Lignin: A Major Barrier in Enzymatic Degradation
Overview of Lignin Structure and Function
Lignin molecules are derived mainly from three monolignols which include p-coumaryl alcohol or H-unit, coniferyl alcohol or G-unit, and sinapyl alcohol or S-unit as shown in Fig. 1.1. These monolignols vary in terms of number of methoxyl groups attached on the aromatic ring: H-unit contains no methoxyl group (zero degree of methoxylation), G-unit contains one methoxyl group (one degree of methoxylation), and S-unit contains two methoxyl groups (two degrees of methoxylation). Monolignols present in softwood lignin are generally composed of approximately 86 % G-units, 12 % H-units, and 1 % S-units (Sjostrom 1993).
The monolignols are synthesized within plant cells via two-step enzymatic process beginning with D-glucose (Sjostrom 1993). During the first step, D-glucose is converted into L-phenylalanine or L-tyrosine via the shikimate pathway.
In the next step, the reaction products from the first step are converted into p-coumaryl alcohol (H-unit), coniferyl alcohol (G-unit), and sinapyl alcohol (S-unit) via the cinnamate pathway (Sjostrom 1993). Once these monolignols get deposited on the plant cell wall, their polymerization is initiated with enzymatic dehydrogenation, whereby phenolic hydrogen is removed and produces resonance-stabilized radical intermediates. The position of a radical electron at the different positions provides an opportunity for several inter-monomer linkages to be formed via radical
Fig. 1.1 Three major building blocks of monolignols of lignin
polymerization and results into C-C and ether linkages providing complexity to the highly heterogeneous lignin structure (Sjostrom 1993).
The most common types of linkages found within softwood lignin molecule include P-O-4, a-O-4, в-5, 5-5, p-1, and p-p as shown in Fig. 1.2. Softwood lignin contains 60-70 % ether bonds (between an aromatic carbon on one monolignol and an aliphatic carbon on another), while carbon-carbon linkages constitute approximately 25 % (Sjostrom 1993). Due to the presence of these ether and carbon-carbon linkages, lignin is a highly heterogeneous and three-dimensional complex polymer and until now, there is no consensus on actual native lignin structure (Sjostrom 1993). It is important to understand that ether bonds are more amenable to disruption during pretreatment than carbon-carbon bonds. This can be explained by the fact that oxygen is more electronegative than carbon atom, creating an imbalance in electronegativity and hence lignin molecule becomes more likely to depolymerize into smaller molecules where the monolignols are attached by ether bonds. Therefore the structures containing ether linkages are referred to as uncondensed, and on the other hand, the structures containing carbon-carbon bonds are referred to as condensed structures (Sjostrom 1993).
Based on the above rationale, it can be understood that the ratio of condensed versus uncondensed lignin is crucial factor in assessing the lignin depolymerization during pretreatment.