Tryptophan isoelectric point

On the acid side of the isoelectric point, Kronman et al. (1965) noted that the a-lactalbumin molecule swells and yields a difference spectra with maxima at 285-286, 292-293, and 230 nm, due largely to changes in the environment of the tryptophan residues. The amplitude of the... 

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)). 

Table 5 presents the amino acid compositions of a number of apoferritins together with their isoelectric point (taken from the literature) where available. There are a number of features whihc are of quite considerable interest. In the first place, although there are clear cut differences between the proteins of different species, a number of very considerable similarities exist. Thus, for all of the apoferritins for which data is available the content of non-polar amino acids (proline, glycine, alanine, valine, methionine, leucine, isoleucine, phenylalanine and tryptophan) remains constant at about 45%. What is even more remarkable is that the content... 

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. 

Analogous effects were observed with aspartic acid in contact with chromium hydroxide particles [24]. However, only negligible amounts of tryptophan adsorbed on the same solid. The isoelectric point of this amino acid is much higher than of the aspartic acid, and it is close to that of chromium hydroxide. Furthermore, the larger aromatic (hydrophobic) tryptophan molecules may not be able to penetrate into the hydrophilic adsorbent. 

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