Tyrosine activation

An outline mechanism for tyrosine activation has been proposed (Fersht, 1975 Fersht et al., 1975a,b Ward and Fersht, 1988a) on the basis of conventional kinetic and binding studies, and this is shown in (49). For the aminoacylation step, some aspects of the reaction are still not known such as the point at which AMP is displaced, but the currently preferred mechanism (Fersht and Jakes, 1975 Ward and Fersht, 1988b) is that given in (50). This is compatible with the observed kinetics which show that two moles of tyrosine bind in each enzyme turnover during which one molecule of Tyr-tRNA appears. 

Fig. 17 Free energy/reaction coordinate diagram for tyrosine activation [see (49)], with wild-type tyrosyl-tRNA synthetase (E) and the Tyr-34 to Phe mutant (E ).

Table 18 Values of and for wild-type and mutant enzymes in tyrosine activation [see (47) and (49)]. 

One of the important consequences of studying catalysis by mutant enzymes in comparison with wild-type enzymes is the possibility of identifying residues involved in catalysis that are not apparent from crystal structure determinations. This has been usefully applied (Fersht et al., 1988) to the tyrosine activation step in tyrosine tRNA synthetase (47) and (49). The residues Lys-82, Arg-86, Lys-230 and Lys-233 were replaced by alanine. Each mutation was studied in turn, and comparison with the wild-type enzyme revealed that each mutant was substantially less effective in catalysing formation of tyrosyl adenylate. Kinetic studies showed that these residues interact with the transition state for formation of tyrosyl adenylate and pyrophosphate from tyrosine and ATP and have relatively minor effects on the binding of tyrosine and tyrosyl adenylate. However, the crystal structures of the tyrosine-enzyme complex (Brick and Blow, 1987) and tyrosyl adenylate complex (Rubin and Blow, 1981) show that the residues Lys-82 and Arg-86 are on one side of the substrate-binding site and Lys-230 and Lys-233 are on the opposite side. It would be concluded from the crystal structures that not all four residues could be simultaneously involved in the catalytic process. Movement of one pair of residues close to the substrate moves the other pair of residues away. It is therefore concluded from the kinetic effects observed for the mutants that, in the wild-type enzyme, formation of the transition state for the reaction involves a conformational change to a structure which differs from the enzyme structure in the complex with tyrosine or tyrosine adenylate. The induced fit to the transition-state structure must allow interaction with all four residues simultaneously. 

Scheme 6.8 Epoxide recognition for epoxide hydrolase that detoxify living cells by catalyzing alcoholysis to water soluble diols. The working model involves the phenolic H-atoms of two tyrosines activating the epoxide for nucleophilic attack. This principle is realized analogously by double hydrogenbonding thiourea catalyst 9 in the natural medium water.

Fig. 14.5 The TCR Complex is associated with a number of T< ll specific membrane-spanning proteins.31 The antigen receptors and the co-receptors, CDS, CD4, and CDS, together with associated enzymes, tyrosine kinases and phosphatases, form the actual signalling complex. The cytoplasmic chains of the co-receptor molecules have characteristic consensus sequences, the ITAMs (immunoreceptor tyrosine activation motife). Each of the invariant -chains and the CD3-y,8, e chains, contain 1-3 copies of the RAM motife. The structure of the TCR-signalling complex still needs to be clarified.

Model compound and difference spectral studies have been carried out on tyrosine and related compounds by Wetlaufer (1956), Laskowski (1957), Edelhoch (1958), Chervenka (1959), Bigelow and Geschwind (1960), Yanari and Bovey (1960), and Foss (1961) (see also Section VI,Z)). Studies of the fluorescence of tyrosine—activation spectrum, fluorescence emission spectrum, and quantum yield—have been reported by Teale and Weber (1957). The pH-dependence of the fluorescence of tyrosine and related compounds was studied by White (1959) fluorescence-polarization studies from the same laboratory were reported by Weber (1960a) for simple compounds, and for proteins (Weber, 1960b). Teale (1960) has carried out extensive studies on the fluorescence characteristics of a score of proteins. 

ITAM immunoreceptor tyrosine activated motive mPDS methylprednisolone... 

EGF-binding site Binding of EGF empty tyrosine activates kinase is inactive, tyrosine kinase. 

Table 5. Protein Tyrosine Activity Inhibitory Activity of Emodin, Physcion and Emodin-0 -p-D-glucoside Against PTK in p56 ck Cells...

Indole and phenol are chemorepellents for Salmonella [412, 746]. It is possible that, due to the overlap in stmeture, tryptophan and tyrosine activate the chemorepellent reeeptors of indole and phenol, respectively, resulting in repulsion. This will eause no harm to the bacteria, because the presenee of serine and aspartate—extremely strong attrac-tants— will clearly overwhelm any mild repulsive action of tryptophan and tyrosine [381],... 

The properties of the purified enzyme correspond closely to the properties observed in crude preparations (see above) and also agree with the properties of the purified enzymes specific for other amino acids (listed in Table IV, as far as they have been determined. In addition to Mg, the tyrosine activating enzyme (133) has a requirement for K+. To these properties ought to be added the ability to transfer the activated amino acid pnto a specific polynucleotide acceptor (158). 

Fuller, B.B., Drake, M.A. and Spaulding, D.T. (2000) Downregulation of tyrosine activity in human melanocyte cell cultures by yohimbine. /. Invest Dermatol, 114, 26S-276. 

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