Sequential Co-immobilization of Enzymes in Metal-Organic Frameworks for Efficient Biocatalytic Conversion of Adsorbed CO2 to Formate.

Yan Li,Yongqin Lv,Liyin Wen,Tianwei Tan

doi:10.3389/fbioe.2019.00394

Abstract

The main challenges in multienzymatic cascade reactions for CO2 reduction are the low CO2 solubility in water, the adjustment of substrate channeling, and the regeneration of co-factor. In this study, metal-organic frameworks (MOFs) were prepared as adsorbents for the storage of CO2 and at the same time as solid supports for the sequential co-immobilization of multienzymes via a layer-by-layer self-assembly approach. Amine-functionalized MIL-101(Cr) was synthesized for the adsorption of CO2. Using amine-MIL-101(Cr) as the core, two HKUST-1 layers were then fabricated for the immobilization of three enzymes chosen for the reduction of CO2 to formate. Carbonic anhydrase was encapsulated in the inner HKUST-1 layer and hydrated the released CO2 to . Bicarbonate ions then migrated directly to the outer HKUST-1 shell containing formate dehydrogenase and were converted to formate. Glutamate dehydrogenase on the outer MOF layer achieved the regeneration of co-factor. Compared with free enzymes in solution using the bubbled CO2 as substrate, the immobilized enzymes using stored CO2 as substrate exhibited 13.1-times higher of formate production due to the enhanced substrate concentration. The sequential immobilization of enzymes also facilitated the channeling of substrate and eventually enabled higher catalytic efficiency with a co-factor-based formate yield of 179.8%. The immobilized enzymes showed good operational stability and reusability with a cofactor cumulative formate yield of 1077.7% after 10 cycles of reusing.

Highlights

High emissions of greenhouse gas such as CO2 into the atmosphere have caused global environmental concern
The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images in Figures 1a,b exhibited the octahedral morphology of MIL-101(Cr) nanocrystals with apparent corners and edges, which were in good agreement with literatures (Férey et al, 2005; Hwang et al, 2008)
We found that the original morphology of MIL-101(Cr) was preserved after loading of amines confirming that the amine functionalization step had little damage to the generic metal-organic frameworks (MOFs) (Figures 1c–f)

Summary

Introduction

High emissions of greenhouse gas such as CO2 into the atmosphere have caused global environmental concern. For enzymatic catalysis of CO2 reduction with NADH regeneration, three enzymes including carbonic anhydrase (CA), formate dehydrogenase (FateDH), and glutamate dehydrogenase (GDH) were immobilized in HKUST-1@amine-MIL-101(Cr) with HKUST-1 in layered structure. For the enzymatic catalysis of stored CO2 to formic acid using immobilized enzymes, the HKUST-1@amine-MIL-101(Cr) nanocomposites were dried using freeze-drying and used for the adsorption of CO2 at 298.15 K and 5 bar for 24 h.

Results

Conclusion