Conformation of Lysozyme Derivatives Modified at Two Carboxyl Groups
M. Z. Atassi 1 and M. C. Rosemblatt 1
From the
1 From the Department of Chemistry, Wayne State University, Detroit, Michigan 48202
Conformational studies have been carried out on two lysozyme derivatives in which the carboxyl groups of aspartic acid 119 and the COOH-terminal leucine 129 were coupled, after carbodiimide activation, with glycine methyl ester (i.e. (GlyMe)2-Lysoz) or with histidine methyl ester (i.e. (HisMe)2-Lysoz). No changes were observed in the spectral and sedimentation behaviors of the two derivatives relative to those of lysozyme. In ORD measurements the rotatory behaviors of lysozyme and (GlyMe)2-Lysoz were identical both at the negative minimum at 233 nm and at the positive 199-nm extremum. Also, their b0 values were identical. On the other hand, (HisMe)2-Lysoz showed a decrease in the ORD parameters of [m']233, [m']199, and b0 values. The rotatory behaviors of the derivatives were also determined over a wide pH range. It was significant that the ORD parameters of (GlyMe)2-Lysoz and lysozyme were equal throughout the pH range 2 to 11. At a given pH, the corresponding parameters of (HisMe)2-Lysoz were lower. In circular dichroism measurements, the [
'] values at the negative ellipticity bands at 208 and 220 nm for lysozyme and (GlyMe)2-Lysoz were equal. The ellipticity values for (HisMe)2-Lysoz were suppressed appreciably relative to lysozyme. Although no conformational differences between lysozyme and (GlyMe)2-Lysoz were observed by ORD and CD measurements, some differences were detectable by chemical monitoring of the conformation. Accessibility to tryptic hydrolysis was appreciably increased in (GlyMe)2-Lysoz (1.15 bonds per mole of protein), and more greatly in the (HisMe)2-Lysoz derivative (3.83 bonds) relative to lysozyme (0.14 bond). Also, there was an appreciable increase in the reducibility of the disulfide bonds in (GlyMe)2-Lysoz (0.29 bond) and (HisMe)2-Lysoz (1.1 bonds) relative to native lysozyme (0.03 bond). The enzymic activity of each derivative retained the same pH optimum as native lysozyme but was decreased drastically. The results are valuable for understanding the immunochemistry of the derivatives. Also, the contribution of the interactions of the modified carboxyl groups to the three-dimensional structure in solution and its relation to the crystalline structure are discussed.
Submitted on November 14, 1973
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