Discovery and Characterization of Enzymes for Degradation of Xyloglucan and Extensin

The aim of this PhD study is based on the concept ‘lean green food’ where application of enzymes replace chemicals in the biomass refinery process to recover labile value-added components and produce biofuels. To achieve this task valuable products like pectin should be isolated from the biomass before the residual polymers are used in the bioethanol production. Therefore, mono-component, substrate-specific enzymes that could selectively degrade or modify plant cell wall components are required. In this PhD study, three enzymes, including two xyloglucan-specific endoglucanases and one metalloprotease, have been discovered and characterized. The xyloglucanase from Xanthomonas citri (XcXGHA) was identified by comparing the phylogenetic relationship with all GH74 enzymes from the CAZy database. XcXGHA had a unique cleavage pattern that it accommodated xylosyl substituted Glcp at subsite -1 and cleaving the xyloglucan in endo-mode. Through sequence alignment with other GH74 xyloglucanases, two residues (D264 and R472) were predicted to contribute to the subsite specificity of XcXGHA. The xyloglucanase from Penicillium marinum (PmXEG12) was discovered by screening a collection of fungi strains for the highest xyloglucanase activity. PmXEG12 was very efficient with a relatively high kcat/KM value. Meanwhile, it had a much lower pH optimum (pH 4.0) than that of other xyloglucanases. PmXEG12 accommodated unbranched Glcp at subsite -1 and also cleaving the xyloglucan in endo-mode. The third enzyme (Prt1) is an extracellular zinc-binding metalloprotease from Protobacterium carotovora identified previously. Prt1 has a pro-peptide functioning as signal peptide for secretion and a pre-peptide functioning as chaperon peptide, which were sequentially removed by the enzyme itself automatically. It has been demonstrated that Prt1 could cleave proline and hydroxyproline-rich proteins, like casein, collagen, lectin and extensins.