Tyrosine, reactions


SPECIAL REACTIONS FOR TYROSINE.  [c.382]

SPECIAL REACTIONS FOR TYROSINE.  [c.382]

The chemical mechanism of other herbicides also involves peroxidative destmction of polyunsaturated fatty acids by starter radicals (37). Fenton reactions produce alkoxy radicals which can spHt into alkyl radicals leading to hydrocarbon gases (102) or can initiate further radical destmction of other chloroplast components (37,97). Potent peroxy and alkoxy radicals and Hpid hydroperoxides are formed (103). Lipid hydroperoxides also decompose to form cytotoxic malondialdehye [542-78-9] (MPA), a compound often used as an index of Hpid peroxidation (97,103). MPA, a significant 2-thiobarbituric reactant, can cause intra- and intermolecular cross-linking of sulfhydryl-containing proteins (97). Proteins can also be fragmented or modified by hydrogen peroxide in the presence of transition metals (97,104). The resulting hydroxyl radicals and the alkoxy radical intermediates from Hpid peroxidation also attack proteins and individual amino acids (104), particularly histidine, cysteine/cystine, methionine, lysine, tyrosine, and tryptophan (97,103,105).  [c.44]

Amino acid-derived hormones include the catecholamines, epinephrine and norepinephrine (qv), and the thyroid hormones, thyroxine and triiodothyronine (see Thyroid AND ANTITHYROID PREPARATIONS). Catecholamines are synthesized from the amino acid tyrosine by a series of enzymatic reactions that include hydroxylations, decarboxylations, and methylations. Thyroid hormones also are derived from tyrosine iodination of the tyrosine residues on a large protein backbone results in the production of active hormone.  [c.171]

Immobilized Enzymes and Metal-Complex Catalysts. Use of enzymes to catalyze reactions in ceU-free systems has been limited by the difficulty of enzyme isolation, labiUty of the enzymes, and difficulty in effecting clean separations of enzymes from reaction mixtures. An approach that has circumvented some of these problems is to attach enzymes to soHd support materials (36). The most frequendy used technique for immobilizing enzymes on a soHd support involves reducing A/-(3-triethoxysilylpropyl)-/)-nitrobenzamide [60871-86-5] groups after attachment to siUca or controUed-pore glass to give aniline derivatives, then converting them to diazonium salt, and effecting coupling through azo linkage to the tyrosine of the proteins (see Enzyt s in organic SYNTTiEsis Enzyt applications, industrial).  [c.73]

These examples serve to illustrate the concept of intramolecular catalysis and the fact that favorable juxtaposition of acidic, nucleophilic, or basic sites can markedly accelerate some of the common reactions of carbonyl compounds. Nature, through evolution, has used optimal placement of functional groups to achieve the catalytic activity of enzymes. The functional groups employed to accomplish this are those present on the amino acid residues found in proteins. The acidic sites available include phenolic or carboxylic acid groups from tyrosine, glutamic acid, and aspartic acid. Basic sites include the imidazole ring in histidine and the co-amino group of lysine. This latter group and the amidine group m arginine are normally protonated at physiological pH and can serve as cationic centers or general acids, as well. Thiol (cysteine) and hydroxyl (threonine and serine) groups and the deprotonated carboxyl groups of glutamic acid and aspartic acid are potential nucleophilic sites.  [c.495]

The protease a-chymotrypsin has been used for transesterification reactions by two groups (Entries 7 and 8) [35, 36]. N-Acetyl-l-phenylalanine ethyl ester and N-acetyl-l-tyrosine ethyl ester were transformed into the corresponding propyl esters (Scheme 8.3-2).  [c.341]

CF3CO2H, PhSCH3, 25°, 3 h. ° The use of dimethyl sulfide or anisole as a cation scavenger was not as effective because of side reactions. Benzyl ethers of serine and threonine were slowly cleaved (30% in 3 h complete cleavage in 30 h). The use of pentamethylbenzene had been shown to increase the rate of deprotection of 0-Bn-tyrosine.  [c.157]

