PLP function

Prolactin-Like Proteins. A number of prolactin-like proteins (PLPs), which ate distinct from the PLs, have been identified in mminants and rodents (11,23). Several cDNA transcripts coding for PLPs in catde have been identified (23). These transcripts code for proteins which possess about 40% sequence homology with bovine PRL 60% if conservative substitutions ate considered. Three glycosylated PLPs, ie, PLP-A, -B, and -C, ate produced during pregnancy in the rat (11). Two additional prolactin-related molecules have been identified in the mouse (24,25), ie, proliferin [92769-12-5] (PLF) and PLF-related protein [98724-27-7]. These ate not found in other rodents and may be unique to the mouse. The functional roles of PLPs remain to be deterrnined. 

Pyridoxal Derivatives. Various aldehydes of pyridoxal (Table 3) react with hemoglobin at sites that can be somewhat controlled by the state of oxygenation (36,59). It is thereby possible to achieve derivatives having a wide range of functional properties. The reaction, shown for PLP in Figure 3, involves first the formation of a Schiff s base between the amino groups of hemoglobin and the aldehyde(s) of the pyridoxal compound, followed by reduction of the Schiff s base with sodium borohydride, to yield a covalendy-linked pyridoxyl derivative in the form of a secondary amine. 

It is noteworthy that the axonal degeneration that occurs in the PNS of MAG-null mice is not observed in the CNS, possibly because other CNS myelin proteins enhance axonal stability. These could include PLP and/or CNP, both of which are needed for axonal stability in the CNS where they are present in much higher concentration. In summary, it appears that the most important function of MAG in the PNS is transmitting a signal from Schwann cells to axons that is needed for the stability of myelinated axons, whereas its principal function in the CNS is to transmit a signal in the reverse direction that promotes efficient myelination and oligodendrocyte vitality. 

A few enzymes, such as the previously mentioned CNP, are believed to be fairly specific for myelin/oligodendro-cytes. There is much more in the CNS than in peripheral nerve, suggesting some function more specialized to the CNS. In addition, a unique pH 7.2 cholesterol ester hydrolase is also enriched in myelin. On the other hand, there are many enzymes that are not myelin-specific but appear to be intrinsic to myelin and not contaminants. These include cAMP-stimulated kinase, calcium/calmodulin-dependent kinase, protein kinase C, a neutral protease activity and phosphoprotein phosphatases. The protein kinase C and phosphatase activities are presumed to be responsible for the rapid turnover of MBP phosphate groups, and the PLP acylation enzyme activity is also intrinsic to myelin. 

PMP-22 gene (Ch. 38). Because the function of PMP-22 is uncertain, the mechanisms by which the point mutations in this protein cause the trembler phenotype are unclear. However, as with the PLP mutations, the pathology may result from an unfolded protein response to abnormal protein that is retained in the endoplasmic reticulum. 

Campagnoni, A. T. and Skoff, R. P. The pathobiology of myelin mutants reveal novel biological functions of the MBP and PLP genes. Brain Pathol. 11,74-91,2001. 

Except the crystallographic asymmetric unit containing one molecule of ACS with AVG (HAY, 1M7Y), all other structures contain two molecules of ACS, which strongly suggests a functional dimeric form of these enzymes. In the dimeric form of both tomato and apple ACS, two independent active sites are formed at the interface of a dimer and share residues from each monomer. It has been well documented that the dimeric form is the basic catalytic unit of most PLP-dependent enzymes and that both monomers within a dimer physically share an active site." ... 

Vitamin B6 occurs naturally in three related forms pyridoxine (6.26 the alcohol form), pyridoxal (6.27 aldehyde) and pyridoxamine (6.28 amine). All are structurally related to pyridine. The active co-enzyme form of this vitamin is pyridoxal phosphate (PLP 6.29), which is a co-factor for transaminases which catalyse the transfer of amino groups (6.29). PLP is also important for amino acid decarboxylases and functions in the metabolism of glycogen and the synthesis of sphingolipids in the nervous system. In addition, PLP is involved in the formation of niacin from tryptophan (section 6.3.3) and in the initial synthesis of haem. 

All aminotransferases have the same prosthetic group and the same reaction mechanism. The prosthetic group is pyridoxal phosphate (PLP), the coenzyme form of pyridoxine, or vitamin B6. We encountered pyridoxal phosphate in Chapter 15, as a coenzyme in the glycogen phosphorylase reaction, but its role in that reaction is not representative of its usual coenzyme function. Its primary role in cells is in the metabolism of molecules with amino groups. 

FIGURE 18-5 Pyridoxal phosphate, the prosthetic group of aminotransferases. (a) Pyridoxal phosphate (PLP) and its aminated form, pyri-doxamine phosphate, are the tightly bound coenzymes of aminotransferases. The functional groups are shaded, (b) Pyridoxal phosphate is bound to the enzyme through noncovalent interactions and a Schiff-base linkage to a Lys residue at the active site. The steps in the formation of a Schiff base from a primary amine and a carbonyl group... 

R. L. Baxter, and L. Sawyer. Biotin synthesis requires three other enzymes (steps b, c, d). Step b is catalyzed by a PLP-dependent transaminase. At the left is thiamin diphosphate, in the form of its 2-(1 -hydroxyethyl) derivative, an intermediate in the enzyme pyruvate decarboxylase (Dobritzsch et al.,. Biol. Chem. 273,20196-20204,1998). Courtesy of Guoguang Lu. Thiamin diphosphate functions in all living organisms to cleave C-C bonds adjacent to C=O groups. 

CHO group greatly enhances the catalytic activity Since certain metal ions, such as Cu2+ and Al3+, increase the rates in model systems and are known to chelate with Schiff bases of the type formed with PLP, it was concluded that either a metal ion or a proton formed a chelate ring and helped to hold the Schiff base in a planar conformation (Fig. 14-6). However, such a function for metal ions has not been found in PLP-dependent enzymes. 

Observation of an abnormally large shift in the position of fluorescent emission of pyridoxal phosphate (PLP) in glycogen phosphorylase answered an interesting chemical question.187188 A 330 nm (30,300 cm ) absorption band could be interpreted either as arising from an adduct of some enzyme functional group with the Schiff base of PLP and a lysine side chain (structure A) or as a nonionic tautomer of a Schiff base in a hydrophobic environment (structure B, Eq. 23-24). For structure A, the fluorescent emission would be expected at a position similar to that of pyridoxamine. On the other hand, Schiff bases of the... 

The first step in this pathway involves SN2 displacement by fluoride on S-adenosine-L-methionine (SAM) catalyzed by the newly discovered enzyme fluor-inase (905-910), which also can function as a chlorinase (912). Fluorinase has been isolated and characterized, and the gene has been cloned (916). Both 5 -fluoro-5 -deoxyadenosine (847) and 5 -fluoro-5 -deoxy-D-ribose-l-phosphate (848) have been identified as intermediates (905-908). Fluoroacetaldehyde (850) is the immediate precursor, presumably via fluororibulose-1-phosphate (849) (915), to both fluoroacetate and 4-fluorothreonine (837) (901). The requisite enzymes fluoroacetaldehyde dehydrogenase (902) and L-threonine transaldolase-PLP (903) have been isolated and purified. The steps from 848 to 850 remain to be established but are based on known biochemistry. The pronounced toxicity of fluoroacetic acid... 

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