Lakshmanan Thiruneelakantan, Sriram Desikan, Priya Kuttikode, Loganathan Duraikkannu
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India.
Biochem Biophys Res Commun. 2003 Dec 12;312(2):405-13. doi: 10.1016/j.bbrc.2003.10.149.
The linkage region constituents, namely, 2-acetamido-2-deoxy-beta-D-glucopyranose and asparagine are conserved in the N-glycoproteins of all the eukaryotes. The present work is aimed at understanding the reasons for the occurrence of GlcNAc and Asn as the linkage region constituents. A total of six sugar amides have been designed as models and analogs of the linkage region and their crystal structures have been solved. This is the first report on the X-ray crystallographic investigation of the effect of systematic changes in the linkage sugar as well as its aglycon moiety on the N-glycosidic torsion, psi(N) (O5-C1-N1-C1(')). This also forms the first report on the crystal structure of a model of L-RhabetaAsn, a variant linkage found in the surface layer glycoprotein of Bacillus stearothermophillus. Among the models and analogs examined, the acetamido derivatives of Man and Xyl, the linkage sugars of O-glycoproteins, show a psi(N) value of -114.5 degrees and -121.2 degrees, respectively, deviating maximum from the value of -89.8 degrees reported for the model compound GlcNAcbetaNHAc. The L-Rha and Gal derivatives also show noticeable deviations. The psi(N) values, -89.5 degrees and -91.0 degrees, of the propionamide derivatives of Glc and GlcNAc (analogs of GlcbetaGln and GlcNAcbetaGln, respectively) agree well with those (-93.8 degrees and -89.8 degrees ) reported for their corresponding acetamide derivatives suggesting Gln could serve as well as Asn as the linkage region amino acid. However, the rotational freedom about the additional C-C bond would lead to altered rigidity of the linkage region. An analysis of packing reveals that the molecular assembly of these compounds is driven by different infinite and finite chains of hydrogen bonds. The double pillaring of hydrogen bonds involving the amide groups at C1 and C2 is seen as a unique packing feature characteristic of beta-1-N-acyl derivatives of GlcNAc. Based on the findings of the present study, it is speculated that the linkage region constituents of the eukaryotic N-glycoproteins appear to fulfill three essential structural requirements: rigidity, planarity, and linearity and these are met by the trisaccharide core and Asn at the linkage region.
连接区域的组成成分，即2-乙酰氨基-2-脱氧-β-D-吡喃葡萄糖和天冬酰胺，在所有真核生物的N-糖蛋白中都是保守的。本研究旨在了解作为连接区域组成成分的N-乙酰葡糖胺（GlcNAc）和天冬酰胺（Asn）出现的原因。总共设计了六种糖酰胺作为连接区域的模型和类似物，并解析了它们的晶体结构。这是关于连接糖及其糖苷配基部分的系统变化对N-糖苷扭转角ψ(N)（O5-C1-N1-C1'）影响的X射线晶体学研究的首次报道。这也是关于嗜热脂肪芽孢杆菌表层糖蛋白中发现的一种变体连接L-RhabetaAsn模型晶体结构的首次报道。在所研究的模型和类似物中，O-糖蛋白连接糖甘露糖（Man）和木糖（Xyl）的乙酰氨基衍生物的ψ(N)值分别为-114.5°和-121.2°，与模型化合物GlcNAcbetaNHAc报道的-89.8°值偏差最大。L-鼠李糖（L-Rha）和半乳糖（Gal）衍生物也表现出明显偏差。葡萄糖（Glc）和N-乙酰葡糖胺（GlcNAc）的丙酰胺衍生物（分别为GlcbetaGln和GlcNAcbetaGln的类似物）的ψ(N)值-89.5°和-91.0°与它们相应的乙酰胺衍生物报道的值（-93.8°和-89.8°）非常吻合，表明谷氨酰胺（Gln）可以和天冬酰胺（Asn）一样作为连接区域的氨基酸。然而，围绕额外C-C键的旋转自由度会导致连接区域刚性的改变。堆积分析表明，这些化合物的分子组装是由不同的无限和有限氢键链驱动的。涉及C1和C2处酰胺基团的氢键双支柱结构被视为GlcNAc的β-1-N-酰基衍生物独特的堆积特征。基于本研究的结果，推测真核生物N-糖蛋白的连接区域组成成分似乎满足三个基本结构要求：刚性、平面性和线性，而连接区域的三糖核心和天冬酰胺满足了这些要求。
