Plants contain a number of well characterised proteases but their exact function is unknown. Proteases in plants may aid the mobilisation of nutrients from the seeds during germination, may aid in fruit ripening, as well as being integral to normal. cellular metabolism. Bromelain is a crude extract of pineapple (Ananas comosus) that contains proteolytic action. Bromelain is absorbed and bioavailable in humans and may be beneficial in fibrinolysis, act as an anti-inflammatory and anti-coagulant, and may play a role in preventing platelet aggregation. Bromelain is a cysteine protease that belongs to a family of cysteine proteases found in plants. Other member of this family include actinidin from kiwifruit (Actinidia chinensis), papain from papaya (Carica papuyu), aulerain, EP-A and EP-B from barley (Hordeum vulgare), and ficin from figs (Ficus sp.). Generally cycteine proteases have a wide substrate specificity that includes most commonly found proteins such as gelatine, casein, albumin, muscle tissue and elastin.
The cysteine proteases are similar in nature to the lysosomal protease, cathepsin B and cathepsin A in humans. In humans these proteins are compartmentalised at a low pH in the lysosome, which prevents unwanted interaction with cellular components. In plants, proteases are located in various parts of the plants including the fruits of figs and kiwifruits, and the stem and fruits of pineapples. In kiwifruit, actinidin can accumulate and account for up to 60% of soluble protein present in the fruit. Cysteine proteases are able to tenderise meat on account of their proteolytic activity, and have been used in the food industry for this purpose. For example, 10% kiwifruit juice is able to effectively tenderise lamb and increase the production of shorter peptides. Papain, fucin and bromelain have all shown to be effective in the tenderising of steaks assessed by both sensory and instrumental analysis.
Cysteine proteases are able to break down protein at particular bonds because of the specific structural characteristics possessed by the enzyme. In their active site, cysteine proteases contain a histidine residue with a basic side chain, that is able to deprotonate an adjacent thiol group (also in the enzyme active site). The deprotonated cysteine now possesses a negatively charged (anionic) sulphur that is able to react with the carbonyl carbon of the substrate protein, forming a thioester bond with its carboxy terminus. The amino terminus of the substrate protein is released, forming a new protein or peptide of reduced length. Hydrolysis of the thioester bond forms a carboxy terminus on the remnants of the old protein and a second shorter protein fragment is released. Release of the second protein fragment regenerates the enzyme and reprotonates the thiol group of the enzyme.
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