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Function(s) and Biological Relevance
Lignostilbene-α,β-dioxygenase A (LsdA) from the bacterium Sphingomonas paucimobilis TMY1009 is a nonheme iron oxygenase that catalyzes the cleavage of lignostilbene, a compound arising in lignin transformation, to two vanillin molecules. LsdA has greatest substrate specificity for lignostilbene. The substrate's 4-hudryoxy moiety is required for catalysis. Phenylazophenol inhibits the cleavage of lignostilbene by LsdA. The breaking down of lignin is essential to the sustainable biorefining of lignocellulose. It is of great relevance to transforming lignocellulose to biofuels.
Broader Implications
Lignin represents 30% of the lignocellulose biomass. It consists of different aromatic building blocks, phenylpropanoids, which are extremely useful. Normally aromatic compounds are extracted from petroleum and are used to manufacture drugs, paint, plastics, etc. Therefore the potential of lignin is very high. Lignin is the most abundant polymer in nature other than cellulose and chitin, and it is the only one that contains such a large number of aromatic compounds.
Structural highlights and structure-function relationships
The of the consists of Phenylalanine59, Tyrosine101, and Lysine134.
The flat surface of the B-sheet is pushing the amino acids up, making it possible for the Phe59, Tyr101, and Lys134 to create interactions with the .
The tertiary structure creates a that are important to the active site. His282 provides pi-stacking, Phe305 provides Hydrophobic contacts, and Tyr101 provides Hydrogen bonding. The tertiary structure also allows the NSL ligand to interact using its 4-hydroxy with the catalytic triad. LsdA can only cleave 4-hydroxystilbenes. The photo below shows the NSL ligand interacting in the binding pocket.
The tertiary structure also creates a metal binding site of Histidines to keep the iron molecule in place. There have been two mechanisms proposed for Lsd's. In one mechanism, the hydroxystillbenoid is activated via the enzyme-catalyzed deprotonation of the 4-hydroxy group, which then allows electron delocalization toward an Fe3+. In the other mechanism, π electron density from the double bond is redistributed to the iron-oxy complex to form an Fe2+ cation intermediate. Deprotonation of the hydroxyl is demanding for both mechanisms and is assisted by Lys134 and Tyr101.
The hydrophobic/hydrophilic view of the ligand in the protein shows that both hydrophilic and hydrophobic residues are important to the ligand in the binding site. The first photo below shows the spacefill look at the protein, and it can be seen that the binding pocket is almost invisible and hard to reach. The second photo shows the hydrophobic/hydrophilic binding pocket up close, showing that the hydrophobic portion of the protein is interacting with the hydrophobic portion of the ligand, and the hydrophilic portion is interacting with the hydrophilic areas of the ligand. The red shows hydrophobic properties, and the green shows hydrophilic properties. In the first photo you can see on chain B the active site is mostly hydrophilic with one red hydrophobic area at the entrance. The ligand is in the middle of the protein and is not very visible from the outside of the protein.
Energy Transformation
