Biological Roles and Production Technologies Associated with Bovine Glycomacropeptide

Glycomacropeptide (GMP) is a casein‐derived whey peptide found in ‘sweet’ whey. The addition of chymosin to milk during cheese making enzymatically hydrolyses or cleaves the milk protein (kappa‐casein) into two peptides, an insoluble peptide (para‐kappa‐ casein) and a soluble hydrophilic glycopeptide (GMP), as shown in Figure 1.1. The larger peptide, para‐kappa‐casein, contains the amino acid residues 1–105 and becomes coagu­ lated and incorporated into the cheese curd. The smaller peptide, which contains the amino acid residues 106–169 (GMP), becomes soluble and is incorporated into the whey (Walstra et al. 2006). GMP is the third most abundant whey protein, after beta‐lacto­ globulin and alpha‐lactalbumin, accounting for approximately 15–25% (1.2–1.5 g L−1) of the total whey protein (Thomä‐Worringer et al. 2006). GMP is highly polar and has unique characteristics due to the absence of phenylalanine, tryptophan, tyrosine, histidine, argi­ nine or cysteine residues (Neelima et al. 2013). The peptide is rich, however, in branched chain amino acids, such as isoleucine and valine (Marshall 2004; Krissansen 2007). At least 13 genetic variants of bovine kappa‐casein have been identified which have different post‐translational modifications (PTMs) and vary in their level of phospho­ rylation and glycosylation (Thomä‐Worringer et al. 2006). The average molecular weight for GMP is 7500 Da, whereas the highest recorded molecular weight is 9631 Da (Mollé and Léonil 2005). It has been suggested that the peptide has the ability to associ­ ate and dissociate under certain pH conditions, possibly explaining why molecular weights of between 14 and 30 kDa are observed via SDS‐PAGE (Galindo‐Amaya 2006; Farías et al. 2010). Given the heterogeneity of GMP, there is no single isoelectric point (pI) assigned to GMP but the pI of the peptide portion is approximately 4, varying with PTM. Approximately 60% of GMP consists of O‐linked carbohydrate chains which are com­ posed of mainly galactose (gal), N‐acetyl galactosamine (GalNAc) and N‐neuraminic acid (Neu5Ac) attached at threonine residues. Saito et al. (1991) determined via high‐ performance liquid chromatography (HPLC) the distribution of monosaccharide, Biological Roles and Production Technologies Associated with Bovine Glycomacropeptide Shane Feeney, Lokesh Joshi, and Rita M. Hickey