Influence of operating conditions and immobilization on activity of alcohol dehydrogenase for the conversion of formaldehyde to methanol

Abstract

Carbon dioxide can be converted to methanol in a multi-enzymatic biocatalytic process performed by three enzymes: formate-, formaldehyde-, and alcohol dehydrogenase (ADH), in a cascade reaction. Efforts to increase the yield of this reaction have been unsatisfactory and to improve the performance, engineering of each enzymatic reaction is required. The present study is focused on studying the last step in the cascade reaction and improving the activity of ADH by immobilizing it in tailored siliceous mesostructured cellular foams (MCF). For this purpose MCF was synthesized and functionalized with chloromethyl, aminopropyl, mercaptopropyl or octyl groups. ADH was successfully immobilized with high enzyme loadings in all versions of the MCFs except in the MCF functionalized with aminopropyl groups. The specific activity of the enzyme increased upon immobilization in MCFs functionalized with octyl groups. In addition, the conditions previously reported to improve the cascade reactions, including raised CO₂ pressure and high concentration of NADH, were evaluated. The raised pressure was found to reduce the activity of ADH and the conversion was also dependent on the formaldehyde concentration. Thus, some of the conditions commonly applied in the cascade reaction were found to be detrimental to the activity of ADH. These findings demonstrate the need for optimizing each step of the cascade reaction separately to improve the conversion of CO₂ to methanol. This study suggests that this may be done by immobilizing each enzyme in materials that are specifically designed for each of them and by running the reaction under optimal operating conditions for each sub-reaction.