title: Untreated Chlorella homosphaera biomass allows for high rates of cell wall glucan enzymatic hydrolysis when using exoglucanase-free cellulases.
abstract: BACKGROUND
  Chlorophyte microalgae have a cell wall containing a large quantity of cellulose Iα with a triclinic unit cell hydrogen-bonding pattern that is more susceptible to hydrolysis than that of the cellulose Iβ polymorphic form that is predominant in higher plants. This study addressed the enzymatic hydrolysis of untreated Chlorella homosphaera biomass using selected enzyme preparations, aiming to identify the relevant activity profile for the microalgae cellulose hydrolysis. Enzymes from Acremonium cellulolyticus, which secretes a complete pool of cellulases plus β-glucosidase; Trichoderma reesei, which secretes a complete pool of cellulases with low β-glucosidase; Aspergillus awamori, which secretes endoglucanases and β-glucosidase; blends of T. reesei-A. awamori or A. awamori-A. cellulolyticus enzymes; and a purified A. awamori β-glucosidase were evaluated.
  
  
  RESULTS
  The highest initial glucan hydrolysis rate of 140.3 mg/g/h was observed for A. awamori enzymes with high β-glucosidase, low endoglucanase, and negligible cellobiohydrolase activities. The initial hydrolysis rates when using A. cellulolyticus or T. reesei enzymes were significantly lower, whereas the results for the T. reesei-A. awamori and A. awamori-A. cellulolyticus blends were similar to that for the A. awamori enzymes. Thus, the hydrolysis of C. homosphaera cellulose was performed exclusively by the endoglucanase and β-glucosidase activities. X-ray diffraction data showing negligible cellulose crystallinity for untreated C. homosphaera biomass corroborate these findings. The A. awamori-A. cellulolyticus blend showed the highest initial polysaccharide hydrolysis rate of 185.6 mg/g/h, as measured by glucose equivalent, in addition to the highest predicted maximum glucan hydrolysis yield of 47% of total glucose (w/w). T. reesei enzymes showed the lowest predicted maximum glucan hydrolysis yield of 25% (w/w), whereas the maximum yields of approximately 31% were observed for the other enzyme preparations. The hydrolysis yields were proportional to the enzyme β-glucosidase load, indicating that the endoglucanase load was not rate-limiting.
  
  
  CONCLUSIONS
  High rates of enzymatic hydrolysis were achieved for untreated C. homosphaera biomass with enzymes containing endoglucanase and β-glucosidase activities and devoid of cellobiohydrolase activity. These findings simplify the complexity of the enzyme pools required for the enzymatic hydrolysis of microalgal biomass decreasing the enzyme cost for the production of microalgae-derived glucose syrups.
authors.affiliation: Federal University of Rio de Janeiro, Institute of Chemistry, Department of Biochemistry, Applied Photosynthesis Laboratory, Athos Avenida da Silveria Ramos, 149-Technology Centre, Block A, Room 532, University City, Rio de Janeiro, RJ 21941-909 Brazil.
authors.forename: Marcoaurélio Almenara
authors.initials: MA
authors.lastname: Rodrigues
authors.affiliation: Federal University of Rio de Janeiro, Institute of Chemistry, Department of Biochemistry, Enzyme Technology Laboratory, 21941-909 Rio de Janeiro, RJ Brazil.
authors.forename: Ricardo Sposina Sobral
authors.initials: RS
authors.lastname: Teixeira
authors.affiliation: National Institute of Technology - Ministry of Science, Technology and Innovation, Biocatalysis Laboratory, 20081-312 Rio de Janeiro, RJ Brazil ; Federal University of Rio de Janeiro, Institute of Chemistry, Department of Biochemistry, Enzyme Technology Laboratory, 21941-909 Rio de Janeiro, RJ Brazil.
authors.forename: Viridiana Santana
authors.initials: VS
authors.lastname: Ferreira-Leitão
authors.affiliation: Federal University of Rio de Janeiro, Institute of Chemistry, Department of Biochemistry, Enzyme Technology Laboratory, 21941-909 Rio de Janeiro, RJ Brazil.
authors.forename: Elba Pinto
authors.initials: EP
authors.lastname: da Silva Bon
keywords: Acremonium cellulolyticus; Chlorella homosphaera; Chlorophyte cell wall crystallinity; Microalgae biomass enzymatic hydrolysis; Microalgae glucose sugar syrups; Trichoderma reesei and Aspergillus awamori cellulases