctica lipase B supported onto silanized Chromosorb P was studied in a continuous solid/gas reactor. In this system the solid phase is composed of a packed enzymatic sample and is percolated by nitrogen as carrier gas, which simultaneously carries substrates to the enzyme while removing reaction products. In this reactor the thermodynamic activity of substrates and effectors can be perfectly adjusted allowing kinetics studies to be performed under different operating conditions. The kinetics obtained for alcoholysis were suggested to fit a Ping Pong Bi Bi mechanism with dead-end inhibition by the alcohol. The values of all apparent kinetic parameters were calculated and the apparent dissociation constant of enzyme for gaseous ester was found very low compared with the one obtained for liquid ester in organic medium, certainly due to the more efficient diffusion in the gaseous phase. The effect of water thermodynamic activity was also investigated. Water was found to act as a competitive inhibitor, with a higher inhibition constant than n-propanol. Thus alcoholysis of gaseous methylpropionate and n-propanol catalyzed by of Candida antarctica lipase B was found to obey the same kinetics mechanism than in other non conventional media such as organic liquid media and supercritical carbon dioxide, but with much higher affinity for the substrates.