Jens-Christian Navarro Poulsen

Stimulation of lignocellulosic biomass hydrolysis by proteins of glycoside hydrolase family 61: structure and function of a large, enigmatic family.

Currently, the relatively high cost of enzymes such as glycoside hydrolases that catalyze cellulose hydrolysis represents a barrier to commercialization of a biorefinery capable of producing renewable transportable fuels such as ethanol from abundant lignocellulosic biomass. Among the many families of glycoside hydrolases that catalyze cellulose and hemicellulose hydrolysis, few are more enigmatic than family 61 (GH61), originally classified based on measurement of very weak endo-1,4-beta-d-glucanase activity in one family member. Here we show that certain GH61 proteins lack measurable hydrolytic activity by themselves but in the presence of various divalent metal ions can significantly reduce the total protein loading required to hydrolyze lignocellulosic biomass. We also solved the structure of one highly active GH61 protein and find that it is devoid of conserved, closely juxtaposed acidic side chains that could serve as general proton donor and nucleophile/base in a canonical hydrolytic reaction, and we conclude that the GH61 proteins are unlikely to be glycoside hydrolases. Structure-based mutagenesis shows the importance of several conserved residues for GH61 function. By incorporating the gene for one GH61 protein into a commercial Trichoderma reesei strain producing high levels of cellulolytic enzymes, we are able to reduce by 2-fold the total protein loading (and hence the cost) required to hydrolyze lignocellulosic biomass.

Insights into the oxidative degradation of cellulose by a copper metalloenzyme that exploits biomass components

The enzymatic degradation of recalcitrant plant biomass is one of the key industrial challenges of the 21st century. Accordingly, there is a continuing drive to discover new routes to promote polysaccharide degradation. Perhaps the most promising approach involves the application of “cellulase-enhancing factors,” such as those from the glycoside hydrolase (CAZy) GH61 family. Here we show that GH61 enzymes are a unique family of copper-dependent oxidases. We demonstrate that copper is needed for GH61 maximal activity and that the formation of cellodextrin and oxidized cellodextrin products by GH61 is enhanced in the presence of small molecule redox-active cofactors such as ascorbate and gallate. By using electron paramagnetic resonance spectroscopy and single-crystal X-ray diffraction, the active site of GH61 is revealed to contain a type II copper and, uniquely, a methylated histidine in the coppers coordination sphere, thus providing an innovative paradigm in bioinorganic enzymatic catalysis.

The molecular basis of polysaccharide cleavage by lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) are copper-containing enzymes that oxidatively break down recalcitrant polysaccharides such as cellulose and chitin. Since their discovery, LPMOs have become integral factors in the industrial utilization of biomass, especially in the sustainable generation of cellulosic bioethanol. We report here a structural determination of an LPMO-oligosaccharide complex, yielding detailed insights into the mechanism of action of these enzymes. Using a combination of structure and electron paramagnetic resonance spectroscopy, we reveal the means by which LPMOs interact with saccharide substrates. We further uncover electronic and structural features of the enzyme active site, showing how LPMOs orchestrate the reaction of oxygen with polysaccharide chains.

Enzymology and Structure of the GH13_31 Glucan 1,6-α-Glucosidase That Confers Isomaltooligosaccharide Utilization in the Probiotic Lactobacillus acidophilus NCFM

Isomaltooligosaccharides (IMO) have been suggested as promising prebiotics that stimulate the growth of probiotic bacteria. Genomes of probiotic lactobacilli from the acidophilus group, as represented by Lactobacillus acidophilus NCFM, encode α-1,6 glucosidases of the family GH13_31 (glycoside hydrolase family 13 subfamily 31) that confer degradation of IMO. These genes reside frequently within maltooligosaccharide utilization operons, which include an ATP-binding cassette transporter and α-glucan active enzymes, e.g., maltogenic amylases and maltose phosphorylases, and they also occur separated from any carbohydrate transport or catabolism genes on the genomes of some acidophilus complex members, as in L. acidophilus NCFM. Besides the isolated locus encoding a GH13_31 enzyme, the ABC transporter and another GH13 in the maltooligosaccharide operon were induced in response to IMO or maltotetraose, as determined by reverse transcription-PCR (RT-PCR) transcriptional analysis, suggesting coregulation of α-1,6- and α-1,4-glucooligosaccharide utilization loci in L. acidophilus NCFM. The L. acidophilus NCFM GH13_31 (LaGH13_31) was produced recombinantly and shown to be a glucan 1,6-α-glucosidase active on IMO and dextran and product-inhibited by glucose. The catalytic efficiency of LaGH13_31 on dextran and the dextran/panose (trisaccharide) efficiency ratio were the highest reported for this class of enzymes, suggesting higher affinity at distal substrate binding sites. The crystal structure of LaGH13_31 was determined to a resolution of 2.05 Å and revealed additional substrate contacts at the +2 subsite in LaGH13_31 compared to the GH13_31 from Streptococcus mutans (SmGH13_31), providing a possible structural rationale to the relatively high affinity for dextran. A comprehensive phylogenetic and activity motif analysis mapped IMO utilization enzymes from gut microbiota to rationalize preferential utilization of IMO by gut residents.