In the HF deprotection of thioethers and many other protective groups, anisole serves as a scavenger for the liberated cation formed during the deprotection process. If cations liberated during this deprotection are not scavenged, they can react with other amino acid residues, especially tyrosine. The authors list 15 protective groups that are cleaved by this method, including some branched-chain carbonates and esters, benzyl estets and ethers, the nitro-protective group in arginine, and 5-benzyl and 5-t-butyl thioethers. They report that 12 protective groups are stable under these conditions, including some straight-chain carbonates and esters, A -benzyl derivatives, and 5-methyl, 5-ethyl, and 5-isopropyl thioethers. Di methyl sulfide, thiocresol, cresol, and thioanisole have also been used as scavengers when strong acids are used for deprotection. A mixture of 5% cresol, 5% p-thiocresol, and 90% HF is recommended for benzyl thioether deprotection. These conditions cause cleavage by an SnI mechanism. The use of low concentrations of HF in dimethyl sulfide (1 3), which has been recommended for deprotection of other peptide protective groups, does not cleave the 5-4-methylbenzyl group. Reactions that use low HF concentrations are considered to proceed via an Sn2 mechanism. The use of low HF concentrations with thioanisole results in some methylation of free thiols. The use of HF in anisole can also result in alkylation of methionine.  [c.280]

Recently, the smdy of the reaction catalyzed by bovine protein tyrosine phosphate (BPTP) using QM-MM methods was reported [30]. This study represents a progression from the techniques applied to the study of TIM and other enzymatic systems, because the reaction was followed by using molecular dynamics techniques and the QM-MM potential (QM-MD). QM-MD studies are a more powerful technique for studying chemical reactions in condensed phases because they allow for sampling of configuration space as the reaction pathway is followed and the generation of statistics that can be used to calculate reaction activation parameters such as the enthalpy and entropy of activation of various steps along the reaction pathway.  [c.230]

The use of QM-MD as opposed to QM-MM minimization techniques is computationally intensive and thus precluded the use of an ab initio or density functional method for the quantum region. This study was performed with an AMi Hamiltonian, and the first step of the dephosphorylation reaction was studied (see Fig. 4). Because of the important role that phosphorus has in biological systems [62], phosphatase reactions have been studied extensively [63]. From experimental data it is believed that Cys-i2 and Asp-i29 residues are involved in the first step of the dephosphorylation reaction of BPTP [64,65]. Alaliambra et al. [30] included the side chains of the phosphorylated tyrosine, Cys-i2, and Asp-i 29 in the quantum region, with link atoms used at the quantum/classical boundaries. In this study the protein was not truncated and was surrounded with a 24 A radius sphere of water molecules. Stochastic boundary methods were applied [66].  [c.230]

CF3CO2H, PhSCH3, 25°, 3 h. The use of dimethyl sulfide or anisole as a cation scavenger was not as effective because of side reactions. Benzyl ethers of serine and threonine were slowly cleaved (30% in 3 h complete cleavage in 30 h). The use of pentamethylbenzene has been shown to increase the rate of deprotection of 0-Bn-Tyrosine.  [c.267]

Cresol, 5% thiocresol, 90% HF. In the HF deprotection of thioethers and many other protective groups, anisole serves as a scavenger for the liberated cation formed during the deprotection process. If cations liberated during this deprotection are not scavenged, they can react with other amino acid residues, especially tyrosine. Dimethyl sulfide, thiocresol, cresol, and thioanisole have also been used as scavengers when strong acids are used for deprotection. A mixture of 5% cresol, 5% p-thiocresol, and 90% HF is recommended for benzyl thioether deprotection. These conditions cause cleavage by an 1 mechanism. The use of low concentrations of HF in dimethyl sulfide (1 3), which has been recommended for the deprotection of other peptide protective groups, does not cleave the 5-4-methylbenzyl group. Reactions that use low HF concentrations are considered to proceed via an 8 2 mechanism. The use of low HF concentrations with thioanisole results in some methylation of free thiols. The use of HF in anisole can also result in alkylation of methionine.  [c.459]


See pages that mention the term Tyrosine, reactions : [c.218]    [c.306]    [c.324]    [c.159]    [c.270]    [c.147]   
Practical organic chemistry (1960) -- [ c.380 , c.382 ]

Practical organic chemistry (1978) -- [ c.380 , c.382 ]