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硫酸乙醯肝素

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硫酸乙醯肝素的結構式

硫酸乙醯肝素Heparan sulfate,可縮寫為HS),是一種廣泛存在於各種動物組織細胞外基質的鏈狀多糖[1]。在生物體中硫酸乙醯肝素一般以蛋白聚糖的形式存在,往往會有兩條或三條硫酸乙醯肝素鏈與細胞表面或細胞外基質蛋白質結合[2][3]。因此,硫酸乙醯肝素能夠與包括Wnt家族蛋白在內的多種蛋白配體結合[4][5][6]並由此調節包括發育血管生成血液凝固、經GrB(Granzyme B)介導抑制細胞脫落活性以及腫瘤轉移等多種生物學過程[7]。一些研究表明,硫酸乙醯肝素還可以作為呼吸道合胞病毒病毒的細胞受體[8]。一項研究表明,在SARS-CoV-2(新冠病毒)感染過程中,冠狀病毒刺突蛋白會同時與硫酸乙醯肝素和血管緊張素轉化酶2分子結合[9]

蛋白聚糖形式

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細胞膜上的硫酸乙醯肝素一般與Syndecan英語Syndecan以及由糖磷脂醯肌醇錨定的Glypican英語Glypican結合形成蛋白聚糖[10][6]。不過,硫酸乙醯肝素鏈也可以與CD44蛋白的V3剪接異構體結合(這種情況主要存在於角質細胞和活化的單核細胞中)[11]TGFBR3英語TGFBR3蛋白結合形成蛋白聚糖[12]

細胞質基質中的硫酸乙醯肝素則一般與Fractone英語Fractone[13]、多結構域Perlecan英語Perlecan[14]聚集蛋白英語Agrin[15]COL18A1英語COL18A1[16]蛋白等核心蛋白結合形成蛋白聚糖。

結構

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硫酸乙醯肝素是一種糖胺聚糖,其結構與肝素非常相似。不過,硫酸乙醯肝素中最常見的二糖單位由葡萄糖醛酸(GlcA)和N-乙醯葡萄糖胺(GlcNAc)組成,通常約占總二糖單位的50%左右。相比之下,肝素中的IdoA(2S)-GlcNS(6S)二糖單位在牛肺肝素中占85%,在豬腸黏膜肝素中約占75%。在定義同時含有「肝素樣」和「硫酸乙醯肝素樣」結構的混合性糖胺聚糖時會出現問題。一種看法是只有當N-硫酸酯基團的含量大大超過N-乙醯基團的含量,並且O-硫酸酯基團的密度超過N-硫酸酯基團時,一種糖胺聚糖才能被歸為肝素,反之則應歸為硫酸乙醯肝素[17]。在生理條件下,硫酸乙醯肝素中的酯基和醯胺硫酸酯基團會去質子化,並吸引正電荷對離子形成鹽。一般認為,肝素硫酸酯在細胞表面以這種形式存在[18]

以下僅列出常見的硫酸乙醯肝素二糖單位,而較少見如包含3-O-硫酸化的葡萄糖胺(GlcNS(3S,6S))或自由胺基(GlcNH3+)的二糖單位則未列出。

縮寫

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  • GlcA = β-D-葡萄糖醛酸
  • IdoA = α-L-異尿酸
  • IdoA(2S) = 2-O-磺酸基-α-L-異尿酸
  • GlcNAc = 2-脫氧-2-乙醯胺基-α-D-葡萄糖吡喃糖
  • GlcNS = 2-脫氧-2-磺醯胺基-α-D-葡萄糖吡喃糖
  • GlcNS(6S) = 2-脫氧-2-磺醯胺基-α-D-葡萄糖吡喃糖-6-O-硫酸酯

生物合成

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不同的細胞合成的硫酸乙醯肝素鏈往往是不同的。雖然硫酸乙醯肝素的生物合成主要依賴糖基轉移酶英語Glycosyltransferase硫酸基轉移酶英語Sulfotransferase差向異構酶等一系列,但合成硫酸乙醯肝素鏈的具體方式很大變數。有學者因此提出了「肝素組」(heparanome)的概念,即特定細胞組織生物體產生的肝素結構的集合[19]

20世紀80年代,傑弗里·埃斯科(Jeffrey Esko)首次分離並研究了與硫酸乙醯肝素生物合成過程相關的突變動物細胞。目前與硫酸乙醯肝素合成相關的許多酶現已得到純化分子克隆表達模式也得到了研究[20]。通過這些研究以及一些使用無細胞系統對肝素以及硫酸乙醯肝素鏈的生物合成進行的早期工作,現在對與硫酸乙醯肝素生物合成中涉及的酶反應的順序和特異性已經有了較為深入的了解[21]

