Fig. 12. Model of Ha-CalpM, based on the crystal structure of mammalian m-calpain (CAPN2) in the absence of Ca²⁺. Identical residues between the lobster and human deduced amino acid sequences are colored (red, acidic residues; blue, basic; yellow, hydrophobic; lavender, uncharged polar). (A) `Top' view of the catalytic domain (domain II) containing the three conserved residues (C141, H299 and N329, in green) forming the catalytic triad of the active site. The residues of the triad are surrounded by conserved nonpolar residues, which form a hydrophobic environment within the catalytic cleft. (B) `Side' view showing an acidic loop (magenta and red) containing 19 acidic residues in a stretch of 20 residues (D452, D455, D457 and D458 are not colored because they did not align with identical residues in the human m-calpain; see Fig. 2). The acidic loop, which includes a stretch of 11 aspartate residues (magenta; residues 460-470) unique to the lobster sequence, is part of a putative calcium-dependent phospholipid-binding C2-like region in domain III. Binding of Ca²⁺ to the acidic loop and to two aspartates (Fig. 2; D232 and D366) in domain II results in closure of the catalytic cleft to allow cooperation between C141, H299 and N329 necessary for catalytic activity (Hosfield et al., 2001; Moldoveanu et al., 2002). The first 60 residues of the N-terminal sequence were excluded, due to its high divergence from the mammalian sequence (Fig. 2). The lobster sequence lacks a calmodulin-like domain (domain IV).