Histidine isoelectric point

Some ammo acids have side chains that bear acidic or basic groups As Table 27 3 indicates these ammo acids are characterized by three values The third pK reflects the nature of the side chain Acidic ammo acids (aspartic and glutamic acid) have acidic side chains basic ammo acids (lysine arginine and histidine) have basic side chains The isoelectric points of the ammo acids m Table 27 3 are midway between the pK values of the zwitterion and its conjugate acid Take two examples aspartic acid and lysine Aspartic acid has an acidic side chain and a pi of 2 77 Lysine has a basic side chain and a pi of 9 74... 

At its isoelectric point, histidine is said to be zwitterionic, as it has both anionic and cationic properties. Most other amino acids are also zwitterionic at neutral pH. Peptides and proteins also have isoelectric points, which can vary widely depending on the composition of the amino acids. 

Calculate the isoelectric point for histidine, aspartic acid, and arginine. Calculate the fractional charge for... 

The bacterial ferredoxins, in general, are similar to each other in amino acid content, but differ in detail with each ferredoxin having a characteristic composition. Each contains about fifty total amino acid residues with an abundance of acidic and a paucity of basic amino acids. The abundance of acidic residues accounts for the affinity of ferredoxin for DEAE-cellulose and its low isoelectric point (Lovenberg, Buchanan, and Rabinowitz (65)). Each of the bacterial ferredoxins lacks histidine, methionine, tryptophan, and at least one additional amino acid, which is characteristic of a particular ferredoxin. The possible significance of these differences in either the conformation or function of these proteins has not been established, although minor differences in enzymic activity do exist (Lovenberg, Buchanan, and Rabinowitz (65)). 

Basic amino acids (histidine, lysine, and arginine) have isoelectric points at pH values of 7.6, 9.7, and 10.8, respectively. These values reflect the weak basicity of the imidazole ring, the intermediate basicity of an amino group, and the strong basicity of the guanidino group. A basic solution is needed in each case to prevent protonation of the basic side chain to keep the amino acid electrically neutral. 

Calculate the isoelectric point for the following polyftmctional amino acids (a) histidine, (b) lysine, and (c) aspartic acid. (See Table 5-1.)... 

The enthalpy of ionization of the titratable histidine groups in myoglobin is 7.5 kcal mol-1 (Antonini and Brunori, 1971). These groups are involved at the isoelectric point level. The solvent effect on the pKa of histidine has been measured. From a value of 6.1 in pure water, it decreases to 5.85 in the mixed solvent, at +20°C. The conditions (AH buffer > AH of the pi) for isoelectric focusing are therefore satisfied. 

The isoelectric point for histidine is the pH value halfway between the pK values for the two nitrogen-containing groups. The p.Aa and pi values of amino acids in peptides and proteins differ somewhat from those of free amino acids, principally because most of the a-amino and a-carboxyl groups are not ionized but are covalently joined in peptide bonds. 

Table 4 examines the amino acid composition of purified SCP2 [21] and compares it with the amino acid compositions of 3 other preparations of interest [47-49], The most abundant amino acid in purified SCP2 is lysine, which accounts for 14 mole%. Since the isoelectric point for SCPj is approximately 8.6, it follows that a substantial portion of the glutamic and aspartic acid residues are amidated. Also of interest is the finding that SCPj contains no arginine or tyrosine, and only small amounts (perhaps 1 residue each) of histidine and tryptophan. 

Fig. 6.9. Titration curve of histidine. The ionic species that predominates in each region is shown below the graph, pi is the isoelectric point (at which there is no net charge on the molecule).

Mathematical Galculate the isoelectric point of each of the following amino acids glutamic acid, serine, histidine, lysine, tyrosine, and arginine. 

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