Rhamnogalacturonan lyase reveals a unique three-domain modular structure for polysaccharide lyase family 4.

Rhamnogalacturonan lyase (RG‐lyase) specifically recognizes and cleaves α‐1,4 glycosidic bonds between l‐rhamnose and d‐galacturonic acids in the backbone of rhamnogalacturonan‐I, a major component of the plant cell wall polysaccharide, pectin. The three‐dimensional structure of RG‐lyase from Aspergillus aculeatus has been determined to 1.5 Å resolution representing the first known structure from polysaccharide lyase family 4 and of an enzyme with this catalytic specificity. The 508‐amino acid polypeptide displays a unique arrangement of three distinct modular domains. Each domain shows structural homology to non‐catalytic domains from other carbohydrate active enzymes.

Short strong hydrogen bonds in proteins: a case study of rhamnogalacturonan acetylesterase

The short hydrogen bonds in rhamnogalacturonan acetylesterase have been investigated by structure determination of an active-site mutant, 1H NMR spectra and computational methods. Comparisons are made to database statistics. A very short carboxylic acid carboxylate hydrogen bond, buried in the protein, could explain the low-field (18 p.p.m.) 1H NMR signal.

Structural and electronic determinants of lytic polysaccharide monooxygenase reactivity on polysaccharide substrates.

Lytic polysaccharide monooxygenases (LPMOs) are industrially important copper-dependent enzymes that oxidatively cleave polysaccharides. Here we present a functional and structural characterization of two closely related AA9-family LPMOs from Lentinus similis (LsAA9A) and Collariella virescens (CvAA9A). LsAA9A and CvAA9A cleave a range of polysaccharides, including cellulose, xyloglucan, mixed-linkage glucan and glucomannan. LsAA9A additionally cleaves isolated xylan substrates. The structures of CvAA9A and of LsAA9A bound to cellulosic and non-cellulosic oligosaccharides provide insight into the molecular determinants of their specificity. Spectroscopic measurements reveal differences in copper co-ordination upon the binding of xylan and glucans. LsAA9A activity is less sensitive to the reducing agent potential when cleaving xylan, suggesting that distinct catalytic mechanisms exist for xylan and glucan cleavage. Overall, these data show that AA9 LPMOs can display different apparent substrate specificities dependent upon both productive protein–carbohydrate interactions across a binding surface and also electronic considerations at the copper active site.Copper-dependent lytic polysaccharide monooxygenases (LPMOs) oxidatively cleave polysaccharides. Here the authors present a structure-function characterization of fungal LPMOs, showing that a particular LPMO cleaves xylan using a mechanism that involves an alternative copper coordination geometry.

From screen to structure with a harvestable microfluidic device

Microfluidic crystallization using the Crystal Former improves the identification of initial crystallization conditions relative to screening via vapour diffusion.

Impact of the physical and chemical environment on the molecular structure of Coprinus cinereus peroxidase.

The structure of the peroxidase from Coprinus cinereus (CiP) has been determined in three different space groups and crystalline environments. Two of these are of the recombinant glycosylated form (rCiP), which crystallized in space groups P2(1)2(1)2(1) and C2. The third crystal form was obtained from a variant of CiP in which the glycosylation sites have been removed (rCiPON). It crystallizes in space group P2(1) with beta approximately 90 degrees; the structure was determined from room-temperature data and low-temperature data obtained from twinned crystals. Two independent molecules of CiP related by non-crystallographic symmetry are contained in the three crystal forms. The packing in the two structures of the glycosylated form of rCiP is closely related, but differs from the packing in the unglycosylated rCiPON. A database search based on small-molecule porphinato iron (III) complexes has been performed and related to observations of the spin states and coordination numbers of the iron ion. The room-temperature structures of CiP and one structure of the almost identical peroxidase from Arthromyces ramosus (ARP) have been used to identify 66 conserved water molecules and to assign a structural role to most of them.

pH, conductivity and long-term stability in the Crystal Screen solutions

Accurate knowledge of pH and ionic strength are very important when crystallization conditions are refined. The pH and conductivity of each of the 50 solutions in the Crystal Screen kit [Jancarik & Kim (1991). J. Appl. Cryst. 24, 409–411] were measured before and after incubation in a sealed chamber for six weeks. Two such kits were tested in this way. The results indicate that the measured pH of the solutions in the Crystal Screen is reproducible between different kits but that the measured value is not always related to the buffer system. Furthermore, it was observed that the protein crystal dye Izit (Hampton Research, Laguna Niguel, California, USA) forms crystals when added to several of the conditions in the crystal screen.