Asparagine-linked glycans (N-glycans) are attached onto nascent glycoproteins in the endoplasmic reticulum (ER) and subsequently processed by a set of processing enzymes in the ER and Golgi apparatus. Accumulating evidence has shown that not all N-glycans on glycoproteins are uniformly processed into mature forms (hybrid and complex types in mammals) through the ER and Golgi apparatus, and a certain set of glycans remains unprocessed as an “immature” form (high-mannose type in mammals). Much attention has been paid to environmental factors regulating N-glycoprotein maturation, such as the expression levels of glycosyltransferases/glycosidases. On the other hand, the influence of the 3D structure of the carrier glycoprotein on N-glycan maturation has been investigated mostly using individual model glycoproteins. To obtain more insights into N-glycoprotein maturation, we herein analyze glycoprotein structures extracted from the Protein Data Bank. We confirm that site-specific N-glycan processing is largely explained by the solvent accessibility of the glycosylated Asn residue and of the conjugated N-glycan. Potential bias of protein structural features toward immature or mature forms was explored within a range of concentric circles of fully folded glycoproteins. There does appear to be bias in amino acid composition and secondary structure. Most notably, γ-branched amino acid residues (Asn+Asp+Leu) occur more frequently on unstructured loop regions of immature glycoproteins. Structural features of the protein surface around the N-glycosylated site do seem to affect N-glycan processing and maturation.

中文翻译：

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分析PDB储存库中N-糖基化位点周围的蛋白质结构，以了解N-糖蛋白加工和成熟的结构基础

天冬酰胺连接的聚糖（N-聚糖）附着在内质网（ER）的新生糖蛋白上，随后在ER和高尔基体中通过一系列加工酶进行加工。越来越多的证据表明，并非所有糖蛋白上的N-聚糖都通过ER和Golgi装置被统一加工成成熟形式（哺乳动物中的杂种和复杂类型），并且某些聚糖仍未加工成“未成熟”形式（高哺乳动物中的甘露糖类型）。已经非常关注调节N-糖蛋白成熟的环境因素，例如糖基转移酶/糖苷酶的表达水平。另一方面，载体糖蛋白的3D结构对N的影响-聚糖成熟已主要使用单个模型糖蛋白进行了研究。为了获得对N-糖蛋白成熟的更多见解，我们在这里分析了从Protein Data Bank中提取的糖蛋白结构。我们证实，位点特异性N-聚糖加工很大程度上是由糖基化Asn残基和结合N的溶剂可及性所解释的。-聚糖。在完全折叠的糖蛋白的同心圆范围内，研究了蛋白质结构特征对未成熟或成熟形式的潜在偏倚。氨基酸组成和二级结构确实存在偏差。最值得注意的是，γ-支链氨基酸残基（Asn + Asp + Leu）在未成熟糖蛋白的非结构化环区域上出现的频率更高。N-糖基化位点周围的蛋白质表面的结构特征似乎影响了N-聚糖的加工和成熟。