08.08.2015   Biomass Recalcitrance

Chemistry of cell wall polymers

4.2.1 Chemistry of cell wall polysaccharides

Cell wall polysaccharides are of two types: the first is the stereo-regular homopolymer type among which cellulose that makes up the microfibrillar phase of all cell walls is the pre­mier example, and the second comprises the stereo-irregular, non-cellulosic polysaccharides represented by the (1—>3,1—>4)-(3 — D-glucans, heteroxylans, heteroglucans, heteromannans, and the pectins (pectic polysaccharides) which are found in the matrix phase of the wall. The chemistry and occurrence of these two polysaccharide types is described in the fol­lowing sections. Table 4.1 summarizes the cell wall components of the vegetative parts of target “cellulosic” ethanol feedstocks. It should be noted that the stereo-irregular polysac­charides other than the pectins are loosely referred to as hemicelluloses in many publications. We prefer to use the term non-cellulosic polysaccharides to describe collectively all these molecules and to refer to the individual polysaccharides specifically by their descriptive chemical names (8). The extension of the term to “hemicellulase” further obfuscates the

Table 4.1 Compositions of the cell walls of vegetative parts of "cellulosic" ethanol feedstocks

Wall type

Plant group

Primary walls

Secondary walls

Angiosperms

Eudicotyledons

e. g., Hardwoods [e. g., willows (Salix spp.) Eucalyptus spp.] and

legumes

Fibrillar phase: cellulose Matrix phase: Pectic polysaccharides > xyloglucans > heteroxylans and (galacto)glucomannans (minor)

(Hardwoods)

Fibrillar phase: cellulose Matrix phase: 4-O-methyl — glucurono-xylans > glucomannans

Ester-linked ferulic and small proportions of p-coumaric acid in "core" Caryophyllales. Known to be linked to RG-I side chains in Amaranthaceae

Lignins: Syringyl and guaiacyl; small proportions of p-hydroxyphenyl units. Acylated with p-hydroxybenzoic acid in Salicaceae and acetic acid in Hibiscus cannabinus

Monocotyledons

Non-commelinids e. g., Sisal (Agave sisalana)

Fibrillar phase: cellulose Matrix phase: As eudicotyledons

Fibrillar phase: cellulose Matrix phase: As eudicotyledons

Commelinids

e. g., Grasses and cereals

(Poaceae), palms (Arecales)

Fibrillar phase: cellulose aMatrix phase:

Glucuronoarabinoxylans (GAXs) > pectic polysaccharides and xyloglucans

[(1^3,1^4)-B-glucans in some Poales families, including

Poaceae]

Fibrillar phase: cellulose Matrix phase:

Glucuronoarabinoxylans > (galacto-)glucomannans (minor)

Ferulic, small proportions of p-coumaric, and sometimes sinapic acids, ester-linked to GAXs

Lignins: As in eudicotyledons. Acylated with p-coumaric acid in Poaceae and Musaceae, and with p-hydroxybenzoic acid in Arecales

Gymnosperms

e. g., Coniferous

(Softwoods)

gymnosperms, e. g., spruce (Picea abies)

Fibrillar phase: cellulose bMatrix phase: As eudicotyledons

Ester-linked ferulic and p-coumaric acids in all families

Fibrillar phase: cellulose Matrix phase:

(Galacto-)glucomannans > 4-O — methyl-glucuronoarabinoxylans Lignins: Usually mostly guaiacyl units;also p-hydroxyphenyl units in compression wood

a The primary walls of palms (Arecales) are an exception; their matrix polysaccharides are pectic polysaccharides > xyloglucans > glucuronoarabinoxylans (minor).

b Detailed dataon the matrix polysaccharides are available only forconifers (Coniferales) and Ginkgobiloba (Ginkgoales).

situation since the enzymes hydrolyzing non-cellulosic polysaccharides are specific and cannot be grouped together (see Chapter 10).