Actinidain is commercially useful as a meat tenderiser[8][9] and in coagulating milk for dairy products, like yogurt and cheese.[10] The denaturation temperature of actinidain is 60 °C (140 °F), lower than that of similar meat tenderising enzymes bromelain from pineapple and papain from papaya.[11]
History
Actinidain was first identified in 1959 when A.C. Arcus looked into why jellies made with kiwifruit don’t solidify.[12] They went on to show that this phenomenon was caused by a proteolytic enzyme attacking gelatin.[12] This enzyme would go on to be named actinidin as it was identified in a fruit in the genus Actinidia.[12] While similar proteins have been found in other fruits, this cysteine protease is unique to the kiwifruit.[13][12] A thiol group was identified to be essential for enzyme activity, which is why it was grouped with enzymes like papain and bromelain.[14][15]
Function
While no clear function has been identified, the enzyme begins to accumulate in the fruit early on and is suspected to be important for fruit development.[16] Actinidain has been found to have a detrimental effect on the larvae of Spodoptera litura, however not enough research has been done into whether the enzyme can be used as a pesticide.[13] It may also be used as a storage protein.[17]
Sequence and structure
Actinidain has an enzyme classification number (EC) of 3.4.22.14. The 3 classifies it as a hydrolase.[18] 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.[18] Actinidain is first produced in the kiwi when it is about half its size and then increases in both protease activity and enzyme production until the fruit is fully matured.[13] The enzyme is encoded by a large gene family and is expressed in most tissues of the kiwifruit plant, not just the fruit itself.[13]
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.[2][14] Electron density mapping shows similar α-helices and overall polypeptide folding.[2][14] 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.[14][15] The active site includes cysteine and histidine residues that are conserved across several other proteins in the fruit peptidase family.[15] 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,[14] 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.[14] Actinidain comprises up to 50% of the kiwifruit’s soluble protein content at harvest.[19] Actinidain is active over a wide range of pH, including very acidic conditions,[20] with a pH optimum from 5-7.[21] 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).[19]
Human health impacts
Actinidain is able to function at low acidities (pH 1-2) that are found in the human GI tract and therefore is found to assist with protein digestion in the stomach and small intestine.[20][22] Actinidain enhances the human body’s ability to digest food, particularly when working together with pepsin and pancreatin, by hydrolyzing food proteins more efficiently than human digestive enzymes.[23] Further work is being done into the usefulness of kiwifruit as a digestive aid.
Actinidain is the major allergen in kiwifruit.[19][20] There does not appear to be any trend when looking at who is allergic to kiwi as it varies within age, geographical differences, and other characteristics clinicians use to track allergens, although the allergy often presents itself as mild symptoms in the mouth.[20] Actinidain provokes both IgG and IgE responses antibody responses, with the IgE binding activity being associated with the severe (anaphylaxis) responses.[19]
Potential applications
Actinidain is used as a high-quality meat tenderizer.[19] When marinating with pork, actinidain was found to tenderize it by affecting the myofibrils and the connective tissue, which are similar to the tissues that are broken down through mechanical tenderization.[24][25]
Studies have shown that actinidain might be a good alternative milk coagulant, replacing chymosin, a common coagulant used in cheese making.[26]
References
^Baker EN, Boland MJ, Calder PC, Hardman MJ (November 1980). "The specificity of actinidin and its relationship to the structure of the enzyme". Biochimica et Biophysica Acta (BBA) - Enzymology. 616 (1): 30–34. doi:10.1016/0005-2744(80)90260-0. PMID7002215.
^ abcKamphuis IG, Drenth J, Baker EN (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. 182 (2): 317–329. doi:10.1016/0022-2836(85)90348-1. PMID3889350.
^Maddumage R, Nieuwenhuizen NJ, Bulley SM, Cooney JM, Green SA, Atkinson RG (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. 61 (3): 728–739. doi:10.1021/jf304289f. PMID23289429.
^Cavic M, Grozdanovic MM, Bajic A, Jankovic R, Andjus PR, Gavrovic-Jankulovic M (October 2014). "The effect of kiwifruit (Actinidia deliciosa) cysteine protease actinidin on the occludin tight junction network in T84 intestinal epithelial cells". Food and Chemical Toxicology. 72: 61–68. doi:10.1016/j.fct.2014.07.012. PMID25042511.
^Bekhit AA, Hopkins DL, Geesink G, Bekhit AA, Franks P (2014). "Exogenous proteases for meat tenderization". Critical Reviews in Food Science and Nutrition. 54 (8): 1012–1031. doi:10.1080/10408398.2011.623247. PMID24499119. S2CID57554.
^Eshamah H, Han I, Naas H, Acton J, Dawson P (April 2014). "Antibacterial effects of natural tenderizing enzymes on different strains of Escherichia coli O157:H7 and Listeria monocytogenes on beef". Meat Science. 96 (4): 1494–1500. doi:10.1016/j.meatsci.2013.12.010. PMID24447905.
^Katsaros GI, Tavantzis G, Taoukis PS (January 2010). "Production of novel dairy products using actinidin and high pressure as enzyme activity regulator". Innovative Food Science & Emerging Technologies. 11 (1): 47–51. doi:10.1016/j.ifset.2009.08.007.
^ abcdefBaker EN (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. 115 (3): 263–277. doi:10.1016/0022-2836(77)90154-1. PMID592367.
^Praekelt UM, McKee RA, Smith H (May 1988). "Molecular analysis of actinidin, the cysteine proteinase of Actinidia chinensis". Plant Molecular Biology. 10 (3): 193–202. doi:10.1007/BF00027396. PMID24277513. S2CID21213015.
^Chalabi M, Khademi F, Yarani R, Mostafaie A (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. 172 (8): 4025–4037. doi:10.1007/s12010-014-0812-7. PMID24604128. S2CID44438930.
^ abcdeDearman RJ, Beresford L, Foster ES, McClain S, Kimber I (May 2014). "Characterization of the allergenic potential of proteins: an assessment of the kiwifruit allergen actinidin". Journal of Applied Toxicology. 34 (5): 489–497. doi:10.1002/jat.2897. PMID23754484. S2CID12609478.
^Kaur L, Rutherfurd SM, Moughan PJ, Drummond L, Boland MJ (April 2010). "Actinidin enhances gastric protein digestion as assessed using an in vitro gastric digestion model". Journal of Agricultural and Food Chemistry. 58 (8): 5068–5073. doi:10.1021/jf903332a. PMID20232890.
^Kaur L, Rutherfurd SM, Moughan PJ, Drummond L, Boland MJ (April 2010). "Actinidin enhances protein digestion in the small intestine as assessed using an in vitro digestion model". Journal of Agricultural and Food Chemistry. 58 (8): 5074–5080. doi:10.1021/jf903835g. PMID20232891.
^Christensen M, Tørngren MA, Gunvig A, Rozlosnik N, Lametsch R, Karlsson AH, Ertbjerg P (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. 89 (9): 1607–1614. Bibcode:2009JSFA...89.1607C. doi:10.1002/jsfa.3633. ISSN0022-5142.
^Anaduaka EG, Chibuogwu CC, Ezugwu AL, Ezeorba TP (2023-04-03). "Nature-derived ingredients as sustainable alternatives for tenderizing meat and meat products: an updated review". Food Biotechnology. 37 (2): 136–165. doi:10.1080/08905436.2023.2201354. ISSN0890-5436. S2CID258559035.