How the walls come crumbling down: recent structural biochemistry of plant polysaccharide degradation
Introduction
A significant amount of attention [1•, 2, 3, 4, 5] is presently focussed on the quest to replace petroleum with plant biomass for chemical and fuel production, widely referred to as the ‘bio-refinery’ concept [6]. Where biomass saccharification followed by microbial fermentation is concerned, polysaccharide-degrading enzymes are of key importance [2]. In this context, the industrial application of enzymes in starch degradation is a mature and fundamentally well-understood technology [7]. Recent attention on the application of enzymes in lignocellulose degradation has almost exclusively centred on improving the efficiency and lowering the production costs of multi-component cellulase mixtures composed of exo-β(1 → 4)-glucanases and endo-β(1 → 4)-glucanases (see [8, 9••, 10] for recent comprehensive reviews). This focus has been motivated by the fact that dilute acid treatment at high temperature, one of the most common pre-treatment steps, hydrolyses nearly all of the hemicellulosic and pectic polysaccharides, exposing the recalcitrant para-crystalline cellulose to enzyme attack [9••]. However, dilute acid hydrolysis of hemicellulose produces relatively low yields of the component monosaccharides and slow reaction kinetics, which limit process economics [9••]. Potential applications of other polysaccharide hydrolases to aid in the extraction or hydrolysis of specific non-cellulosic polysaccharides have been largely unexplored. However, these enzymes and their appended non-catalytic carbohydrate-binding modules (CBMs) are likely to become increasingly important in the near future for both the analysis [11••, 12] and breakdown of selected and engineered plants for biofuel production [9••]. Thus, this review will focus on recent advances regarding the structure and function of proteins involved in the degradation of some key hemicelluloses from dicotolydenous and woody plants, namely the xyloglucans (XGs), galactoglucomannans (GGMs) and glucuronoarabinoxylans (GAXs). New developments in the characterisation and application of CBMs and cellulosomes will also be discussed.
Section snippets
Bacteria and fungi: masters of lignocellulose saccharification
Structurally complex and biologically recalcitrant, the plant cell wall and its metabolism are currently poorly understood. In particular, the nanoscale organisation of the component polymers — polysaccharides, polypeptides and polyphenolics — is essentially unknown [13]. Microbes interact with plant cell walls in many contexts, for example, as pathogens, symbionts or saprotrophs, and as a consequence have evolved numerous catalytic strategies to attack wall components, the majority of which are
Enzyme modularity and substrate targeting: CBMs and cellulosomes
Enzyme secretion by plant cell wall degrading micro-organisms can be divided along two lines. The majority secrete mixtures of individual enzymes into their surroundings, many of which possess a multi-modular structure composed of a catalytic module linked to one or more CBMs [15, 18•, 19]. The latter function to improve enzyme efficacy by targeting the catalytic module to surfaces of insoluble substrates [19] and possibly through substrate disruption [20••]. An alternative approach for
Hemicellulose degradation: a diversity of linkages and a multitude of enzymes
Pectins notwithstanding [14, 22], the hemicellulosic polysaccharides present an amazing array of sugars connected by bonds of diverse stereochemistry and regiochemistry. Although many of the CAZyme mechanisms are well established, the past two years have witnessed an impressive amount of effort in the structural analysis of polysaccharidases to elucidate the molecular basis of substrate recognition leading to catalysis. The following sections provide a brief tour of recent advances in the
Future prospects
It is widely established that enzyme access is the major limitation preventing efficient saccharification of the plant cell wall. Indeed, the access problem is the primary reason why the economic viability of lignocellulose-derived biofuels is limited by the cost of enzymes deployed in the process. There is thus an urgent need to discover new, efficient enzymes that display novel activities against the complex matrix polysaccharides for the further refinement of non-chemical methods for plant
References and recommended reading
Papers of particular interest, published within the annual period of the review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Protein representations were created using PyMOL v0.99rc6 (http://pymol.sourceforge.net/). HS thanks the Swedish Research Council (VR), the Swedish Agency for Innovation Systems (VINNOVA) for financial support. HB and HJG acknowledge funding from Formas and the Forestry Commission, respectively, under the WoodWisdom-Net framework. HB is a fellow (Rådsforskare) and grantee of the Swedish Research Council.
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