Acidity continued histidine

Heterocyclic amino acids (continued) CH,—CHCO fim 0 Histidine His H 2.1%... 

The number of known or presumed mononuclear, non-heme iron oxygenases and related enzymes continues to grow. This is due to intensive biochemical research and especially based on sequence data derived from genome research projects i.14). For several of these enzymes structural data are available by now from protein crystallography (12-14). In many of the iron oxygenases the iron is facially bound by two histidines and one carboxylate donor, either glutamic acid or aspartic acid. Thus, the term 2-His-l-carboxylate facial triad has been introduced by L. Que Jr. for this motif (19). 

The genetic code refers to specific sequences of RNA bases (or DNA bases) that encode specific amino acids. Kach of these sequences is composed of a triplet of bases (three bases in a row). Hence, any mRNA molecule can be considered a continuous polymer of successive triplets. For example, the triplet UUU codes for phenylalanine, CAU encodes histidine, GAG encodes glutamate, AAA encodes lysine, and AUG codes for methionine. DNA is used for information storage, while mRNA is used for information transfer. 

Such a procedure adapted from Gross and Morell (1966) and Blumenfeld and Gallop (1962) is as follows. Visser et al. (1971) treated 2.5 to 10 mg of a modified elastase with 1 N NHjOH-HCl, adjusted to pH 9 by addition of sodium hydroxide, for 2 hr at 25°C. The excess hydroxylamine was removed either by dialysis or by precipitation of the protein at pH 3.0. The protein was then dissolved, brought to pH 8.0 by addition of NaOH and treated with an equal volume of a 1 % solution of l-fluoro-2,4-dinitrobenzene in ethanol. The pH of the solution was maintained at 8 by the continuous addition of NaOH. The reaction is complete when no additional alkali must be added for 5 min. The mixture is then extracted three times with ether and the aqueous phase subjected to the conditions of the Lossen rearrangement (i.e. heating to 100°C under alkaline condition (0.1 N NaOH) for 10 min). Acid hydrolysis followed by amino acid analysis permits the identification of either diaminopropionic or diaminobutyric acid which would result from either aspartate or glutamate modification, respectively. Diaminopropionic and diaminobutyric acids may be estimated on the short column of the amino acid analyzer. Diaminopropionic acid emerges with histidine. (Color values do not seem to be available.)... 

Fig. 10.4 (continued) tide bond cleavage. In the cysteine (and also serine and threonine) proteases, the nucleophile is the protease type amino acid (in this case cysteine) which forms a covalent bond with the carbon atom of the bond to be cleaved (covalent catalysis) in contrast to the metalloprotei-nases and aspartic proteases which use an activated water molecule to attack the carbon atom to be cleaved (noncovalent catalysis). In covalent catalysis, a nearby histidine residue normally functions as a base to activate the mechanism, whereas in noncovalent catalysis, the protease type serves as an acid and base, with an ancillary histidine (aspartate proteases) or aspartate or glutamate residue acting as the nucleophile (Fig. 8.2b) (Modified from Fig. 9.18 in Berg., et al., Biochemistry, 5th Ed. 2002, W.H. Freeman Co., New York)...

The complexation of the Group IIB metals by amino-acid and related derivatives continues to be a subject of interest stability constants reported include those for histidine and its derivatives, histamine, glycylhistamine, aspartic and glutamic acids, aspargine, glutamine, glycine, cysteine, and alanine. " ... 

With histidine and arginine, the problem is the intrinsic polarity of the derivative. This affects the solubility of the derivative in the solvent (ethyl acetate or chlorobutane) used to transfer the derivative from the reaction cell to the conversion flask. With some systems, trifluoroacetic acid (TFA) is added to this solvent to help counteract the problem, although the tenacious attraction of TFA to proteins/peptides and their derivatives indicates that His and Arg derivatives have already formed the TFA salt during the cleavage reaction and continue to... 

---