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Logo of biotbiofuelBioMed CentralBiomed Central Web Sitesearchsubmit a manuscriptregisterthis articleBiotechnology for Biofuels
 
Biotechnol Biofuels. 2012; 5: 21.
Published online Apr 11, 2012. doi:  10.1186/1754-6834-5-21
PMCID: PMC3431977
Two structurally discrete GH7-cellobiohydrolases compete for the same cellulosic substrate fiber
Fernando Segato,1,4 André R L Damasio,1,5 Thiago Augusto Gonçalves,4 Mario T Murakami,3 Fabio M Squina,4 MariadeLourdesTM Polizeli,6 Andrew J Mort,2 and Rolf A Pradecorresponding author1,4
1Department of Microbiology & Molecular Genetics, Oklahoma State University, Stillwater, OK, USA
2Department of Biochemistry and Molecular Biology, Oklahoma State University, Stillwater, OK, USA
3Laboratório Nacional de Biociências (LNBio), Campinas, Sao Paulo, Brazil
4Laboratório Nacional de Ciência e Tecnologia do Bioetanol (CTBE), Centro Nacional de Pesquisas em Energia e Materiais, Campinas, Sao Paulo, Brazil
5Department of Biochemistry, Ribeirão Preto School of Medicine, Ribeirão Preto, Sao Paulo, Brazil
6Biology Department, FFCLRP, Universidade de São Paulo, Ribeirão Preto, Sao Paulo, Brazil
corresponding authorCorresponding author.
Fernando Segato: fernando.segato/at/bioetanol.org.br; André R L Damasio: andre.damasio/at/bioetanol.org.br; Thiago Augusto Gonçalves: thiago.goncalves/at/bioetanol.org.br; Mario T Murakami: mario.murakami/at/lnbio.org.br; Fabio M Squina: fabio.squina/at/bioetanol.org.br; MariadeLourdesTM Polizeli: polizeli/at/ffclrp.usp.br; Andrew J Mort: andrew.mort/at/okstate.edu; Rolf A Prade: prade/at/okstate.edu
Received December 14, 2011; Accepted March 30, 2012.
Abstract
Background
Cellulose consisting of arrays of linear beta-1,4 linked glucans, is the most abundant carbon-containing polymer present in biomass. Recalcitrance of crystalline cellulose towards enzymatic degradation is widely reported and is the result of intra- and inter-molecular hydrogen bonds within and among the linear glucans. Cellobiohydrolases are enzymes that attack crystalline cellulose. Here we report on two forms of glycosyl hydrolase family 7 cellobiohydrolases common to all Aspergillii that attack Avicel, cotton cellulose and other forms of crystalline cellulose.
Results
Cellobiohydrolases Cbh1 and CelD have similar catalytic domains but only Cbh1 contains a carbohydrate-binding domain (CBD) that binds to cellulose. Structural superpositioning of Cbh1 and CelD on the Talaromyces emersonii Cel7A 3-dimensional structure, identifies the typical tunnel-like catalytic active site while Cbh1 shows an additional loop that partially obstructs the substrate-fitting channel. CelD does not have a CBD and shows a four amino acid residue deletion on the tunnel-obstructing loop providing a continuous opening in the absence of a CBD. Cbh1 and CelD are catalytically functional and while specific activity against Avicel is 7.7 and 0.5 U.mg prot-1, respectively specific activity on pNPC is virtually identical. Cbh1 is slightly more stable to thermal inactivation compared to CelD and is much less sensitive to glucose inhibition suggesting that an open tunnel configuration, or absence of a CBD, alters the way the catalytic domain interacts with the substrate. Cbh1 and CelD enzyme mixtures on crystalline cellulosic substrates show a strong combinatorial effort response for mixtures where Cbh1 is present in 2:1 or 4:1 molar excess. When CelD was overrepresented the combinatorial effort could only be partially overcome. CelD appears to bind and hydrolyze only loose cellulosic chains while Cbh1 is capable of opening new cellulosic substrate molecules away from the cellulosic fiber.
Conclusion
Cellobiohydrolases both with and without a CBD occur in most fungal genomes where both enzymes are secreted, and likely participate in cellulose degradation. The fact that only Cbh1 binds to the substrate and in combination with CelD exhibits strong synergy only when Cbh1 is present in excess, suggests that Cbh1 unties enough chains from cellulose fibers, thus enabling processive access of CelD.
Keywords: Cellobiohydrolase, Cellobiohydrolase I, Cellobiohydrolase D, Aspergillus niveus, Aspergillus fumigatus, Crystalline cellulose breakdown, Biofuels, Cellulases, Biomass decomposition
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