Chitin Synthesis

Chitin synthesis occurs throughout the insect's life cycle and is under hormonal control of ecdysteroids. Bursts of synthetic activity that are associated with the buildup of the new cuticles occur in particular at the last phase of embryonic development, and as larvae or pupae molt. Chitin synthesis is the end result of a cascade of interconnected biochemical and biophysical events that link the mobilization of substrate molecules, polymerization by the enzyme chitin synthase, and translocation of the nascent amino polymer across the plasma membrane (Fig. 2). Individual chitin chains coalesce outside the plasma membrane, forming fibril crystallites by intramolecular hydrogen bonds. The UDP-A^-acetyl-D-glucosamine substrate is the end point of a series of biochemical transformations that include successive steps of phosphorylation, amination, and acetylation of starting precursors such as trehalose or glucose. Chitin synthase is a relatively large membrane-bound enzyme with multiple transmembrane segments. The active site of the enzyme faces the cytoplasm, and the catalysis involves linking together dimer amino sugar substrates. The question of how chitin polymers are translocated across the cell membrane remains unresolved. Hydrophobic transmembrane segments of chitin synthase are implicated in this process.

The complete chitin synthase cDNA and deduced amino acid sequences of the insects Drosophila melanogaster and Lucila cuprina, and the nematode Caenorhabditis elegans have

Chitin synthase (CS) and product Synthesis of CS Integration of CS into the plasma membrane

Substrate formation Trehalose, glucose Phosph ory lation

Animation

Acetylation UDP -N-acetylglucosamine

Catalysis polymerization

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