Wood is built from three major components, which are physically mixed and chemically bonded together:

• Cellulose, a linear polymer of glucose, which is the most abundant organic compound on earth

  Cellulose

• Hemicellulose, a branched polymer, heterogeneous, with different composition in hardwoods and softwoods:

  Hemicellulose

• Lignin, an irregular, branched phenolic polymer of hypothetical structure, and the most abundant aromatic polymer on earth

  Lignin

Although the polymerization that forms lignin is initiated by enzymes (some of the same ones that degrade it), it is not guided by them: once the radicals are formed they are on their own. [Ralph et al., Rec. Adv. Polyphenol Res., 2008, 88, 2153; Plant Cell, 2010, 22, 1035; Plant Physiol. 2010, 153, 895]

An organism that proposes to make a living eating wood must be able to digest at least one of these three components.

Needlesss to say, humans are not such an organism. However, cows and some other mammals, termites, fungi, and bacteria have learned to do this - cows and termites with the aid of intestinal bacteria.

• The intestinal bacteria hydrolyze the cellulose and hemicellulose, providing the sugars

• White and brown rot basidomycete fungi are the only known organisms that are capable of degrading lignin.

• Soil bacteria are capable of eating the aromatics released by degradation of lignin

We shall take a look here at some of the enzymes that have evolved to degrade cellulose, hemicellluloses, and lignin.

• These enzymes are a key part of the global carbon cycle, a component that is often neglected.

• There is a yearly breakdown and recycling of plant lignocellulose accounting for 15% of the total atmospheric carbon, achieved mainly by bacterial and fungal cellulases.
  

• The possible harnessing of these processes to provide renewable sources of carbon for fuels and polymeric materials is currently the subject of major research efforts.