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PubMed1975

Titration study of acetylated lysozyme.

To study the interaction between carboxyl groups and amino groups in native lysozyme [EC 3.2.1.17], and to identify the positions and the pK values of the abnormal carboxyl groups, N-acetylated lysozyme was prepared. The acetylation did not affect the molecular shape of the enzyme, but changed six amino groups to a non-ionizable form, leaving one amino group free; this was determined to be Lys 33. In addition, pH titration of the acetylated lysozyme in 0.2 or 0.02 M KCl aqueous solution indicated fewer titratable groups with pK(int) of 7.8 or 10.4 compared with the native protein, though the number of titratable carboxyl groups was not affected by the acetylation. From the pH titration results and structural considerations, the unititratable carboxyl groups were suggested to be Asp 48, Asp 66, and Asp 87. On the other hand, spectrophotometric titration in 0.2 M KCl showed that all three tyrosine residues are titratable in the acetylated protein, although an abnormal tyrosine residue exists in the native state. Tyr 20 was suggested to be untitratable in the pH range of 8-12.6.

Acetylation↗
PubMed1976

A re-examination of the conformation of secretin in water.

An examination of a series of peptides corresponding to partial sequences of secretin and the application of empirical conformational parameters to the sequence has reopened the question about the position of the short helical stretch in the folded chain. In earlier studies, this was tentatively placed near the N-terminus, while newly accumulated evidence points to the C-terminal area. The possible role of ion pairs in the stabilization of the folding was investigated by the preparation and examination of synthetic analogues of the C-terminal tricosapeptide part of secretin. The carboxyl groups of glutamic acid (in position 9) and of aspartic acid (in position 15) were replaced by carboxamides. The 9-glutamine and the 15-asparagine analogues show a significant decrease in helical character. This loss of "structure" is even more pronounced in the 9-glutamine-15-asparagine tricosapeptide. Thus, ion-pair formation is indeed implicated as one of the forces which stabilize the folded conformation of the chain. Possible correlations between biological activity and secretin-like architecture were studied on several smooth muscle preparations.

Amino Acid Sequence↗