Tyrosine modification

Directing the iodination reaction toward histidine residues in proteins, as opposed to principally tyrosine modification, is possible simply by increasing the pH of the lodobeads reaction from the manufacturer s recommended pH 7.0-8.2 (Tsomides et ai, 1991). No reducing agent is required to stop the iodination reaction as is the case with chloramine-T and other methods. 

Directing the iodination reaction toward histidine residues in proteins, as opposed to principally tyrosine modification, is possible simply by increasing the pH of the... 

Froschle, M., Ulmer, W. and Jany, K.D. (1984). Tyrosine modification of glucose dehydrogenase from Bacillus megaterium. Effect of tetranitromethane on the enzyme in the tetrameric and monomeric state. European Journal of Biochemistry, 142, 533-540. 

The two most widely utilized procedures for tyrosine modification, nitration and iodination, are discussed below. 

The role of certain residues in the enzyme mechanism has been confirmed by chemical modification studies, notably for tyrosine. " Modification of tyrosyl residues (for example acetylation or nitration) leads to loss of peptidase activity and enhancement of esterase activity. The presence of the inhibitor -phenylpropionate protects two tyrosine residues from acetylation. Those are Tyr-248 and probably Tyr-198, which is also in the general area of the active site. The modified apoenzyme has lower affinity for dipeptides, as might be expected from the loss of hydrogen bonding between Tyr-248 and the peptide NH group. 

Zhang W, Luo Y, Wang Y, Zhong R. Influence of metallic ions on tyrosine modification by peroxynitrite. Chinese J Inorg Chem. 2006 22 1113-7. 

Van der Vliet, Eiserich JP, O Neill A, Halliwell B. Cross C E. Tyrosine modification by reactive, nitrogen species a closer look. Arch Biochem Biophys 1995 319 341-9. 

A more selective modification, with concomitant loss of activity, has been achieved using diazotized [ °S]sulfanilic acid (130,194). The free thiol groups were protected by mercuric chloride before the tyrosine modification was performed. It was found that an incorporation of 1.5 radioactive sulfur atoms per subunit inhibited the activity. The labeled peptide was isolated and partially sequenced. By comparison with the dogfish LDH sequence the labeled tyrosine corresponds to residue 220. However, this tyrosine is nowhere near the active center of the subunit. It is most improbable that it can be associated with catalysis. 

The above studies emphasize the ability of diazonium-coupling reactions to modify proteins with extremely high efficiency, but one of the limitations of this method is the lack of selectivity that can be obtained when there are multiple tyrosines on the surface of a single protein. This has not been problematic for the viral capsids shown above, as only one tyrosine is accessible on each monomer, but many applications demand higher levels of selectivity than allowed by these coupling reactions. To address this need, and to increase the substrate scope for bioconjugation reactions in general, a versatile Mannich-type reaction has been developed for tyrosine modification, Fig. 10.3-5 [25]. In this reaction, aldehydes and anilines are mixed to form... 

Fig. 10.3-5 Tyrosine modification using a when proteins are treated alone with either three component Mannich-type reaction. component, (b) The reaction conversion is (a) Aldehydes and anilines condense to listed for a number of anilines and aliphatic form imines in situ, which react with tyrosine amines using a-chymotrypsinogen A as the residues through an electrophilic aromatic substrate and formaldehyde as the aldehyde substitution reaction. No reaction occurs component.

Fig. 10.3-6 Tyrosine modification using commercially available lysine-reactive probes, (a) The aliphatic amino group reacts chemoselectively with NHS esters, leaving the aniline amino group free to participate in the Mannich reaction. On addition of formaldehyde and a protein target, tyrosine residues are modified, (b) Modification of...

Fig. 10.3-8 Tyrosine modification using palladium jr-allyl chemistry, (a) Allylic acetates (shown), carbonates, and carbamates can be activated by palladium(O) in aqueous solution to yield electrophilic 7r-allyl complexes. These species alkylate tyrosine residues with high...

The chemistry of the brain and central nervous system is affected by a group of substances called neurotransmitters, substances that carry messages across a synapse from one neuron to another Several of these neurotransmitters arise from l tyrosine by structural modification and decarboxylation as outlined m Figure 27 5... 

Histone phosphorylation is a common posttranslational modification fond in histones, primarily on the N-terminal tails. Phosphorylation sites include serine and threonine residues, tyrosine phosphorylation has not been observed so far. Some phosphorylation events occur locally whereas others occur globally throughout all chromosomes during specific events like mitosis. Histone phosphorylation is catalyzed by kinases. Removal of the phosphoryl groups is catalyzed by phosphatases. 

After their synthesis (translation), most proteins go through a maturation process, called post-translational modification that affects their activity. One common post-translational modification of proteins is phosphorylation. Two functional classes of enzymes mediate this reversible process protein kinases add phosphate groups to hydroxyl groups of serine, threonine and tyrosine in their substrate, while protein phosphatases remove phosphate groups. The phosphate-linking... 

Protein tyrosine kinases (PTKs) are enzymes (EC 2.7.1.112) that catalyze the transfer of the y-phosphate group of ATP to tyrosine residues of protein substrates. The activity of PTKs is controlled in a complex manner by posttranslational modifications and by inter- and intramolecular complex formations. 

Neurotoxins present in sea snake venoms are summarized. All sea snake venoms are extremely toxic, with low LD5Q values. Most sea snake neurotoxins consist of only 60-62 amino acid residues with 4 disulOde bonds, while some consist of 70 amino acids with 5 disulfide bonds. The origin of toxicity is due to the attachment of 2 neurotoxin molecules to 2 a subunits of an acetylcholine receptor that is composed of a2 6 subunits. The complete structure of several of the sea snake neurotoxins have been worked out. Through chemical modification studies the invariant tryptophan and tyrosine residues of post-synaptic neurotoxins were shown to be of a critical nature to the toxicity function of the molecule. Lysine and arginine are also believed to be important. Other marine vertebrate venoms are not well known. 

There is only one tyrosine residue in some sea snake neurotoxins. This residue is usually quite difficult to modify, but once it is modified, the toxicity is lost (9). Histidine seems not to be essential as the chemical modification of this residue does not affect the toxicity 10). 

Originally, for preparation of such conjugates the hydroxyl groups of monomethoxy-PEG (mPEG) were activated with cyanuric chloride, and the resulting compound then coupled with proteins (10). This approach suffers from disadvantages, such as the toxicity of cyanuric chloride and its limited applicability for modification of proteins having essential cysteine or tyrosine residues, as manifested by their loss of activity. 

Chemical modifications of proteins (enzymes) by reacting them with iV-acylimidazoles are a way of studying active sites. By this means the amino acid residues (e.g., tyrosine, lysine, histidine) essential for catalytic activity are established on the basis of acylation with the azolides and deacylation with other appropriate reagents (e.g., hydroxylamine). 

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