2012年10月31日 星期三

暗反應的卡式循環將二氧化碳還原成碳水化合物

探索生命《生物學》55
The Dark Reactions of the Calvin-Benson Cycle Reduce Carbon Dioxide to Carbohydrate
1.        到目前為止我們討論的都是光合有機體的反應:利用光能合成O2ATPNADPH。光反應產生的ATPNADPH是將二氧化碳合成碳水化合物的能源。
Rachmaninov - piano concerto No.2 (Adagio sostenuto)

2.        合成碳水化合物的反應稱為暗反應,因為他們能夠在黑暗無光的環境下進行,只有植物體有足夠的NADPHATP就可以進行暗反應。合成碳水化合物的反應需要光反應的「產物」來獲取能量,不是直接使用光能。
3.        不過,多數的植物是在白天有光的時候才進行碳水化合物的合成反應,因為那時候植物體正在大量生產ATPNADPH
4.        缺乏能量的二氧化碳CO2要進行還原反應,製造出高能量的醣,必須經歷許多步驟,每個步驟都有酵素協助催化反應。事實上,二氧化碳被推進一個節節高昇的能量階梯,過程經歷一連串的中間產物化合物,其中有些是不穩定的,一直到最後製造出高能量、穩定的碳水化合物最終產物。
5.        二氧化碳歷經節節高昇的能量階梯而製造出碳水化合物的過程,我們可比喻成一個人拖一大箱重物從一樓爬樓梯到二樓,他每往上爬一個階梯,就要把這個笨重的箱子拉到夠高的位置才能擺上下一個階梯,擺好之後稍微喘氣、恢復力氣一下,再繼續拉箱子到下一階。
6.        如果在過程中,箱子還沒有擺穩就放手,箱子一定會掉下去。樓梯讓人可以一點一點用力提高能階,但是爬樓梯過程中的每一步卻是不太穩定的,最穩定的狀態就是抵達二樓,或是回到一樓。
7.        箱子從一樓移到二樓,爬過一階又一階不穩地狀態的力量,來自於人。葉綠體將二氧化碳合成碳水化合物的能量,來自於光反應製造出來的ATPNADPH
8.        特別留意,暗反應需要的ATPNADPH是類囊體外的基質區域製造出來的,合成碳水化合物也是在基質進行。
9.        二氧化碳還原成碳水化合物的系列步驟稱為卡式循環Calvin-Benson cycle,這個名稱是為了紀念發現它的科學家Melvin Calvin Andrew Benson
10.     這個循環的一開始是,從空氣而來的二氧化碳與五碳醣「二磷酸核酮醣ribulose bisphosphate/簡稱RuBP」結合成高度不穩定的六碳化合物,接著六碳醣化合物斷裂成二個三碳醣分子,稱為磷酸甘油酸phosphoglyceric acid,或PGA
11.     催化卡式循環啟動反應的酵素是二磷酸核酮醣羧化酶bisphosphate carboxylase,或簡寫為Rubisco;這是世界上數量最多的酵素。卡式循環的初始步驟也是光合作用裡最重要的生物合成反應。
12.     接下來,藉由ATP分子提供的能量與磷原子團,每個PGA分子增加磷酸原子團,再來,帶磷酸原子團的PGA分子與NADPH反應,獲得NADPH提供的氫原子與電子,帶磷酸原子團的PGA分子被還原。
13.     光反應在卡式循環(暗反應)中扮演的角色就是提供光合作用的產物給暗反應,充滿能量的三碳化合物稱為磷酸甘油醛phosphoglyceraldehyde,簡稱PGAL
14.     磷酸甘油醛PGAL是醣類,它是光合作用的穩定產物。大部分的PGAL分子用來合成新的二磷酸核酮醣RuBP,由ATP提供能量啟動的一系列複雜反應。
15.     但是其中有部分的PGAL用來合成葡萄醣,葡萄醣又可進一步變成澱粉儲存在葉綠體內,有些PGAL被運送到細胞質,在細胞質PGAL經歷一系列的反應步驟重組與結合成蔗醣,蔗醣再運送到植物體的其他地方。
16.     雖然傳統上認為葡萄醣是光合作用的產物,實際上植物體細胞內的葡萄醣量很少。植物體內的PGAL大多用來合成澱粉、脂肪酸、胺基酸和核醣酸,或者打斷PGAL分子釋放裡面的能量來作功。
17.     PGAL合成的葡萄醣,通常被當成建造蔗醣、纖維、澱粉或其他多醣的基礎單元。
n   翻譯編寫 Carol H. McFadden and William T. Keeton Biology》;圖片來源/Southtexascollege.eduDaviddarling.info

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