Sandbox 43: Difference between revisions

α-lactalbumin is composed of nine <scene name='Sandbox_43/Adfa/1'>α-helices</scene> (purple), eleven beta turns, two beta hairpin turns, and three strands.  The three strands make up an antiparallel <scene name='Sandbox_43/Beta_sheets/2'>β-sheets</scene> (blue), the nine helices are involved in 6 helix-helix interactions.<ref name="iv"> </ref>  The primary structure can also been seen in the picture below.  <scene name='Sandbox_43/Secondary_structures/1'>Secondary structure</scene> can be seen here in reference to the overall tertiary structure.  α-helices are represented by the pink rockets, while the β-sheets are shown as golden arrows.

α-lactalbumin is a small protein with calcium ions as cofactors.  The binding of the calcium ion increases the stability of the protein in its native conformation and makes the folding of the protein much faster.  The binding of the calcium ion acts of a nucleus for the stabilization of the tertiary structure in the protein, without it the process is much slower.<ref>Natalia A. Bushmarina, Clement E. Blanchet, Gregory Vernier, and Vincent Forge. 2006. Cofactor effects on the protein folding reaction: Acceleration of a-lactalbumin refolding by metal ions ''Protein Science'' 15:659–671</ref>  In α-lactalbumin native conformation the calcium ion is bound to a unique binding loop.<ref name="ii"> </ref>  The calcium binding site is located in the β-domain and is formed by three Asp side chains and two mainchain carbonyls. This site the calcium ion has a pentagonal bypyramidal coordination.<ref name="iv"> </ref> <ref name="iii"> </ref>  A secondary calcium binding site involves the residues Thr, Gln, Leu, and Asp.  In this site the calcium ion has a tetrahedral coordination.<ref name="iv"> Protein Data Bank. 2009. European Bioinformatics Institute. <http://www.ebi.ac.uk/thornton-srv/databases/pdbsum/> Retrieved Sept.19,2009.</ref>  The <scene name='Sandbox_43/Cal_res/1'>residues</scene> that come into contact with the calcium ion are shown to the right.  Protection from thermal, guanidine HCL and urea denaturation is provided by the stability given to the protein from the calcium ion binding.  The calcium-binding site has also been shown to weakly bind Mg2+, Na2+, and K+ also.  Removal of the calcium ion has shown to induce a conformational change.  In the presence of denaturants or absence of calcium ions α-lactalbumin adopts the molten globule state and characterized by the conserved secondary structure but fluctuating tertiary structure.<ref name="ii"> </ref>  α-lactalbumin, in its native state, possesses a relatively strong Zn2+ site causing subtle changes in α-lactalbumin structure upon binding to the calcium loaded protein.  The Zn2+ is important in the binding of glucose in the lactose synthase complex.<ref name="ii"> </ref>