Molecular Origins of Reduced Activity and Binding Commitment of Processive Cellulases and Associated Carbohydrate-Binding Proteins to Cellulose III

Shishir Chundawat, Bhargava Nemmaru, Markus Hackl, Sonia Brady, Mark Hilton, Madeline Johnson, Sungrok Chang, Matthew Lang, Hyun Huh, Sang-Hyuk Lee, John Yarbrough, Cesar Lopez, Sandrasegaram Gnanakaran

Research output: Contribution to journalArticlepeer-review

11 Scopus Citations

Abstract

Efficient enzymatic saccharification of cellulosic biomass into fermentable sugars can enable production of bioproducts like ethanol. Native crystalline cellulose, or cellulose I, is inefficiently processed via enzymatic hydrolysis but can be converted into the structurally distinct cellulose III allomorph that is processed via cellulase cocktails derived from Trichoderma reesei up to 20-fold faster. However, characterization of individual cellulases from T. reesei, like the processive exocellulase Cel7A, shows reduced binding and activity at low enzyme loadings toward cellulose III. To clarify this discrepancy, we monitored the single-molecule initial binding commitment and subsequent processive motility of Cel7A enzymes and associated carbohydrate-binding modules (CBMs) on cellulose using optical tweezers force spectroscopy. We confirmed a 48% lower initial binding commitment and 32% slower processive motility of Cel7A on cellulose III, which we hypothesized derives from reduced binding affinity of the Cel7A binding domain CBM1. Classical CBM–cellulose pull-down assays, depending on the adsorption model fitted, predicted between 1.2- and 7-fold reduction in CBM1 binding affinity for cellulose III. Force spectroscopy measurements of CBM1–cellulose interactions, along with molecular dynamics simulations, indicated that previous interpretations of classical binding assay results using multisite adsorption models may have complicated analysis, and instead suggest simpler single-site models should be used. These findings were corroborated by binding analysis of other type-A CBMs (CBM2a, CBM3a, CBM5, CBM10, and CBM64) on both cellulose allomorphs. Finally, we discuss how complementary analytical tools are critical to gain insight into the complex mechanisms of insoluble polysaccharides hydrolysis by cellulolytic enzymes and associated carbohydrate-binding proteins.

Original languageAmerican English
Article numberArticle No. 100431
Number of pages17
JournalJournal of Biological Chemistry
Volume296
DOIs
StatePublished - 1 Jan 2021

Bibliographical note

Publisher Copyright:
© 2021 THE AUTHORS.

NREL Publication Number

  • NREL/JA-2700-78361

Keywords

  • carbohydrate-active enzymes
  • carbohydrate-binding proteins
  • cellulases
  • cellulose
  • lignocellulosic biofuels
  • molecular dynamics
  • optical tweezers force spectroscopy

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