Holo-SlPPO1 and the pH Optimum of the PPO Enzyme
The mass spectra of latent SlPPO1 provide information about the charge states of the PPO enzyme. This enzyme is a critical component in the enzymatic degradation of polysaccharides. A few recent studies have examined the holo-SlPPO1 structure and the optimum pH of the enzyme. The effect of a substrate stabilizing agent on PPO enzyme activity is also discussed.
X-ray structure analysis of holo-SlPPO1
The holo-SlPPO1 protein exhibited the highest affinity for phloretin. However, the binding affinity was not as high as that for phloretin. This observation has several implications. We conducted docking studies on the holo-SlPPO1 protein to identify binding poses for phenolic and monophenolic substrates. We set the exhaustiveness of docking at 100.
The overall structure of holo-SlPPO1, compared with the apo-SlPPO1, is slightly different from that of the apo-SlPPO1 protein. The copper ions bridge the oxygen moiety in apo-SlPPO1 while the water molecule is situated beneath it in holo-SlPPO1. This structure also reveals that the gatekeeper residue is different in apo-SlPPO1, which is not present in the holo-form.
The holo-SlPPO1 protein’s structure was determined with a resolution of 1.85 A. The crystal structure closely resembles other plant PPOs such as the catalytic enzyme Ipomoea batatas, the aurone synthase in Vitis vinifera, and the apo-SlPPO1 protein from Coreopsis Grandiflora.
Another exciting feature of the holo-SlPPO1 structure is that it displays a strong electron density peak near the water molecule equivalent. This peak is quickly assigned to a hydrogen atom and has a unique elliptical shape, while all the other hydrogen atoms nearby are spherical. The structure was deposited in the Protein Databank (PDB) as 3O4P.
pH optimum of SlPPO1
The pH optimum for SlPPO1 is found in fruits such as avocado, mango, green bean, and jonagored apple. The enzyme is most stable at these pH values. Then, pH values gradually increase from two to four and five to eight and then decrease at nine. However, it is possible to achieve higher levels of PPO activity by adjusting the pH of these fruits.
This pH optimum was found in an experiment involving the SlPPO1 enzyme. PPO activity was measured for a pH range from 4.0 to 8.0 using catechol as a substrate. The optimal pH was discovered to be 4.5-5.5. For PPO I and PPO II, the optimal pH was 5.5. The pH optimum for PPO activity varies according to the enzyme source, substrate, buffer system, and substrate. The literature contains several reports on pH optimums for PPO activity.
The substrate specificity of SlPPO1 PPO is determined using a series of buffer solutions. Various levels of pH influence the ionization state of the substrate and enzyme combination. In one study, BRP activity was influenced by pH using pyrogallol as a substrate. The pH optimum of BRP was 8.0, which was considered the optimal pH for sweet melon BRP.
Comparison of SlPPO1 and SlPPO2
Two different PPO enzymes are found in S. Lycopersicum, and their activity is quite different. SlPPO1 contains a serine residue at Ser240, which contradicts the notion that the TYR enzyme needs asparagine to be active. On the other hand, SlPPO2 contains alanine or glycine at this position. The similarities between the two enzymes are minimal, and the differences are not significant.
The main differences between the two enzymes lie in their structure. SlPPO1 has a disulfide bond at its center, while SlPPO2 has two. SlPPO1 also lacks a thioether bridge in its central region. Moreover, it lacks a sizeable solvent-exposed loop and a Cys97-Leu117 loop. This makes it difficult to differentiate the two enzymes, despite their structural similarities.
Both enzymes are capable of generating chromophoric quinones from diphenols. In addition to this, the enzymes can convert phenolic compounds into flavonoids. The reaction between these molecules generates diphenols, namely tyramine, dopamine, and caffeic acid. The kinetic measurements were performed in a 96-well microplate containing 200 ul total volume and 50 mM Tris-HCl buffer. Various molarities of substrates and enzymes were used for the experiments. 1.5 mM SDS was used for activation.
Both SlPPO1 and SlPIPO2 enzymes exhibit high specificity for monophenolic substrates. They are faster and more specific than tyramine but slower than phloretin, a more demanding and sterile compound. Nevertheless, both have similar PPo2 activity. However, SlPPO1 is more specific than SlPPO2.
Effect of substrate-stabilizing effect on PPO enzyme activity
Substrates with a negative impact on food quality are affected by the presence of PPO enzymes. The negative impact of these enzymes is often attributed to their role in post-harvest browning. Quinone reactions in the presence of PPOs produce a brownish color in plants and fresh produce. These reactions may have positive effects in some instances, such as preserving protein in forage crops. However, browning reactions are generally thought to affect food processing negatively, and much research has been conducted on them.
Despite PPO enzyme activity being important to plant, fungal, bacterial, and animal physiology, it has many biotechnological applications. Applications range from food sensors to the treatment of processing waste streams. Some examples of biotechnological applications of PPO include:
PPO activity in plant cells depends on a range of conditions, including the pH, concentration, and purity of the enzyme preparation. A lower pH and higher concentration of PPO are more likely to inhibit the activity of PPO enzymes. The more increased the temperature is, the greater the chance of reducing free quinones. The reducing agents used in the biochemical reactions should be as pure as possible to reduce the risk of PPO inactivation.
During the PPO enzyme extraction process, ascorbic acid interferes with the spectrophotometric measurements of the enzyme activity. In addition, ascorbic acid inhibits the production of quinones by PPO, but it also hinders the formation of catechol. Ultimately, ascorbic acid inhibits PPO activity, but it does not protect it from degradation.
PPO enzymes in plants cause browning.
Polyphenol oxidase, or PPO, is a plant enzyme that catalyzes the oxidation of phenolic compounds to o-quinones. These compounds react with other components to form melanin, the dark-brown pigment found in hair, skin, and eyes. Because PPO enzymes cause browning in fruits and vegetables, it is essential to avoid exposure to oxygen when processing or storing fruit and vegetables.
Although PPO enzymes are primarily known for their role in post-harvest browning, they also have other functions. They can help preserve protein content in forage crops. Nevertheless, most people associate browning with negative aspects of food processing. The enzyme is responsible for up to 50% of post-harvest losses in some crops. To control this process, it is necessary to understand the mechanism of PPO enzymes, how they react with substrates, and the inhibitors’ role.
Polyphenol oxidase is responsible for the browning reaction in many fruits and vegetables. Abnormal conditions trigger this process, and tissue damage facilitates the reaction. As a result, plants produce more ethylene and respiration. This process also reduces the amounts of carbohydrates, vitamins, and organic acids. This is why enzymatic browning is an issue for the food industry.
The present study used the PPO enzyme in bioremediation for contaminated soils. This enzyme degrades tyrosine to form L-DOPA, an essential component of cellular membranes. However, to test the potential of the PPO enzyme for bioremediation, different experimental conditions were used. The following are the results of the experiments. The PPO enzyme is a versatile tool in bioremediation. It can even be utilized in combination with other enzymes.
The partially purified PPO enzyme shows excellent potential in bioremediation, especially in wastewater treatment. Further process optimization is needed to achieve better results. This enzyme is found in high concentrations in apple waste, which is a source of phenols. Further, it can be used to treat wastewater contaminated with phenols. The enzyme was successfully tested in wastewater by reducing a range of phenols from soil samples.
The enzyme PPO can be used for bioremediation because it efficiently detects phenol derivatives. An enzyme is a promising tool for biodegradation, as it degrades toxins in the environment. This enzyme is particularly effective in reducing PPO concentration in polluted water. The enzyme is also effective for the biodegradation of cadmium and other metals.