Histidine reaction with

The dinitrophenyl group has been used to protect the imidazole — NH group in histidines (45% yield)" by reaction with 2,4-dinitrofluorobenzene and potassium carbonate. Imidazole —NH groups, but not a-amino acid groups, are quantitatively regenerated by reaction with 2-mercaptoethanol (22°, pH 8, 1 h)." The 2,4-... 

The results supported the proposal of Glu-165 as the general base and suggested the novel possibility of neutral histidine acting as an acid, contrary to the expectation that His-95 was protonated [26,58]. The conclusion that the catalytic His-95 is neutral has been confinned by NMR spectroscopy [60]. The selection of neutral imidazole as the general acid catalyst has been discussed in terms of achieving a pX, balance with the weakly acidic intermediate. This avoids the thermodynamic trap that would result from a too stable enediol intermediate, produced by reaction with the more acidic imidazolium [58]. 

Similarly, the rate of inhibition of phosphoenzyme formation by diethylpyrocarbonate (DEPC) was much slower than the loss of ATPase activity [368], Even when the reaction approached completion with more than 90% inhibition of ATP hydrolysis, about 70% of the Ca -ATPase could still be phosphorylated by ATP (2.3nmoles of E P/mg protein). The remaining 30% of E P formation and the corresponding ATPase activity was not reactivated by hydroxylamine treatment, suggesting some side reaction with other amino acids, presumably lysine. When the reaction of the DEPC-modified ATPase with P-ATP was quenched by histidine buffer (pH 7.8) the P-phosphoenzyme was found to be exceptionally stable under the same conditions where the phosphoenzyme formed by the native ATPase underwent rapid hydrolysis [368]. The nearly normal phosphorylation of the DEPC-trea-ted enzyme by P-ATP implies that the ATP binding site is not affected by the modification, and the inhibition of ATPase activity is due to inhibition of the hydrolysis of the phosphoenzyme intermediate [368]. This is in contrast to an earlier report by Tenu et al. [367], that attributed the inhibition of ATPase activity by... 

Figure 4.33 Benzidine can be diazotized with sodium nitrite and HC1 for reaction with proteins through their tyrosine, histidine, or lysine side-chain groups.

A sulfonyl chloride group rapidly reacts with amines in the pH range of 9-10 to form stable sulfonamide bonds. Under these conditions, it also may react with tyrosine —OH groups, aliphatic alcohols, thiols, and histidine side chains. Conjugates of sulfonyl chlorides with sulf-hydryls and imidazole rings are unstable, while esters formed with alcohols are subject to nucleophilic displacement (Nillson and Mosbach, 1984 Scouten and Van der Tweel, 1984). The only stable derivative with proteins therefore is the sulfonamide, formed by reaction with e-lysine... 

ROS can modify amino acid side chains, with histidine, tryptophan, cysteine, proline, arginine, and lysine among those most susceptible to attack (Brown and Kelly 1994). As a result, carbonyl groups are generated, and these carbonyl concentrations can be measured directly in plasma by using atomic absorption spectroscopy, fluorescence spectroscopy, or HPLC following reaction with 2,4-dinitrophenylhydrazine. 

Activation parameters for the reaction of NO with metMb, Eq. (15), were determined in this laboratory and in collaboration with van Eldik and Stochel (Table II) (23). Comparison of these activation parameters with those determined for reactions of NO with the water soluble ferri-heme complexes Fem(TPPS)(H20)2 and Feni(TMPS)(H20)2 (Table II) demonstrate that the latter compounds represent reasonable models for the kinetics for the analogous reaction with metMb. For example, the kon step would appear to be defined largely by the H20 lability of metMb(H20), although it is clear that the diffusion through protein channels, the distal residues and the proximal histidine binding to the Fe(III) center must all influence the NO binding kinetics (23,24). These properties may indeed be reflected in the lower AS values for both the on and off reactions on metMb. In a related study, Cao et al. recently... 

Trispyrazolylborates are models for tris-histidine active sites in zinc enzymes, e.g., the matrix metalloproteinases involved in breakdown of extracellular matrices. Inhibition of these metalloproteinases may prove valuable in the treatment of, inter alios, cancer and arthritis, so efforts are being made to find appropriate ligands to block the zinc active site. The search has recently moved on from hydroxamates to hydroxypyridinones - l-hydroxy-2-pyridinone is a cyclic analogue of hydroxamic acid. As reported in Section II.B.2 earlier, hydroxypyridinones form stable five-coordinate complexes on reaction with hydrotris(3,5-phenylmethylpyrazolyl)borate zinc hydroxide. Modeling studies suggest that hydroxypyridinonate ligands should be able to access the active site in the enzyme with ease (110). 

Chemical Reviews paper. We can only discuss a small number of these here, but some important categories are (1) synthetic Fe(II)-Cu(I) complexes and their reactions with O2, (2) oxidized heme-copper models (Fe(III)-X-Cu(II) complexes, where X equals 0x0- and hydroxo-bridged complexes, cyanide-bridged complexes, or other X-bridged complexes), (3) crosslinked histidine-tyrosine residues at the heme-copper center, and (4) Cua site synthetic models. 

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