Tyrosine residues protonated

H2O O2 -F 4e -F 4H+). The electrons are transferred to P680 through a redox active tyrosine residue, protons are ejected into the luminal space, and molecular oxygen is released into the atmosphere. Protons and electrons are ultimately used to convert carbon dioxide to organic molecules that constitute the biosphere and the oxygen released supports most life on earth. 4i... 

Tyrosine fluorescence emission in proteins and polypeptides usually has a maximum between 303 and 305 nm, the same as that for tyrosine in solution. Compared to the Stokes shift for tryptophan fluorescence, that for tyrosine appears to be relatively insensitive to the local environment, although neighboring residues do have a strong effect on the emission intensity. While it is possible for a tyrosine residue in a protein to have a higher quantum yield than that of model compounds in water, for example, if the phenol side chain is shielded from solvent and the local environment contains no proton acceptors, many intra- and intermolecular interactions result in a reduction of the quantum yield. As discussed below, this is evident from metal- and ionbinding data, from pH titration data, and from comparisons of the spectral characteristics of tyrosine in native and denatured proteins. 

Rat oncomodulin, a parvalbumin-1 ike tumor protein that has two tyrosine residues but no tryptophan, exhibits fluorescence emission at 301 and 345 nm.(135) Upon binding two moles of Ca2+ per mole of oncomodulin, the 301-nm intensity increases while the 345-nm band decreases. These results were explained in terms of acidic side chains involved in either binding Ca2+ or accepting a proton on excited-state generation of tyrosinate. The cloned... 

The finding that the hydrolytic activity of the enzyme is retained after replacement of a tyrosine residue by phenylalanyl challenges the notion that a tyrosine acts as a general acid catalyst in peptide hydrolysis. It has been suggested that either the protonated Glu270 moiety or the zinc-water complex could perform the proton transfer [77]. 

The apparent epitope map based on STDs can exhibit rather significant dependence on which particular protein proton(s) is being saturated. For example, saturation of resolved methyl proton resonances from two separate residues may result in STD spectra with different relative and/or absolute intensities. Similarly, saturation of a specific tyrosine ring proton vs. a specific methyl group can result in different STD spectra. 

Ribonuclease is an enzyme with 124 amino acids. Its function is to cleave ribonucleic acid (RNA) into small fragments. A solution containing pure protein, with no other ions present except H+ and OH- derived from the protein and water, is said to be isoionic. From this point near pH 9.6 in the graph, the protein can be titrated with acid or base. Of the 124 amino acids, 16 can be protonated by acid and 20 can lose protons to added base. From the shape of the titration curve, it is possible to deduce the approximate pATa for each titratable group.1-2 This information provides insight into the environment of that amino acid in the protein. In ribonuclease, three tyrosine residues have "normal values of pATa(=10) (Table 10-1) and three others have pA a >12. The interpretation is that three tyrosine groups are accessible to OH, and three are buried inside the protein where they cannot be easily titrated. The solid line in the illustration is calculated from pA"a values for all titratable groups. 

Holtje and Sippl selected a representative training set of twelve superimposed structures. The aforementioned Y AK process resulted in a pseudoreceptor that consists of six amino acid residues (Figure 5). In this pseudoreceptor model, the imidazole ring of the ligands is bound to a tyrosine residue that donates a proton to the imidazole and an asparagine residue that accepts a proton from the imidazole. Hence, two distinct H-bond have been found, which explain the strict binding mode of the imidazole. It has to be noted that the position of these receptor residues relative towards the imidazole is a consequence of... 

In an important step to mimic the natural photosystem, tyrosine residues tethered to a Rubpy sensitizer as in 15 have been shown to reduce the Rum center obtained after oxidative quenching with methylviologen or [Co(NH3)5C1]2+.187 Formation of the resulting tyrosyl radical is a proton coupled process and it has been shown to be a concerted process in which the reorganization energy associated with deprotonation can be tuned by H-bonding and pH.188191 Similar results are observed for tyrosyl residues tethered to Re(I)diimine based chromophores.192... 

Figure 19.15. A Plausible Scheme for Oxygen Evolution from the Manganese Center. A possible partial structure for the manganese center is shown. The center is oxidized, one electron at a time, until two bound H2O molecules are linked to form a molecule of O2, which is then released from the center. A tyrosine residue (not shown) also participates in the coupled proton-electron transfer steps. The structures are designated Sq through S4 to indicate the number of electrons that have been removed.

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