Discovery and characterization of a novel PKD-Fn3 domains containing GH44 endoglucanase from a Tibetan metagenomic library

AIMS

To explore novel microbial endoglucanases with unique properties derived from extreme environments by using metagenomics approach.

METHODS AND RESULTS

A Tibetan soil metagenomic library was applied for screening cellulase-active clones by function-based metagenomics. The candidate genes in the active clones were identified through bioinformatic analyses and heterologously expressed using an Escherichia coli system. The recombinant endoglucanases were purified and characterized using enzyme assays to determine their bioactivities, stabilities, substrate specificities, and other enzymatic properties. A novel endoglucanase gene Zfeg1907 was identified, which consisted of a glycoside hydrolase family 44 (GH44) catalytic domain along with a polycystic kidney disease (PKD) domain and a fibronectin type Ⅲ (Fn3) domain at the C terminal. Recombinant enzyme ZFEG1907 and its truncated mutant ZFEG1907t (ΔPKDΔFn3) were successfully expressed and purified. The two recombinants exhibited catalytic activities toward carboxymethyl cellulose, konjac glucomannan (KGM), and lichenan. Both enzymes had an optimal temperature of 50°C and an optimal pH value of 5.0. The catalytic activities of both recombinant enzymes were promoted by adding Zn2+ and Ca2+ at the final concentration of 10 mM. The Km value of ZFEG1907 was lower, while the kcat/Km value of ZFEG1907 was higher than those of of ZFEG1907t when using carboxymethyl cellulose, KGM, and lichenan as substrates. Structure prediction of two recombinants revealed that PKD-Fn3 domains consisted of a flexible linker and formed a β-sandwich structure.

CONCLUSIONS

A novel endoglucanase ZFEG1907 contained a GH44 catalytic domain and a PKD-Fn3 domain was characterized. The PKD-Fn3 domains were not indispensable for the activity but contributed to the enzyme binding of the polysaccharide substrates as a carbohydrate-binding module (CBM).

Sulfur metabolism in Rhodococcus species and their application in desulfurization of fossil fuels

Organosulfur compounds in fossil fuels have been a major concern in the process of achieving zero-sulfur fuel production. Biodesulfurization (BDS) is an environmentally friendly strategy for the removal of refractory organosulfur compounds from fossil fuels. Even though researchers are committed to engineering the desulfurization-specific pathway for improving BDS efficiency, the industrial application of BDS is still difficult. Recently, the sulfur metabolism of Rhodococcus has begun to attract attention due to its influences on the BDS process. In this review, we introduce the sulfur metabolism in Rhodococcus, including sulfur absorption, reduction, and assimilation; and summarize desulfurization in Rhodococcus, including the desulfurization mechanism, the regulation mechanism of the 4S pathway, and the strategies of optimizing the 4S pathway to improve BDS efficiency. In particular, the influence of sulfur metabolism on BDS efficiency is discussed. In addition, we consider the latest genetic engineering strategies in Rhodococcus. An improved understanding of the relationship between sulfur metabolism and desulfurization will enable the industrial application of BDS.

Efficient Bioproduction of Indigo and Indirubin by Optimizing a Novel Terpenoid Cyclase XiaI in Escherichia coli

Blue indigo dye, an important natural colorant, is used for textiles and food additives worldwide, while another red isomer, indirubin, is the major active ingredient of a traditional Chinese medicine named "Danggui Longhui Wan" for treating various diseases including granulocytic leukemia, cancer, and Alzheimer's disease. In this work, we constructed a new and highly efficient indigoid production system by optimizing a novel terpenoid cyclase, XiaI, from the xiamycin biosynthetic pathway. Through introducing the flavin-reducing enzyme Fre, tryptophan-lysing and -importing enzymes TnaA and TnaB, and H₂O₂-degrading enzyme KatE and optimizing the fermentation parameters including temperature, the concentration of isopropyl-β-d-thiogalactopyranoside, and feeding of the l-tryptophan precursor, the final maximum productivity of indigoids by the recombinant strain Escherichia coli BL21(DE3) (XiaI-Fre-TnaAB-KatE) was apparently improved to 101.9 mg/L, an approximately 60-fold improvement to that of the starting strain E. coli BL21(DE3) (XiaI) (1.7 mg/L). In addition, when the fermentation system was enlarged to 1 L in the flask (feeding with 5 mM tryptophan and 10 mM 2-hydroxyindole), the indigoid productivity further increased to 276.7 mg/L at 48 h, including an indigo productivity of 26.0 mg/L and an indirubin productivity of 250.7 mg/L, which has been the highest productivity of indirubin so far. This work provided a basis for the commercial production of bio-indigo and the clinical drug indirubin in the future.