Polyphenol Oxidases (PPO) (E.C.1.10.3.1), also termed catechol oxidases, catecholases, diphenol oxidases, ortho-diphenolases, phenolases, and tyrosinases [1]. They are a group of copper-containing enzymes that catalyze the o-hydroxylation of monophenols to o-diphenols as well as the oxidation of o-diphenols to quinones in the presence of oxygen [2]. Polyphenol oxidase is broadly distributed among animals, fungi, and plants, although the studies are more extensive in plants [3, 4]. The study of PPOs in plants has focused primarily on their role in the process of postharvest browning, whereby cut or damaged plant tissues turn brown due to the polymerization of PPO-generated quinones, generating phyto-melanins [5]. The enzyme can catalyze the oxidation of polyphenols and result in the browning of damaged or cut plant, which seriously affects quality and reduce the market value of the crop [2, 6]. Polyphenol oxidase is widely present in plants, play an important role in the growth, development, and stress responses. Many studies have reported that PPO and peroxidases are induced in response to biotic and abiotic stress in plants, and it has been implicated in several functional processes such as participating in plant defense and the synthesis of plant-specific metabolites [7, 8].
Plant PPO generally contain three conserved regions, N-terminal cTP, aCuA and CuB domain and a C-terminus [9], which are responsible for thylakoid lumen localization and enzyme activity. Polyphenol oxidase is found in many plant species such as banana [10], apple [11], potato [12], eggplant [13], strawberry [14], red cocoyam [15], bitter leaf [16] etc. The function and distribution of PPO differ in various plants [9]. Most PPO are transported to the thylakoid membrane in the chloroplast, or in cytosol and other organelles [17], whereas the phenolic compounds are localized to the vacuoles. Because of the different localization of the enzyme and its substrates, their interaction requires destruction of the cell and mechanical damage [18]. Plant PPOs have reported to possess various applications such as synthesis of drugs and other organic compounds [19].
Turmeric (Curcuma longa) is a perennial rhizomatous crop of the Zingiberaceae, a world-wide known spice whose medicinal properties has received interest from both the medical and scientific world as well as culinary enthusiasts, as it is the major source of the polyphenol curcumin [20]. Turmeric contains 3 to 6% polyphenolic compounds which is known as curcuminoids [21]. It aids in the management of oxidative and inflammatory conditions, metabolic syndrome, arthritis, anxiety, and hyperlipidemia. Most of these benefits can be attributed to its antioxidant and anti-inflammatory effects [20]. Like several plants, it suffers a reduction in its sensory qualities and health benefits as a result of spoilage which occurs due to browning. Recent research of novel anti-PPO systems is focused on mild alternatives to conventional treatments which could impair not only the sensory and nutritional properties of agro-food products but also the consumer health [22]. Milder processes of controlling enzymatic browning can be discovered as PPO studies are being carried out and this will help improve the shelf-life of turmeric and its products. Many researches have been conducted using turmeric rhizomes in many forms of investigation in the area of its characteristics, functionality, and applications [23]. Specifically, several studies have reported that turmeric possesses potent multiple properties such as anti-inflammatory, antioxidant, antitumor, antibacterial and anticoagulant, and antidiabetic based on its free-radical-scavenging activity expressed by the domicile bioactive compound. Among the bioactive components of turmeric, curcumin is the most frequently studied, it is fat soluble bioactive compound, whose characteristics and functionality is equable to its popularly reported medicinal, pharmacological (hypoglycemic, insulinotropic, and hypolipidemic) [24]. The potency of turmeric anti-oxidant and anti-inflammatory properties improves symptoms of depression, arthritis and Alzheimer’s disease. It contains natural antioxidants, polyphenols and phytochemicals which confer significant protection against free radicals’ damage [19, 25].
Different sources of polyphenol oxidase have been reported including bacteria, fungi, animal and plants [26]. The PPO found in humans is responsible for skin pigmentation including development of freckles [27]. Plant PPO has an important role in plant stress resistance and physiological metabolism. Most of the PPO in plants are found in the chloroplasts of photosynthetic cells and the leucoplasts of storage cells [28]. Polyphenol oxidase is one of the most important industrial enzymes, considering its wide applications in several industrial and biotechnological application. There is need for continual searching for cheaper and readily available sources of the enzyme. Sufficient information on the relative occurrence and some physio-chemical properties of polyphenol oxidase from turmeric had been earlier reported [19]. However, in that work, the much-needed information on kinetics, inhibition studies and molecular docking of some important inhibitors and/or anti-browning agents of the purified turmeric PPO was lacking and greatly necessary. This was with a view to providing information on its mechanism of catalytic reactions, specificity towards substrates and binding interactions with inhibitors and anti-browning agents through molecular docking. Hence, the reason for this study. These could be deployed in several biotechnological applications and primarily for understanding the strategy in the control of browning in turmeric food products.