Tyrosine dimer

For proteins such as tyrosine-kinase receptors, dimerization (the association of two receptors to form a new species... 

I topoisomerase of mammals is a 100 kD monomeric protein whose activity is ATP-independent. This enzyme binds to double-stranded DNA and cleaves one of the DNA strands of the duplex, simultaneously forming an enzyme-DNA covalent bond between a tyrosine residue and the 3 -phosphate of the cleaved DNA. The type II topoisomerases are dimeric enzymes, which are ATP-dependant. Two isoforms of topoisomerase II exist, topoisomerase a and (3, with apparent molecular weights of 170 and 180 kD. Topoisomerase... 

II cleaves the two complementary strands of DNA four base pairs apart and the resulting 5 -phosphoryl groups become covalently linked to a pair of tyrosine groups, one in each half of the dimeric topoisomerase II enzyme. Several groups of drugs are known that selectively inhibit topoisomerases in bacteria (quino-lones) or mammalian cells (etoposide, tenoposide). Quinolones are used to treat bacterial infections inhibitors of mammalian topoisomerases are cytostatic drugs used for the treatment of cancer. 

Phenoxazines — The two main types of phenoxazines are the ommochromes and the microbial phenoxazines. The biosynthesis of ommochromes occurs via the kynurenine pathway. The tryptophan amino acid is converted to formylkynurenine and then to kynurenine and 3-hydroxykynurenine. Not all the steps of ommochrome synthesis are completely elucidated yet. Ommatins are dimers and ommins are oligomers of 3-hydroxykynurenine. - The papiliochromes are derived from tyrosine as well as from the tryptophan pathway. The key intermediate in the formation of papiliochromes is N-beta-alanyldopamine (NBAD). Papiliochromes are synthesized in special wing scale cells, before melanins. " "... 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

FIGURE 4.2 Receptor tyrosine kinase dimerization. Binding of monomeric or dimeric ligands to RTK monomers leads to formation and stabilization of activated RTK dimers. Cytoplasmic domains of RTK are trans-phosphorylated by active FTK. 

In principle, RTK autophosphorylation could occur in cis (within a receptor monomer) or in trans (between two receptors in a dimer). In the first case, ligand binding would cause a change in receptor conformation that would facilitate c/ s-autophosphorylation of tyrosine residues located within or outside the PTK domain. In the second case, no conformational change must occur upon dimerization. The simple proximity effect would provide sufficient opportunity for trans-phosphorylation of tyrosines in the cytoplasmic domain by a second RTK. 

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