Calcium ions structure stabilizers

The family 11 and family 111 cellulases consist of a similar structure that is much larger than family 1 cellulose binding domains (100-170 amino acid residues) with three aromatic amino acid residues, mainly tryptophan, and five to seven polar residues. Again these amino acids bind to the cellulose with the aromatic residues associated with the sugar face and the polar residues with the hydroxyl groups. Families 11 and 111 cellulose binding domains also require a calcium ion to stabilize the binding domain and to function properly [12]. 

The basic structural unit of these two-sheet p helix structures contains 18 amino acids, three in each p strand and six in each loop. A specific amino acid sequence pattern identifies this unit namely a double repeat of a nine-residue consensus sequence Gly-Gly-X-Gly-X-Asp-X-U-X where X is any amino acid and U is large, hydrophobic and frequently leucine. The first six residues form the loop and the last three form a p strand with the side chain of U involved in the hydrophobic packing of the two p sheets. The loops are stabilized by calcium ions which bind to the Asp residue (Figure S.28). This sequence pattern can be used to search for possible two-sheet p structures in databases of amino acid sequences of proteins of unknown structure. 

The use of alkali and alkaline earth group metal ions, especially those of sodium, potassium, magnesium, and calcium, for maintenance of electrolyte balance and for signaling and promotion of enzyme activity and protein function are not discussed in this text. Many of these ions, used for signaling purposes in the exciting area of neuroscience, are of great interest. In ribozymes, RNAs with catalytic activity, solvated magnesium ions stabilize complex secondary and tertiary molecular structure. Telomeres, sequences of DNA at the ends of chromosomes that are implicated in cell death or immortalization, require potassium ions for structural stabilization. 

Chen B, Costantino HR, Liu J, Hsu CC, Shire SJ. Influence of calcium ions on the structure and stability of recombinant human deoxyribonuclease I in the aqueous and lyophilized states. J Pharm Sci 1999 88(4) 477 182. 

Both cations play a stabilizing role in various biological structures. Calcium ions are necessary to link the 24-subunit aggregates of the dioxygen carrier hemocyanin of Limulus polyphemus into the native 48-subunit molecule.121 The divalent character of Ca2+ probably allows it to crosslink... 

Aqualysin I is an alkaline serine protease, extracellularly secreted by Thermus aquaticus YT-1,14) an extremely thermophilic, Gram-negative bacterium. The enzyme is thermostable. Optimum temperature for the proteolytic activity of aqualysin I is 80° C in the presence of 1 mM Ca2+ and 70°C in its absence (Fig. 12.1).l5) 85% of the original activity remains after treatment at 80°C for 3 h in the presence of 1 mM Ca2+, but only about 20% remains in its absence.15) These results indicate that calcium ion is essential for the structural stability of the enzyme. 

The catalytic activity of porcine pancreatic a-amylase, another member of the glucosyl hydrolase family, is metal cofactor-dependent [53]. A calcium ion-binding site is located at the interface of an antiparallel f-sheet, inserted in one of the loop regions of the /ia-barrel, and the core structure. In the calcium-bound state, the insertion stabilizes the substrate-binding site, and indirectly constrains part of the active site in a catalytically competent conformation. 

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