Borén K, Freskgård P O, Carlsson U
IFM-Department of Chemistry, Linköping University, Sweden.
Protein Sci. 1996 Dec;5(12):2479-84. doi: 10.1002/pro.5560051210.
The CD spectra of human carbonic anhydrase I and II and bovine carbonic anhydrase III were recorded and analyzed. The 3D structures of these isoenzymes are known, showing very similar secondary structure and polypeptide-chain fold. The tryptophan content, however, differs between the isoenzymes, i.e., isoenzymes I, II, and III possess 6, 7, and 8 tryptophans, respectively. All of the tryptophans except the additional tryptophans in isoenzymes II and III, i.e., W245 and W47, are conserved. Despite the fact that X-ray structure determinations showed that the isoenzymes had highly similar secondary structure, the contents of alpha-helix and beta-sheet structure differed considerably when using different CD algorithms for estimation of the fractions of various secondary structural elements. This shows that aromatic amino acids also interfere in the wavelength region (far-UV) used to calculate the amount of secondary structure. Such interference is especially problematic when analyzing proteins like carbonic anhydrase, which consist mainly of beta-structure that gives rise to weak ellipticity bands, compared to the bands arising from alpha-helical structure.
记录并分析了人碳酸酐酶I和II以及牛碳酸酐酶III的圆二色光谱。这些同工酶的三维结构是已知的，显示出非常相似的二级结构和多肽链折叠。然而，这些同工酶中的色氨酸含量不同，即同工酶I、II和III分别含有6、7和8个色氨酸。除了同工酶II和III中额外的色氨酸（即W245和W47）外，所有色氨酸都是保守的。尽管X射线结构测定表明同工酶具有高度相似的二级结构，但在使用不同的圆二色算法估计各种二级结构元件的比例时，α-螺旋和β-折叠结构的含量有很大差异。这表明芳香族氨基酸也会干扰用于计算二级结构数量的波长区域（远紫外）。在分析像碳酸酐酶这样的蛋白质时，这种干扰尤其成问题，因为与α-螺旋结构产生的谱带相比，碳酸酐酶主要由β-结构组成，会产生较弱的椭圆率谱带。
