Tyrosine phenolate

L-tyrosine phenol + pyruvic acid + NH P-tyrosinase Erwinia herbicola ... 

Fig.S. Correlation between the integrated area of the seven line ESR spectrum (of Cu-H-Y, Cu-MCM-22 and Cu-VPl-5) and conversion of L-tyrosine, phenol, o-cresol and m-cresol (Curves A-D, respectively).

BSA possesses a total of 59 lysine e-amine groups (with only 30-35 of these typically available for derivatization), 1 free cysteine sulfhydryl (with an additional 17 disulfides buried within its three-dimensional structure), 19 tyrosine phenolate residues, and 17 histidine imidazole groups. The presence of numerous carboxylate groups gives BSA its net negative charge (pi 5.1). 

Figure 28.21 The reactions of R u (11) pby 3 + are catalyzed by light at 452 nm that begins by forming an excited state intermediate. In the presence of persulfate, a sulfate radical is formed concomitant with the oxidative product Ru(III)bpy33+. This form of the chelate is able to catalyze the formation of a radical on a tyrosine phenolic ring that can react along with the sulfate radical either with a nucleophile, such as a cysteine thiol, or with another tyrosine side chain to form a covalent linkage. The result of this reaction cascade is to cause protein crosslinks to form when a sample containing these components is irradiated with light.

Silk fibroin contains no cystine and the content of lysine and histidine is also low (about 1% in total), but it does contain tyrosine phenolic (13%) and serine alcoholic (16%) sidechains. Since glycine accounts for 44% of the total aminoacid content, an N-terminal glycine residue is reasonably representative of most of the primary amino dyeing sites in silk fibres. Amino acid analysis of hydrolysed reactive-dyed silk indicates that the reaction between fibroin and reactive dyes takes place mainly at the e-amino group of lysine, the imino group of histidine and the N-terminal amino group of the peptide chain. In an alkaline medium, the hydroxy groups of tyrosine and serine also react [114]. 

Inhibition of pyridoxal phosphate enzymes by fluoroalanines has been widely studied. Among the numerous examples, alanine racemase, tyrosine phenol... 

Inhibition of enzymes by fluorinated derivatives of tyrosine have been the focus of many investigations. The inhibition of tyrosine phenol lyase,of... 

The solvent pH and polarity will affect the absorbance and fluorescence properties of a protein. A notable example of pH effects on absorbance is seen with tyrosine residues, where a change in pH from neutral to alkaline results in a shift of the absorbance maximum to a longer wavelength and an increase in absorptivity due to dissociation of the tyrosine phenolic hydroxyl group (Freifelder, 1982 Fasman, 1989). An example of solvent polarity effects on fluorescence is observed with tryptophan, where a decrease in solvent polarity... 

It is of interest to compare the tertiary structure of AspAT with that of other PLP-dependent enzymes. Some PLP enzymes whose primary structures are quite different from AspAT exhibit similar tertiary structures. Such enzymes are a>-amino acid pyruvate aminotransferase,341 phosphoserine aminotransferase351 and tyrosine-phenol lyase361 (Phillips, R., personal communication). Similarity in tertiary structure among these PLP enzymes may lead to the idea that many PLP-dependent enzymes share the same ancestor protein. There are PLP enzymes belonging to its own category, such as glycogen phosphorylase and tryptophan synthase.37 381 These enzymes do not share any similarities in either primary or tertiary structures with AspAT. 

The three-dimensional and primary structures of the closely related enzyme, tyrosine phenol-lyase, have recently been determined.28 Since 43% of the amino acid sequences of this enzyme and tryptophanase is identical, their three-dimensional structures should be very similar. The structure of the lyase is like a butterfly. The tetrameric molecule can be considered from a crystallographic point of view as an a2a 2-structure consisting of two... 

There is at the moment no compelling evidence for either of these mechanisms. An important experiment which needs to be done with enzymes of this class is to probe for internal transfer of the a-hydrogen from one enantiomer to the other under single turnover conditions with trapping of the product. An experimental design to accomplish this is currently being explored with tyrosine phenol-lyase and will be discussed below. Demonstration of any internal return of the a-hydrogen... 

The closely related bacterial enzyme tyrosine phenol-lyase [137] has an even wider substrate and reaction specificity than tryptophanase, including the remarkable ability to cleave both D- and L-tyrosine and to interconvert D- and L-alanine. As already discussed and summarized in Tables 1 and 2, the stereochemistry at C-/J in all the a,/3-elimination and -replacement reactions of this enzyme studied so far is always retention [108,109,129]. This includes the a, -elimination of L- as well as of D-tyrosine. The fate of the a-hydrogen of L-tyrosine in this reaction has been probed in preliminary experiments (H. Kumagai, E. Schleicher and H.G. Floss, unpublished results), and the results tentatively suggest transfer of deuterium from the a-position to C-4 of the resulting phenol. Attempts to demonstrate intramolecularity of this transfer have so far been inconclusive. The base abstracting H-a in this enzyme may be histidine [138]. 

A system has recently been developed to examine the question whether any internal return of the H-a can be demonstrated in the interconversion of L- and D-alanine catalyzed by this enzyme. L-[2-2H]Alanine in HzO or unlabeled L-alanine in 2H20 are converted into D-alanine with tyrosine phenol-lyase in the presence of excess D-amino acidiacetyl-coenzyme A acetyl transferase [139] and acetyl-coenzyme A to ensure single turnover conditions. The resulting TV-acetyl-D-alanine is analyzed for its deuterium content by mass spectrometry and NMR. Initial data indicate 12% transfer of a- H from L-alanine to the D-isomer in 2HzO (S.-j. Shen, H. Kumagai and H.G. Floss, unpublished results), but these results need to be verified. If they are confirmed, racemization by a single base mechanism would be indicated. 

Figure 1. Nucleophilic groups of proteins that are susceptible to acylation. The a-and e-amino groups are most reactive the tyrosine phenolic groups generally have a higher pK and are usually more protected from reaction than the amino groups the histidine and cysteine residues are seldom acylated because the reaction products hydrolyz in aqueous solution and the serine and threonine hydroxyl groups, being weak nucleophiles, are not easily acylated in aqueous solution.

Fig. 8. Tyrosine coordination modes in galactose oxidase-copper complex. Tyrosine phenolate bond angles (0) and ring torsion angles (t) are indicated. (Based on protein coordinates PDB ID IGOG.)...

Dissociation of the aldehyde product would leave a low-coordinate, Cu(I) redox center associated with two protonated tyrosine phenols in the active site. This complex is known to be very reactive toward dioxygen, the second-order kinetic constant for reoxidation of the reduced enzyme by O2 being nearly 8x10 s (Borman et aL, 1997 Whit-... 

Another example of how catalysis plays a key role in enabling our lives is in the synthesis of pharmaceuticals. Knowles s development, at Monsanto in the early 1970s, of the enantioselective hydrogenation of the enamide precursor to L-DOPA (used to treat Parkinson s disease), using a Rh-chiral phosphine catalyst (Section 3.5), led to a share in the Nobel prize. His colaureates, Noyori and Sharpless, have done much to inspire new methods in chiral synthesis based on metal catalysis. Indeed, the dramatic rise in the demand for chiral pharmaceutical products also fuelled an intense interest in alternative methodologies, which led to a new one-pot, enzymatic route to L-DOPA, using a tyrosine phenol lyase, that has been commercialized by Ajinomoto. 

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