Tyrosine phosphate ester

Covalent modifications of proteins serve many purposes (1-4). Some are structural and affect the three dimensional structure of proteins, such as disulfide bonds or cross linking of collagen chains via allysine side chains. There are a many different modifications that allow for the attachment of a variety of nonpeptide prosthetic groups to the protein. The attachment of the heme group to cysteine in c type cytochromes and that of biotin or pyridoxal phosphate to lysine are but a few examples. Some processes, such as cysteine isoprenylation or N myristoylation, allow proteins to become tightly associated with membranes. In other situations, a protein may be regulated by a reversible reaction, such as phosphorylation. The best know examples of this are serine, threonine, and tyrosine phosphate esters. In many other cases, the function of a particular modification is less evident. 

While esters of sulfuric acid do not play as central a role in metabolism as do phosphate esters, they occur widely. Both oxygen esters (R-0-S03 , often referred to as O-sulfates) and derivatives of sulfamic acid (R-NH-SOg, N-suIfates) are found, the latter occurring in mucopolysaccharides such as heparin. Sulfate esters of mucopolysaccharides and of steroids are ubiquitous and sulfation is the most abundant known modification of tyrosine side chains. Choline sulfate and ascorbic acid 2-sulfate are also found in cells. Sulfate esters of phenols and many other organic sulfates are present in urine. 

On the other hand, a particular protein function can be realized with different protein folds, and an example of this are protein phosphatases. Protein phosphatases feature two distinctively different catalytic mechanisms for hydrolytically cleaving phosphorylated amino acid residues. The active sites of serine/threonine protein phosphatases (PPs) contain two metal centers that directly activate a water molecule for nucleophilic attack of the phosphate ester bond. In contrast, protein tyrosine phosphatases (PTPs) [105] possess a Cys residue present in the active site loop containing the conserved PTP signature motif HCXXXXXRS. The Cys sidechain acts as the attacking nucleophile in the formation of a phosphocysteine intermediate, which is eventually hydrolyzed by a water molecule [106], The same catalytic mechanism is also shared by dual-specificity phosphatases (see below). 

In the first step, we showed by analytical studies that compound 28 was a donor substrate for transketolase in the presence of D-ribose-5-phosphate as acceptor substrate and that in the second step, the hydroxylated aldehyde released 29 led to the P-elimination of protected L-tyrosine. We showed that the free L-tyrosine can thus be obtained by enzymatic deprotection of N-acetyl-L-tyrosine ethyl ester using acylase and subtilisine. In this conditions, it should be possible to carry out this assay in vivo in the presence of host cells overexpressing transketolase and auxotrophic for L-tyrosin. This strategy should offer the first stereospecific selection test of transketolase mutants. The principle of this assay may be extended to other enzymes that can release aldehydes P-substituted by L-tyrosine. 

Covalent modification (adenylylation) - A specific tyrosine residue in glutamine synthetase can react with ATP to form a phosphate ester with AMP (see here). 

Figure 17-7 Two alternative mechanisms utilized by phosphatases to carry out hydrolysis of phosphate esters. The phosphoenzyme intermediate mechanism utilizes an amino acid (represented as -X] as a nucleophile to attack the phosphate ester, transferring the phosphoryi group and producing a short-lived phosphoenzyme intermediate. In the second step, water serves as the nucleophile, hydrolyzing the phosphoenyzme intermediate and regenerating the enzyme. This mechanism is used by the tyrosine phosphatases (nucleophile = cysteine) and E. coli alkaline phosphatase (active site nucleophile = Ser 102). The metallophosphatases do not proceed by formation of a phosphoenzyme intermediate but rather carry out hydrolysis by direct transfer of the phosphoryi group to a metal-coordinated water molecule.

One of the most common control mechanisms for enzymes is by phosphorylation. The side-chain hydroxyl groups of serine, threonine, and tyrosine can all form phosphate esters. Transport across membranes provides an important example, such as the sodium-potassium ion pump, which moves potassium into the... 

Some phosphatases are nonspecific, catalyzing the hydrolysis of a wide variety of substrates. Other phosphatases are small-molecule specific and hydrolyze a particular small-molecule phosphate ester or structurally similar substrates. Phosphoprotein-specific phosphatases accept particular phosphorylated proteins or peptides as substrates. The phosphoprotein-specific phosphatases can be subdivided into subgroups those specific for proteins phosphorylated on tyrosine those specific for proteins phosphorylated on serine or threonine and the so-called dual-specific enzymes, which accept both classes of phosphorylated proteins. More recently, a group of protein histidine phosphatases has been described. 

Muller et al.understand better the role of tyrosine in the structure and biological function of MDH. Resolution of the protein absorption spectrum, using iV-acetylphenylalanine ethyl ester in dioxane and A-acetyltyrosine ethyl ester in dioxane or 0.1 M phosphate buffer to model the effect of the local environments of the chromophoric groups, indicated that both the pig and the... 

Novozymes, a subtilisin produced by Bacillus licheniformis, was used by Chen et al ° to carry out a dynamic kinetic resolution of benzyl, butyl, or propyl esters of DL-phenylalanine, tyrosine, and leucine. The hydrolysis was performed at pH 8.5 in 2-methyl-2-propanol/water (19 1) and the freed L-amino acids precipitated. The key feature bringing about continual racemization of the remaining D-amino acid esters was the inclusion of 20 mmol 1 pyridoxal phosphate. 

Figure 2.6 By resolution of df-amino acid esters under conditions of dynamic resolution 100% of a single enantiomer may be produced. Using catalytic amounts of pyiidoxyl-5-phosphate, which forms a Schiff s base with the ester and not the acid, the unreacted D-ester may be racemised in situ and for instance L-tyrosin has been obtained in 97% ee and 95% yield.

The procedure, based on the phosphoramidite method, is illustrated in Scheme 7.11. Nn-Fmoc-L-tyrosine was temporarily protected as its ferf-butyldimethylsilyl ester (see section 6.6). Phosphitylation with dimethyl tyN-diethylphosphorami-dite followed by in situ oxidation with /erf-butyl hydroperoxide gave the phos-phodiester 11 3 The labile silyl ester hydrolysed during the sodium metabisulfite workup used to destroy excess /erf-butyl hydroperoxide to give 11.4 in 57% overall yield, By the same procedure, the dkerf-butyl phosphate 11.5, dibenzyl phosphate 11.6 and diallyl phosphate (not shown) were prepared. 

The formation of vanadate esters with hydroxyl groups in aqueous solutions has been studied in detail for methanol (53), ethylene glycol (57), phenol, and tyrosine (58). Vanadate is able to form cyclic complexes when there are adjacent hydroxyls in the molecule (59) and interacts with luidine, adenosine monophosphate (60), glutathione disulphide (61), and phosphate (62). Rehder (63) studied the interaction of amino acids and dipeptides with vanadate. He concluded that complexes are formed in which the peptide function and the iV-terminal amino group are involved. Similarly, it was reported (64) that vanadate... 

Phenylalanine catabolism, 468 chemical structure, 19 plasma conoentralion, 46o sparing by tyrosine, 467,469 Phenylketonuria, 467,469 Phenylpyruvic acid, 469 FhIP,889,B90 Phlebotomy, 759 Phlorizin hydrolase, 109-110 Phorbol esters, cancer and, 916 Phosphatases, 54, 66 Phosphate, 694 in biologLcal fluids, 696 in bore, 697... 

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