Like other invertebrates, shrimp posses an efficient defense system that permits recognition and elimination of pathogens. One of the main components of shrimp immunity is prophenoloxidase proPO system that is located inside hemocyte granules and is released to plasma under microbial stimulus. Outside the cell, the system is activated by the effect of plasma Ca2+ on the proPO activating enzyme (PPAE), a serine type proteinase. PPAE cleaves proPO, producing a 7-kDa peptide and the 107-kDa active phenoloxidase that catalyzes the initial reaction for melanization. Recognizing components are also part of the defense system. Two recognizing proteins have been detected in shrimp plasma: Lipopolysaccharide binding agglutinin (LPS-BA) and beta glucan binding protein (BGBP). These proteins activate defense cellular functions, after reaction with the ligand. LPS-BA is a 180-kDa protein able to agglutinate Gram(-) bacteria through surface LPS and acts as an opsonin stimulating phagocytosis. BGBP is a 100-kDa protein that extends the β glucan-induced proPO system stimulation. This protein is highly conserved with similar amino acid composition, molecular weight, antigenicity and N-terminal sequence in marine shrimp and freshwater crayfishes. LPS also stimulates the shrimp coagulation system by inducing the release of a transglutaminase (TGase) from hyaline hemocytes. This TGase polymerizes the clotting protein (CP), which is a 400-kDa glycoprotein formed by two identical subunits. CP amino acid composition and N- terminal sequence is similar to lobster and crayfish homologues. Microbial products can stimulate cellular reactions directly or through recognizing proteins, activating mechanism and effectors like proPO and coagulation which in turn produce melanization, encapsulation and physical entrapping.
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