【原】O-GlcNAcase：出现机制、底物识别和小分子抑制剂。

O-GlcNAcase: Emerging Mechanism, Substrate Recognition and Small-Molecule Inhibitors.

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|核心内容：

O-GlcN酰化是核质蛋白在丝氨酸/苏氨酸残基上发生的动态的、普遍存在的翻译后糖基化反应，与细胞周期调控有关。

这种蛋白质修饰主要受两种酶的调控：O-GlcNAc转移酶(OGT)将N-乙酰氨基葡萄糖部分加到受体蛋白上，O-GlcNAcase(OGA)水解蛋白受体上的糖部分，即O-GlcNAc转移酶(O-GlcNAc Transfer ase，OGT)和O-GlcNAc转移酶(O-GlcNAcase，OGA)。

不规则的O-GlcN酰化与多种疾病有关，包括癌症、糖尿病和神经退行性变。

近年来，小分子OGA抑制剂的发现使O-GlcN酰化的生理功能得以研究。

然而，由于到目前为止还没有发现人类OGA的完整结构数据，高效和选择性抑制剂的设计面临着一些挑战。

此外，还有许多机械上相似的相关酶，如十六烷基己糖苷酶(β-Hexosamidases，Hex)，这些酶的伴随抑制会导致不良的溶酶体储存障碍。

本文综述了近年来对人O-GlcNAcase及其异构体结构的研究进展。

重点研究了催化机理和OGA对底物的识别。

此外，它还介绍了小分子OGA抑制剂的最新概况，无论是碳水化合物还是非碳水化合物支架。

我们讨论了抑制剂的结构，结合模式，对酶的选择性，以及在细胞水平上探测O-GlcN酰化的潜在结果。

原文摘要：

O-GlcNAcylation is the dynamic and ubiquitous post-translational glycosylation of nucleocytoplasmic proteins on serine/threonine residues; it is implicated in regulation of the cell cycle. This protein modification is mainly governed by a pair of enzymes: O-GlcNAc transferase (OGT) adds the N-acetylglucosamine moiety to acceptor proteins, and O-GlcNAcase (OGA) hydrolyses the sugar moiety from protein acceptors. Irregular O-GlcNAcylation is linked to several diseases including cancer, diabetes and neurodegeneration. Recently, the discovery of small-molecule OGA inhibitors has enabled the physiological function of O-GlcNAcylation to be investigated. However, the design of highly potent and selective inhibitors faces several challenges as no full structural data of human OGA has been discovered to date. Moreover, there are a number of mechanistically similar related enzymes such as β-hexosaminidases (Hex), and the concomitant inhibition of these enzymes leads to undesirable lysosomal-storage disorders. This review highlights recent insights into the structure of human O-GlcNAcase and its isoforms. We focus on the catalytic mechanism and substrate recognition by OGA. In addition, it presents an updated overview of small-molecule OGA inhibitors, with either carbohydrate or noncarbohydrate scaffolds. We discuss inhibitor structures, binding modes, and selectivity towards the enzyme, and potential outcomes in probing O-GlcNAcylation at cellular levels.

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● Introduction ●

The post-translational modifications (PTMs) of proteins including  phosphorylation, methylation, ubiquitylation and acetylation, have  gained significant attention over the past few years.[1,2] 

Among  these modifications, O-GlcNAcylation that was first discovered by  Torres and Hart,[3] it is a β-O-glycosidic coupling of a O-linked Nacetylglucosamine (O-GlcNAc) monosaccharide moiety to serine  and threonine residues of intracellular proteins.[4] 

Protein OGlcNAcylation is regulated through the hexosamine biosynthetic  pathway (HBP, Figure 1). 

The pathway involves nutrient flux of  carbohydrate, protein and lipid metabolism to culminate uridine  diphosphate GlcNAc (UDP-GlcNAc), the high-energy donor  substrate for O-GlcNAcylation. 

O-GlcNAc is not only a nutrient dependent substrate but also it is highly responsive to different  types of cellular stress, including hypoxia and heat shock.[2] 

There  are many evidences confirming O-GlcNAcylation which can  interplay with other PTMs, especially phosphorylation and can  also target the same residues of proteins. 

Phosphorylation is  controlled by a massive number of kinases and phosphatases.  

However, only two enzymes are responsible for regulating OGlcNAcylation: O-GlcNAc transferase (OGT) that transfers OGlcNAc from the donor sugar UDP-GlcNAc to naked proteins; and  O-GlcNAcase (OGA) that cleaves this moiety and returns proteins  back to their unmodified state.[1] 

Unusual patterns of cellular OGlcNAc levels have been physiologically implicated in many  chronic human diseases such as diabetes, cancer and  neurodegeneration.[5–13] 

Giving the growing interest in targeting protein O-GlcNAcylation  for medicinal purposes, different approaches of increasing  intracellular levels of O-GlcNAc have been applied including  overexpression of OGT, enhancing UDP-GlcNAc levels,  knockdown of OGA, and small molecule inhibitors of OGA.  

Compared to genetic methods, small molecule OGA inhibitors  have been widely used to increase O-GlcNAc levels in cultured  cells and in vivo to analyze their phenotypic effects.[14] 

They  supply a rapid and reversible response to adjust the enzyme  function. 

However, due to the mechanistic similarity between  different families of glycosyl hydrolase, inhibitors are usually  poorly selective producing confounding effects.[15]

In this minireview, we discuss the potential mechanism that allows  OGA to recognize a large number of protein substrates, describe the partial structure of human OGA (hOGA) and present different  classes of OGA inhibitors covering their complexes with bacterial  and eukaryotic OGA. 

Many reviews have explained the diverse  functions of O-GlcNAcylation as well as the structural  characterization and catalytic mechanisms of its cycling  enzymes. [1,2,16–20] We specifically focused on an updated comprehensive overview of OGA inhibitors and offers an insight of merging these facts into unifying concepts towards developing  highly potent and selective OGA inhibitors.

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参考文献：https:///10.1002/cmdc.202000077

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