Glycomacropeptide

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Glycomacropeptide (GMP) is a small, sugar-containing piece of milk protein that forms when milk curdles with rennet. It comes from sweet whey. If milk is curdled with acid (as in yogurt or acid-curdled cheese) without rennet, GMP is not produced. The unglycosylated form is called caseinomacropeptide (CMP). GMP and CMP occur in roughly similar amounts in whey.

Together GMP and CMP make up about 20–25% of whey protein, making them the third largest part of whey protein isolate after alpha-lactalbumin and beta-lactoglobulin. GMP forms when the enzyme chymosin cuts the casein micelle and removes the 64 terminal peptides of kappa-casein, leaving para-kappa-casein behind. Vegetable rennets cut at the same spot and also produce GMP.

GMP is unique because kappa-casein is the only glycosylated casein protein, and GMP itself is glycosylated. The attached sugars make GMP water-soluble, so it is the part that dissolves into whey after curdling. Additionally, GMP is said to be the only easily obtainable dietary peptide that does not contain any aromatic amino acids, making it a safe source for people with phenylketonuria (PKU).