起始與延伸

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圖中上方的示意圖為硫酸乙醯肝素鏈的結構

硫酸乙醯肝素生物合成的第一步是木糖基轉移酶英語xylosyltransferase(XT)將UDP-木糖分子上的木糖基團經O-連接糖基化英語O-linked glycosylation與核心蛋白的絲氨酸殘基相連。之後,兩個半乳糖(Gal)基團經半乳糖基轉移酶英語GalactosyltransferaseI和II(GalTI和GalTII)以及一個葡萄糖醛酸殘基經由葡萄糖醛酸基轉移酶英語GlucuronosyltransferaseI(GlcATI)分別被添加到新生成的糖鏈上,形成以下結構[22]

βGlcUA-(1→3)-βGal-(1→3)-βGal-(1→4)-βXyl-O-Ser

這種常見的四糖連接結構是硫酸乙醯肝素/肝素、硫酸軟骨素(CS)和硫酸皮膚素英語dermatan sulfate(DS)的生物合成共通步驟,但之後這些分子的合成途徑各不相同。β-1,2-N-乙醯氨基葡萄糖轉移酶I(GlcNAcT-I)會主導硫酸乙醯肝素/肝素糖鏈的合成,而N-乙醯氨基半乳糖轉移酶I(GalNAcT-I)則會主導硫酸軟骨素與硫酸皮膚素的合成[23]

一般認為,木糖與核心蛋白質的連接被認為發生在內質網(ER),連接區域和鏈的其餘部分的組裝則發生在高爾基體[22][23]

在四糖連接結構合成之後,葡萄糖醛酸殘基與乙醯葡萄糖胺會輪流插入糖鏈的末端,肝素鏈由此不斷延長。具有糖基轉移酶活性EXT基因家族下的蛋白參與肝素鏈合成過程[22]。EXT1-3上的基因突變會造成肝素鏈無法合成,使人患上遺傳性多發性外生骨疣英語Hereditary multiple exostoses(MHE),一種影響青少年長骨發育的軟骨瘤[24]

修飾

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硫酸乙醯肝素與肝素合成上的主要不同是硫酸基轉移酶和差向異構酶會對硫酸乙醯肝素進行修飾,加上來自3'-磷酸腺苷-5'-磷酸硫酸鹽英語3'-Phosphoadenosine-5'-phosphosulfate(PAPS)的硫酸基團[25][26]

硫酸乙醯肝素的修飾過程中,首先,經由四種2-脫氧-2-乙醯胺基-α-D-葡萄糖吡喃糖(GlcNAc)N-脫乙醯化酶/N-磺基轉移酶(NDSTs)中的一種或多種酶的催化,GlcNAc殘基會經歷端(N-端)的脫乙醯化與硫化,轉變為 2-脫氧-2-硫磺胺基-α-D-葡萄糖吡喃糖(GlcNS)。四種NDST都能進行氮端脫乙醯化與氮端磺基轉移,但是活性各有不同。這一步對硫酸乙醯肝素的後續修飾而言是必不可少的。這一修飾過程是隨機發生的,也就是說肝素鏈上的所有GlcNAc的N-端都有一定機率被脫乙醯化與硫化[27][28]

上述的GlcNAc氮端脫乙醯化酶以及氮端磺基轉移過程是由同一種酶完成的,但是一些物種中也發現了兩種過程的解偶聯現象,在硫酸乙醯肝素中發現了2-脫氧-2-氨基-α-D-葡萄糖吡喃糖(GlcNH2)殘基的存在[29]

最後,硫酸乙醯肝素還會發生與2、3,以及6號位碳原子上的O-硫酸基化[30][31]。其中,3號位碳原子的O-硫酸基化可能與癌症發生過程中Wnt信號通路的調節相關[6]

配體

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硫酸乙醯肝素能與許多蛋白發生相互作用,包括細胞外基質的組成蛋白、凝血因子,以及大多數生長因子細胞因子趨化因子成形素英語Morphogen[32]

γ-干擾素

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γ-干擾素的細胞表面受體結合區與硫酸乙醯肝素結合區重疊,都在其的碳端(C-端)附近。因此,硫酸乙醯肝素的結合會阻斷γ-干擾素的受體結合位點,使γ-干擾素與硫酸乙醯肝素形成的複合物難以與γ-干擾素的其他配體結合[33]

Wnt信號通路

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硫酸乙醯肝素的一種核心蛋白GPC3英語Glypican_3能與WNT捲曲受體(Frizzled)相互作用形成複合物並觸發下游信號傳導[4][6]。實驗證明Wnt識別GPC3上的IdoA2S和GlcNS6S等硫酸乙醯肝素模體。GlcNS6S3S的3-O-磺酸化增強了Wnt與硫酸乙醯肝素的結合[5]

類似物

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硫酸乙醯肝素類似物是指具有與硫酸乙醯肝素相同的性質,但是在創口等蛋白水解環境下依然能保持穩定的物質[34][35]。在創口等炎性環境下,硫酸乙醯肝素會逐漸被肝素酶等酶水解,但硫酸乙醯肝素類似物卻能保持穩定,因此有助於傷口恢復等過程[36]。另外,硫酸乙醯肝素類似物因比較穩定,也能在體外用於研究肝素鏈與蛋白之間的相互作用[37]

參見

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參考文獻

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