Histidine titration curve

If the histidine titration curves are composite in the free protein as observed by Ruterjans and Witzel but not in the substrate complexes, the ks/Km curve should be distorted but not the ke curve. The opposite is observed. 

With multiple ionizable groups, such as in amino acids and proteins, each group titrates separately according to its pKa. The titration curves shown in Fig. 23-5 are for the amino acids glycine, histidine, and glutamate. 

FIGURE 3-12 Titration curves for (a) glutamate and (b) histidine. The pKa of the R group is designated here as ptfR. 

Figure 3-1 Titration curve for histidine. The solid line represents the uncorrected titration curve for 3 mM histidine monohydrochloride titrated with 0.2 M HCL to lower pH or with 0.2 M NaOH to higher pH assuming pKa values of 1.82, 6.00, and 9.17. The dashed line represents the corrected curve showing the number of protons bound or lost per mole of histidine monohydrochloride.

Using the pKa values from problem 3, construct the theoretical titration curve showing the equivalents of H+ or OH reacting with 1 mol of glycine as a function of pH. Note that the shape of this curve is independent of the pfCa. Sketch similar curves for glutamic acid (pK./s equal 2.19,4.25, and 9.67), histidine (pfCa s equal 1.82,6.00, and 9.17) and lysine (pfCa s equal 2.18,8.95, and 10.53). 

Titration curves of glutamic acid, lysine, and histidine. In each case, the pK of the R group is designated pKR. 

Titration curve of /3-lactoglobulin. At very low values of pH (<2) all ionizable groups are protonated. At a pH of about 7.2 (indicated by horizontal bar) 51 groups (mostly the glutamic and aspartic amino acids and some of the histidines) have lost their protons. At pH 12 most of the remaining ionizable groups (mostly lysine and arginine amino acids and some histidines) have lost their protons as well. 

Figure 2-6. Titration curves for glycine (A) and histidine (6). The molecular species of glycine present at various pHs are indicated by the molecules above the curve. For histidine, pKa is the dissociation constant of the imidazole (side chain) group.

Unusual features of the titration curves for charged groups, such as deviation from the expected Henderson-Hasselbalch pattern, indicating mutual interaction or the failure of a residue, e.g., histidine or tyrosine to titrate with the expected p/C, which can be taken to reflect local interactions. 

Recent work with kirromycin (85, mocimycin) has included study of its interactions with EF-Tu. NMR has been used to examine the EF-Tu kirromycin complex. Small changes in the pH titration curves for E. coli EF-Tu histidine residues were noted in the presence of 85, and the acid stability of EF-Tu was increased slightly [237]. In another study, the 1 1 deuterated EF-Tu 85 complex was examined by H NMR. Differences in the spectrum of the complex were consistent with induction of a conformation similar to that of EF-Tu-GTP [238], Results from... 

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