CHEM3501 Lab3: Isolation and Characterization of Proteins
Casein is globular protein, and one of the predominant types of protein found in milk (Brown, 2008). It consists of four proteins – alpha, beta, kappa, and gamma caseins that arrange themselves into large random coils (Brown, 2008). They account for almost 80% of the protein in milk can react as either acids or bases depending on the pH conditions (Brown, 2008). Like most proteins, casein’s integrity and structure could be altered through changes in pH and temperature. Casein is considered a phosphoprotein and acts as a storage protein, carrier for Ca2+ and PO43- ions due to its special structure (de Zoete, 2010). The side chains of amino acids in casein, like serine and threonine, have phosphate groups attached to them and become phosphorylated at the natural pH of milk of 6.6 (Anonymous, 2010). The normally three-negatively charged phosphate PO43- ion becomes a dianion (two-negatively charged). The dianion attracts calcium cation. Therefore, casein, phosphate anions PO43-, and calcium Ca2+ form a compound called calcium caseinate, or calcium phosphocaseinate, a type of salt. Formation of Calcium Caseinate:
At natural pH of milk, 6.6, casein is negatively charged and remains highly soluble (de Zoete, 2010). Calcium ions surround casein-phosphate; this action prevents caseins from coagulating into curd (Brown, 2008).However, as pH lowers casein become less negatively charged when the hydrogen ions H+ neutralize the phosphate ions. When the pH of milk reaches 4.6, casein becomes isoelectric (zero net charge), non-polar, and highly insoluble in milk. (Milk is 80% water). The calcium ions detach from the formerly negatively charged casein, which leads to precipitation of casein proteins and formation of curds. In addition, as pH decreases, peptide bonds are weakened because the oxygen in the carbonyl group forms bond with excess hydronium ions H3O+. The peptide bond eventually...
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