Actinidain

History

Actinidain was first identified in 1959 when A.C. Arcus examined why jellies made with kiwifruit did not solidify, an effect caused by a proteolytic enzyme acting on gelatin. This enzyme was named actinidin as it was identified in a fruit of the genus Actinidia (Actinidia chinensis). While similar proteins have been found in other fruits, this cysteine protease is unique to the kiwifruit. A thiol group was identified to be essential for enzyme activity, which is why it was grouped with enzymes like papain and bromelain.

Function

While no clear function has been identified, the enzyme begins to accumulate in the immature fruit and is suspected to be important for fruit development. Actinidain has a detrimental effect on the larvae of Spodoptera litura, although its use as a pesticide has not been established. It may also be used as a storage protein.

Sequence and structure

Actinidain has an enzyme classification number (EC) of 3.4.22.14. The 3 classifies it as a hydrolase. It is further classified as acting on peptide bonds, also known as a peptidase (3.4). The .22 represents the cysteine endopeptidases and then the .14 is actinidain’s unique identifier within that group. Actinidain is first produced in the kiwifruit when it is about half its size and then increases in both protease activity and enzyme production until the fruit is fully matured. The enzyme is encoded by a large gene family and is expressed in most tissues of the kiwifruit plant, not just the fruit itself.

Actinidain is similar to papain in size, shape, active site location and conformation, as well as in kinetic studies, which is especially interesting as they only share 48% amino acid similarity. Electron density mapping shows similar α-helices and overall polypeptide folding. While the electron density map indicates 218 amino acids, further sequencing work suggests 220 amino acids with the extra two being found at the C-terminus. The active site includes cysteine and histidine residues that are conserved across several other proteins in the fruit peptidase family. Electron density mapping indicates a double crossover with domain 1 being made up of AA 19-115 and 214-218 and domain II composing of AA 1-18 and 116-213, with both the N-terminal and the C-terminal ends crossing over into both domains. Domain 1 has several α-helices whereas domain 2 is primarily made up of one anti-parallel β-sheet. Actinidain comprises up to 50% of the soluble protein content in kiwifruits at harvest. Actinidain is active over a wide range of pH, including very acidic conditions, with a pH optimum from 5-7. At least ten different isoforms that all have the same molecular weight and cysteine protease activity as actinidain have been identified but they vary in isoelectric point from acidic (pI 3.9) to basic (pI 9.3).

Allergic potential

Actinidain is the major allergen in kiwifruit, with the reaction presenting as mild symptoms in the mouth.

Food applications

Actinidain is used as a high-quality meat tenderizer. When marinating with pork, actinidain was found to tenderize it by affecting the myofibrils and connective tissue, which are similar to the tissues that are broken down through mechanical tenderization.

Studies have shown that actinidain might be a good alternative milk coagulant, replacing chymosin, a common coagulant used in cheese making.

References

References

1. (November 1980). "The specificity of actinidin and its relationship to the structure of the enzyme". Biochimica et Biophysica Acta (BBA) - Enzymology.
2. (March 1985). "Thiol proteases. Comparative studies based on the high-resolution structures of papain and actinidin, and on amino acid sequence information for cathepsins B and H, and stem bromelain". Journal of Molecular Biology.
3. (1987). "Active Sites of Enzymes". John Wiley and Sons.
4. (May 2006). "Temperature-dependences of the kinetics of reactions of papain and actinidin with a series of reactivity probes differing in key molecular recognition features". The Biochemical Journal.
5. (January 2013). "Diversity and relative levels of actinidin, kiwellin, and thaumatin-like allergens in 15 varieties of kiwifruit (Actinidia)". Journal of Agricultural and Food Chemistry.
6. (2014). "Exogenous proteases for meat tenderization". Critical Reviews in Food Science and Nutrition.
7. (April 2014). "Antibacterial effects of natural tenderizing enzymes on different strains of Escherichia coli O157:H7 and Listeria monocytogenes on beef". Meat Science.
8. (January 2010). "Production of novel dairy products using actinidin and high pressure as enzyme activity regulator". Innovative Food Science & Emerging Technologies.
9. (2008). "Ingredients in meat products properties, functionality and applications". Springer.
10. (May 1959). "Proteolytic enzyme of ''Actinidia chinensis''". Biochimica et Biophysica Acta.
11. (June 2005). "Effects of kiwifruit (''Actinidia deliciosa'') cysteine protease on growth and survival of Spodoptera litura larvae (Lepidoptera: Noctuidae) fed with control or transgenic avidin-expressing tobacco". New Zealand Journal of Crop and Horticultural Science.
12. (September 1977). "Structure of actinidin: details of the polypeptide chain conformation and active site from an electron density map at 2-8 A resolution". Journal of Molecular Biology.
13. (July 1978). "The amino acid sequence of the tryptic peptides from actinidin, a proteolytic enzyme from the fruit of Actinidia chinensis". The Biochemical Journal.
14. (May 1988). "Molecular analysis of actinidin, the cysteine proteinase of Actinidia chinensis". Plant Molecular Biology.
15. (April 2014). "Proteolytic activities of kiwifruit actinidin (Actinidia deliciosa cv. Hayward) on different fibrous and globular proteins: a comparative study of actinidin with papain". Applied Biochemistry and Biotechnology.
16. "Information on EC 3.4.22.14 - actinidain".
17. (May 2014). "Characterization of the allergenic potential of proteins: an assessment of the kiwifruit allergen actinidin". Journal of Applied Toxicology.
18. (December 2018). "The nutritional and health attributes of kiwifruit: a review". European Journal of Nutrition.
19. (June 1970). "Anionic proteinase from Actinidia chinensis. Preparation and properties of the crystalline enzyme". European Journal of Biochemistry.
20. (July 2009). "Injection of marinade with actinidin increases tenderness of porcine M. biceps femoris and affects myofibrils and connective tissue". Journal of the Science of Food and Agriculture.
21. (2023-04-03). "Nature-derived ingredients as sustainable alternatives for tenderizing meat and meat products: an updated review". Food Biotechnology.
22. (2011-03-22). "Actinidin: A Promising Milk Coagulating Enzyme". European Journal of Nutrition & Food Safety.