The Chemistry of Continental Solids 101Hemicelluloses
Hemicelluloses are the second most common
class of plant polymer. Hemicellluloses are
more complex than cellulose, being polymers
of hexose sugars (six-atom ring, e.g. glucose,
galactose, mannose), and pentose sugars
(five-atom ring, e.g. ribose, xylose and
arabinose) as well as uronic acids including
glucuronic and galacturonic acid. An example
of the degradation of a hemicellulose is that
of pectin (Fig. 2), which is a polymer of
galacturonic acid. Pectin is important in plant
structure, being involved in plant cell wall
formation. Decomposition of pectin occurs in
three stages. Initially, pectin esterase enzymes
attack esters (COOCH 3 ) on side-chains (Fig.
2a) resulting in carboxylic acid (COOH) where
esters were originally (Fig. 2b). The second
stage of degradation is depolymerization to
form glucuronic acid monomers (Fig. 2c).
These monomers are then mineralized to CO 2
and H 2 O.LigninLignin is the third most common plant
polymer after cellulose and hemicelluloses.
Lignin gives wood its toughness and
structural rigidity. Lignin is formed from the
metabolic processing of glucose (non-O O
OHOHCOOCH 3OO
COOCH 3OHnO
OHOHCOOHOH OH¥ n4O 2
5CO 2 + 5H 2 O + energyStage 1: Pectin esterases attack side-chainsOHO O
OHOHCOOCH 3O O
OHOHCOOCH 3OO
COOCH 3OHOHOO O
OHOHCOOHOO
COOHOHn
Stage 2: The polymer is attacked by depolymerasesOHO O
OHOHCOOHO O
OHOHCOOHOO
COOHOHOHOStage 3: Mineralization of galacturonic acid monomersGalacturonic acid monomers
Galacturonic acid oxidase(c)(a)(b)Fig. 2Steps in the aerobic degradation of the hemicellulose pectin.
(continued on p. 102)