A phosphatase is an enzyme capable of dephosphorylating a corresponding substrate by removing a phosphate group on a substrate molecule by hydrolyzing a phosphate monoester to form a phosphate ion and a free hydroxyl group. The action of phosphatase is the opposite of that of a kinase, which is a phosphorylating enzyme that can use an energy molecule, such as ATP, to add a phosphate group to a corresponding substrate molecule. A phosphatase that is ubiquitous in many organisms is alkaline phosphatase.
Phosphatase can be divided into two classes: acid phosphatase and alkaline phosphatase.
Figure 1. Structure of phosphatase protein.
Inducing and secreting acid phosphatase is one of the important adaptive responses of plants to low phosphorus environment. Acid phosphatase can hydrolyze phosphate groups from different organic phosphorus substrates for plant uptake and utilization. Most plant acid phosphatases have no significant substrate specificity, and substrates that can be hydrolyzed include RNA, DNA, 3-phosphoglycerate, hexose phosphate, etc.
The activity of the acid phosphatase can be judged by the color of the reaction product. Using this method, mutants with abnormal responses to low phosphorus stress can be screened. Low-phosphorus-induced acid phosphatases can be divided into two categories: acid phosphatase acting on cells and acid phosphatase secreted outside the cell. The combined effect of the two ensures that plants are better able to cope with low phosphorus stress. Intracellular acid phosphatase can achieve phosphorus recycling in vivo through two pathways: one is to convert the organic phosphorus in the plant vacuole into Pi. Under normal circumstances, most of the phosphorus in plants is stored in the vacuole, and the Pi content in the cytoplasm is maintained within a certain range. When the plant is exposed to low phosphorus stress, the Pi content in the plant is continuously decreased, and the enzyme activity of the acid phosphatase which is normally inhibited by the high phosphorus environment in the vacuole is restored, and the organic phosphorus stored in the hydrolyzed bubble is transported through the phosphorus on the tonoplast. Protein is secreted into the cytoplasm, maintaining a dynamic balance of Pi content in the cytoplasm. Another way to achieve phosphorus cycling in the body is to activate and reuse phosphorus in senescent tissues to younger tissues. Robinson et al. reported that Arabidopsis purple acid phosphatase AtPAP26 is involved in the reuse of phosphorus in aging tissues. The main role of secreted acid phosphatase is to decompose the organic phosphorus substrate in the soil environment to release Pi that can be directly absorbed and utilized by plants. Normally, secreted acid phosphatase is more stable than intracellular acid phosphatase. The pH range of the secreted acid phosphatase (activity greater than 50%) is 4.0-7.6, and the temperature activity range (activity above 80%) is 22 °C - 48 °C. This ensures that they can function more efficiently and sustainably in complex soil media. It has been reported that plants in the soil can directly absorb the utilized Pi, and 80% are derived from the decomposition of the organic phosphorus substrate in the soil by the acid phosphatase secreted to the outside of the cell, which is sufficient to see the importance of acid phosphatase.
Alkaline phosphatase is a non-specific phosphomonoesterase that catalyzes the hydrolysis of almost all phosphate monoesters to form inorganic phosphates and corresponding alcohols, phenols, and sugars. etc., it can also catalyze the transfer reaction of phosphate groups, and E. coli AP is also a phosphite-dependent hydrogenase. AP exists in almost all organisms except higher plants, and can directly participate in phosphorus metabolism, playing an important role in the process of digestion, absorption, secretion and ossification of calcium and phosphorus.
Protein phosphatases (PPs)
Protein phosphatases (PPs) are structurally and functionally diverse enzymes which are represented by three distinct gene families. Two of these, the PPP and PPM families, dephosphorylate phosphoserine and phosphothreonine residues, whereas the protein tyrosine phosphatases (PTPs) dephosphorylate phosphotyrosine. The most abundant protein serine/ threonine phosphatases of the eukaryotes, PP1, PP2A and PP2B, belong to the PPP family whereas PP2C and the related mitochondrial pyruvate dehydrogenase phosphatase are members of the PPM family. Within each family, although the catalytic domains are highly conserved, suggesting similarities in tertiary structure and catalytic mechanisms, considerable structural and functional diversity of individual protein phosphatases is created as a result of a combination of associated regulatory domains and subunits.
Protein serine/threonine phosphatases of the PPP family play numerous roles in mediating intracellular signaling processes. In addition to PP1, PP2A and PP2B, related novel protein phosphatases have recently been characterized that occur in low abundance and in a tissue and developmental specific manner. PP1 and PP2A are specifically inhibited by a variety of naturally occurring toxins such as okadaic acid, a diarrhetic shellfish poison and strong tumor promotor, and microcystin, a liver toxin produced by blue green algae. Whereas PP2B is only poorly inhibited by the toxins that affect PP1 and PP2A, it was recently defined as the immunosuppressive target of FK506 and cyclosporin in association with their major cellular binding proteins, the cis-trans peptidyl propyl isomerases FKBP12 and cyclophilin, respectively. The structural complexity of PP1 and PP2 holoenzymes in vivo resolves the seemingly paradoxical situation that a relatively small number of protein phosphatase catalytic subunits are responsible for the specific dephosphoiylation of a variety of cellular proteins, and that both PP1 and PP2A have been implicated in regulating many diverse cellular functions, including glycogen metabolism, muscle contraction, control of the cell cycle and RNA splicing. As the individual catalytic subunits of PP1 and PP2A catalyze the dephosphoiylation of a broad and overlapping range of substrates in vitro, specificity in vivo is generated either by altering the selectivity of the enzyme towards a particular substrate or by targeting the phosphatase to the subcellular location of its substrates. This is achieved by regulatory or targeting subunits that bind to the phosphatase catalytic subunits. In addition, the regulatory subunits allow the activity of the PPPs to be modulated by reversible protein phosphorylation and second messengers.