Proteins enzymatic crosslinking

Lorenzen, P.C., Mautner, A., and Schlimme, E. 2000. Enzymatic crosslinking of proteins in the production of milk products In Proceedings of the 1st International Symposium on Enzymatic Protein Processing (H. Gruppen and W. van Hartingsveldt eds), pp. 163—169. TNO Nutrition and Food Research Institute, Zeist, The Netherlands. 

Formaldehyde fixes proteins in tissue by reacting with basic amino acids— such as lysine,5 7—to form methylol adducts. These adducts can form crosslinks through Schiff base formation. Both intra- and intermolecular cross-links are formed,8 which may destroy enzymatic activity and often immunoreactiv-ity. These formaldehyde-induced modifications reduce protein extraction efficiency and may also lead to the misidentification of proteins during proteomic analysis. 

F g- 13.13 Principle of protein crosslinking by enzymatic oxidation of lactose coupled with the Maillard reaction [35-37]. 

Selenium plays a special role in development and protection of spermatozoa (Chapter 15). Tire selenoprotein phospholipid hydroperoxide glutathione peroxidase (PHGPx Eq. 15-58, Table 15-4) has a high activity in the testis and in spermatids. However, in mature spermatozoa it forms an enzymatically inactive oxidatively crosslinked capsular material around the midpiece of the cell perhaps providing mechanical stability.268 A similar 34-kDa selenoprotein is present in sperm nuclei and may be essential for condensation of DNA.269 Sperm tails contain specialized cytoskele-tal proteins which form "outer dense fibers."270 In contrast to mammalian spermatozoa, nematode sperm move by ameboid motility that depends upon a specialized actin-like molecule.271 Sperm cells are unusually rich in polyamines, most of which are bound to RNA and DNA (Chapter 24). 

Formaldehyde is a toxic substance that reacts spontaneously with amino groups of proteins and nucleic acids, hydroxymethylating them and forming methylene-bridge crosslinks between them. Free formaldehyde therefore wreaks havoc in living cells and could not serve as a useful hydroxymethylating agent. In the form of A5,A10-methylenetetrahydrofolate, however, its chemical reactivity is attenuated but retained in a potentially available form where needed for specific enzymatic action. Formate, how-... 

Fibrin polymerization is initiated by the enzymatic cleavage of the fibrinopeptides, converting fibrinogen to fibrin monomer (Fig. 1). Then, several nonenzymatic reactions yield an orderly sequence of macromolec-ular assembly steps. Several other plasma proteins bind specifically to the resulting fibrin network. The clot is stabilized by covalent ligation or crosslinking of specific amino acids by a transglutaminase, Factor XHIa. 

Konno, K., Hirayama, C., Yasui, H., and Nakamura, M., Enzymatic activation of oleuropein A protein crosslinker used as a chemical defense in the privet tree, Proc. Nat. Acad. Sci. USA, 96, 9159, 1999. 

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