Genomics Review of Holocellulose Deconstruction by Aspergilli

Abstract

SUMMARY Biomass is constructed of dense recalcitrant polymeric materials: proteins, lignin, and holocellulose, a fraction constituting fibrous cellulose wrapped in hemicellulose-pectin. Bacteria and fungi are abundant in soil and forest floors, actively recycling biomass mainly by extracting sugars from holocellulose degradation. Here we review the genome-wide contents of seven Aspergillus species and unravel hundreds of gene models encoding holocellulose-degrading enzymes. Numerous apparent gene duplications followed functional evolution, grouping similar genes into smaller coherent functional families according to specialized structural features, domain organization, biochemical activity, and genus genome distribution. Aspergilli contain about 37 cellulase gene models, clustered in two mechanistic categories: 27 hydrolyze and 10 oxidize glycosidic bonds. Within the oxidative enzymes, we found two cellobiose dehydrogenases that produce oxygen radicals utilized by eight lytic polysaccharide monooxygenases that oxidize glycosidic linkages, breaking crystalline cellulose chains and making them accessible to hydrolytic enzymes. Among the hydrolases, six cellobiohydrolases with a tunnel-like structural fold embrace single crystalline cellulose chains and cooperate at nonreducing or reducing end termini, splitting off cellobiose. Five endoglucanases group into four structural families and interact randomly and internally with cellulose through an open cleft catalytic domain, and finally, seven extracellular β-glucosidases cleave cellobiose and related oligomers into glucose. Aspergilli contain, on average, 30 hemicellulase and 7 accessory gene models, distributed among 9 distinct functional categories: the backbone-attacking enzymes xylanase, mannosidase, arabinase, and xyloglucanase, the short-side-chain-removing enzymes xylan α-1,2-glucuronidase, arabinofuranosidase, and xylosidase, and the accessory enzymes acetyl xylan and feruloyl esterases.

Lignocellulases: a review of emerging and developing enzymes, systems, and practices

“…For exoglucanases, they have long loops and affinity for the crystalline sites along cellulose chains and yield primarily cellodextrin (Segato et al 2014). Most often, the loops form a tunnel around the catalytic residues; therefore, substrates usually are directed from the end of the tunnel to encounter the active site of the enzyme (Juturu and Wu 2014).…”

“…Laccases have four copper atoms present in their active sites which are distributed at three different copper centers, namely: Type-1 (blue copper center), Type-2 (normal copper), and Type-3 (coupled binuclear copper centers) (Dwivedi et al 2011). These copper atoms serve as a catalytic metal and reducing agent for the oxidation of various carbons (C-1, C-4, and C-6) in the polymeric structure (Segato et al 2014). Ascorbate oxidase, ferroxidases, ceruloplasmin, and bilirubin oxidases are examples of members in the multi-copper protein family.…”

Clonagem, expressão e purificação de glicosil hidrolases (GH5 e GH45) provenientes do fungo <i>Gloeophyllum trabeum</i> e estudo da ação das proteínas como auxiliares na hidrólise de polissacarídeos

“…Esta etapa de degradação promove uma rápida diminuição no grau de polimerização da celulose e expõe novas extremidades das cadeias de celulose. Atualmente as endoglucanases estão classificadas no CAZy nas famílias GH 5, 6, 7, 8, 9, 12, 44, 45, 48, 51, 61, 74 e 124 (VLASENKO et al, 2010;VAN DEN BRINK;DE VRIES, 2011;HORN et al, 2012;SEGATO et al, 2014).…”

“…Apesar de ser classificada como uma única família no CAZy, as GHs5 atuam em diferentes substratos, o que pode ser atribuído à origem das proteínas que derivaram de um ancestral comum, preservando alguns domínios proteicos (PAYNE et al, 2015). As enzimas pertencentes à essa família podem atuar em substratos diversos, apresentando atividade típica de endoglucanases, mananases, xilanases, xiloglucanases e arabinofuranosidases (ASPEBORG et al, 2012;SEGATO et al, 2014).…”

“…As EGs da família GH5 são as principais celulases de fungos, sendo que estão codificadas tanto no genoma de ascomicetos, como no genoma de fungos de degradação branca e parda (FLOUDAS et al, 2012;SEGATO et al, 2014). As GHs5 provenientes de fungos estão agrupadas em 18 das 51 subfamílias e, até o momento, apenas 4 estruturas tridimensionais de EGs de origem fungica foram determinadas a partir da difração de cristais proteícos (VLASENKO et al, 2010;PAYNE et al, 2015).…”