An extract on #alkaline
In Gram-negative bacteria, such as Escherichia coli (E.coli), alkaline phosphatase is located in the periplasmic space, external to the inner cell membrane and within the peptidoglycan portion of the cell wall. With the periplasmic gap being more prone to environmental variation than the inner cell, alkaline phosphatase is suitably resistant to inactivation, denaturation, or degradation. This characteristic of the enzyme is uncommon to many other proteins.
The precise structure and function of the isozyme in E.coli is solely geared to supply a source of inorganic phosphate when the environment lacks this metabolite.
While the outer membrane of E.coli contains porins that are permeable to phosphorylated compounds, the inner membrane does not. Then, an issue arises in how to transport such compounds across the inner membrane and into the cytosol. Surely, with the strong anionic charge of phosphate groups along with the remainder of the compound they are very much immiscible in the nonpolar region of the bilayer. The solution arises in cleaving the phosphate group away from the compound via ALP. In effect, along with the concomitant compound the phosphate was bound to, this enzyme yields pure inorganic phosphate which can be ultimately targeted by the phosphate-specific transport system (Pst system) for translocation into the cytosol. As such, the main purpose of dephosphorylation by alkaline phosphatase is to increase the rate of diffusion of the molecules into the cells and inhibit them from diffusing out.
Alkaline phosphatase is a zinc-containing dimeric enzyme with the MW: 86,000 Da, each subunit containing 429 amino acids with four cysteine residues linking the two subunits. Alkaline phosphatase contains four Zn ions and two Mg ions, with Zn occupying active sites A and B, and Mg occupying site C, so the fully active native alkaline phosphatase is referred to as (ZnAZnBMgC)2 enzyme. The mechanism of action of alkaline phosphatase involves the geometric coordination of the substrate between the Zn ions in the active sites, whereas the Mg site doesnt appear to be close enough to directly partake in the hydrolysis mechanism, however, it may contribute to the shape of the electrostatic potential around the active center. Alkaline Phosphatase has a Km of 8.4 x 10^-4.
Alkaline phosphatase in E.coli is uncommonly soluble and active within elevated temperature conditions such as 80 degrees Celsius. Due to the kinetic energy induced by this temperature the weak hydrogen bonds and hydrophobic interactions of common proteins become degraded and therefore coalesce and precipitate. However, upon dimerization of ALP the bonds maintaining its secondary and tertiary structures are effectively buried such that they are not affected as much at this temperature. Furthermore, even at more elevated temperatures such as 90 degrees Celsius ALP has the uncommon characteristic of reverse denaturation. Due to this, while ALP ultimately denatures at about 90 degrees it has the added ability to accurately reform its bonds and return to its original structure and function once cooled back down.
Alkaline phosphatase in E. coli is located in the periplasmic space and can thus be released using techniques that weaken the cell wall and release the protein. Due to the location of the enzyme, and the protein layout of the enzyme, the enzyme is in solution with a smaller amount of proteins than there are in another portion of the cell. The proteins' heat stability can also be taken advantage of when isolating this enzyme (through heat denaturation). In addition, alkaline phosphatase can be assayed using p-Nitrophenyl phosphate. A reaction where alkaline phosphatase desphosphorylates the non-specific substrate, p-Nitrophenyl phosphate in order to produce p-Nitrophenol(PNP) and inorganic phosphate. PNP's yellow color, and its max at 410 allows spectophotometry to determine important information about enzymatic activity. Some complexities of bacterial regulation and metabolism suggest that other, more subtle, purposes for the enzyme may also play a role for the cell. In the laboratory, however, mutant Escherichia coli lacking alkaline phosphatase survive quite well, as do mutants unable to shut off alkaline phosphatase production.
The optimal pH for the activity of the E. coli enzyme is 8.0 while the bovine enzyme optimum pH is slightly higher at 8.5. Alkaline Phosphatase accounts for 6% of all proteins in depressed cells.