幾丁質酶

幾丁質酶（英語：Chitinase；EC編號：3.2.1.14）是一種催化幾丁質水解的酶，它能夠斷開幾丁質中的糖苷鍵。^[1]

大麥種子中的幾丁質酶

自然界分布

幾丁質生物包括許多細菌^[2]（例如氣單胞菌目、芽孢桿菌屬、弧菌屬^[3]等）。這些細菌具有致病性或者腐食性。它們入侵活的節肢動物、浮游動物或真菌，也可能會降解這些生物的殘留物。

真菌，例如球蟲科球菌，也具有降解性的幾丁質酶，這與它們作為節肢動物病原體以及自身的潛力有關。

幾丁質酶也存在於植物中（例如大麥種子中的幾丁質酶：PDB：1CNS），其中一些是致病性（PR）蛋白被誘導系統性獲得抗性的一部分，表達則是由NPR1基因和水楊酸途徑介導的，兩者均涉及對真菌和昆蟲的抵抗力。其他植物幾丁質酶可能需要產生真菌共生酶。^[4]

儘管哺乳動物不產生幾丁質，但它們具有兩種功能性幾丁質酶：殼三糖苷酶（CHIT1）和酸性哺乳動物幾丁質酶（AMCase），以及具有高度序列相似性但缺乏幾丁質酶活性的類似幾丁質酶的蛋白質（例如YKL-40）。^[5]

功能

像纖維素一樣，幾丁質是一種相對耐降解的生物聚合物，^[6] 儘管某些魚類能夠消化幾丁質，但通常不會被動物消化。^[7] 目前認為，動物的幾丁質消化需要細菌共生體和長時間的發酵，類似於反芻動物的纖維素消化。然而，幾丁質酶已經從包括人類在內的某些哺乳動物的胃中分離出來。^[8]

幾丁質酶活性還可以在人血^[9][10][10]和軟骨^[11]中檢測到。與植物幾丁質酶一樣，這可能與病原體抗性有關。^[12][13]

臨床意義

人體產生的幾丁質酶（稱為「人幾丁質酶」）可能與過敏有關，哮喘與幾丁質酶表達水平提高有關。^{[14][15][16][17][18]}

人類幾丁質酶可以解釋某些最常見的過敏（塵蟎，黴菌孢子等都含有幾丁質）和蠕蟲（寄生蟲）感染之間的聯繫，這是衛生學假說的一種形式^[19][20][21]（蠕蟲具有幾丁質的口器以固定腸壁）。最後，植物中的幾丁質酶和水楊酸之間的聯繫已經建立，但是水楊酸與人類過敏之間存在假想的聯繫。^[22]

真菌調節

調節因物種而異，並且在生物體內，具有不同生理功能的幾丁質酶將處於不同的調節機制下。例如，參與維護（比如重塑細胞壁）的幾丁質是構成表達的。但具有專門功用的酶，如降解外源性幾丁質或參與細胞分裂的，需要精確的基因時空調控。^[23]

木霉的內切幾丁質酶的調控依賴於N-乙酰葡萄糖酶，數據表明，在反饋循環中，幾丁質的分解會產生N-乙酰葡萄糖胺，這有可能被採取並觸發對chitinbiosidases的上調節。^[24]

在釀酒酵母和ScCts1p（釀酒酵母幾丁質酶1）的調節中，幾丁質酶其中之一是通過降解隔膜中的幾丁質參與細胞分裂後的細胞分離。^[25] 由於這些幾丁質酶在細胞分裂中很重要，因此必須進行嚴格的調節和激活。具體而言，在有絲分裂後期必須在子細胞中激活Cts1的表達，並且該蛋白必須位於隔膜的子位置。^[26] 為此，必須與其他控制細胞不同階段的調節網絡進行協調，如Cdc14早期相位釋放（FEAR）、有絲分裂退出（MEN）和Ace2p（轉錄因子）和細胞形態生成（RAM）的調節。^[27] 總的來說，不同調節網絡的整合使幾丁質酶降解細胞壁的功能取決於細胞周期的階段以及子細胞之間的特定位置。^[23]

食物中的分布

幾丁質酶天然存在於許多常見食品中。例如，香蕉，栗子，奇異果，鱷梨，木瓜和西紅柿都含有大量的幾丁質酶，可以抵抗真菌和一些無脊椎動物的入侵。壓力或環境信號（例如乙烯氣體）可能會刺激幾丁質酶產量的增加。

幾丁質酶分子的某些部分，在植物防禦中的功能相似，在結構上與橡膠乳膠中的促肝素或其他蛋白質幾乎相同，可能會引發一種稱為乳膠-水果綜合徵（latex-fruit syndrome）的過敏交叉反應。^[28]

應用

幾丁質酶具有廣泛的應用，其中一些已經被工業實現。這包括將幾丁質生物轉化為有用的產品（例如肥料），生產不過敏，無毒，可生物相容和可生物降解的材料（已經生產出具有這些質量的隱形眼鏡，人造皮膚和縫合線）以及增強的殺蟲劑和殺菌劑。^[29]

幾丁質酶未來可能應用包括作為食品添加劑以延長保質期，哮喘和慢性鼻竇炎的治療劑，抗真菌藥物，抗腫瘤藥物以及蛋白質工程中所使用的一般成分。^[29]

另見

• 幾丁質
• 木質素酶

參考資料

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外部連結

• 醫學主題詞表（MeSH）：Chitinase
• The X-ray structure of a chitinase from the pathogenic fungus Coccidioides immitis