Protein Cross-linking: Enzyme Resistant Bond in Proteins

It happened due to action of high temperatures and drying. And there is production of non-peptide bonds, highly resistant to proteolysis (unnatural covalent bonds), resulting in lower digestibility and bioavailability products.

DEHIDROALANINE is a very important product, because it is a very reactive molecule. It is a strong electrophile (lacks electron). So when you introduce into the reaction a molecule with excess of electron with the molecule that lacks an electron, you will produce a covalent bond.  A compound with excess of electron (nucleophile) would be Nitrogen, meaning that a protein can be a good electron donating (lysine). So lysine (N-Standing amine group, with excess of electrons) can react with dehydroalanine, creating an adduct and producing LISINOALANINE, it is not a peptide (not a dipeptide because there isn’t a peptide bond), it is a dimer or di amino acid, but nod a peptide, meaning that it cannot be digested (enzymes cannot break it).

Ornitine can react also with dehidroalanine and get ornithoalanine.

Cysteine can react with the sulfridril group and produce LANTIONINE (Adduct between Cysteine and dehydroalanine). So by doing this you are producing protein crosslinks, producing dimers, trimmers, polymers.

Consequences: produce non peptide bonds, so it becomes undigestable. Loosing essencial aminoacids (loose alanine). So nutritional value decreses (1. Reduce digestability, and 2. reducingessencial aminoacids). Lisinoalanine is shown to be toxic for red blood cells in animal studies.

There is another kind of protein crosslinking reaction. For example glutamin can react with lysine (amonia is lost) producing an adduct lisinoglutamate, this is an amine bond but not a peptide bond. This is produced by heat and drying.  It also can happen with asparagine. The consequences are the same.

The reaction can also happen enzymatically, transglutaminase (meat glue). It is possible to glue together pieces of meat by crosslinking proteins using transglutaminese.

By making bigger these proteins, it is possible to increase its allergenicity.

In maillard reaction we will also be confronted with protein crosslinking