Chapter 7 PartII 第十一組 0993B006 楊育真 0993B037 劉家薰

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1 Chapter 7 PartII 第十一組 0993B006 楊育真 0993B037 劉家薰
生物化學重點整理 Chapter 7 PartII 第十一組 0993B006 楊育真 0993B037 劉家薰

2 心智圖 Sugar natural cryoprotectant Trehalose Oligosaccharides Sucrose
nonreducing sugar Homopolysaccharide(one type) Sugar Heteropolysaccharide(two↑ types) Amylose(no branches) Nomenclature Starch Amylopectin(branches) Polysaccharides Storage glycogen(animal) Dextran(Alpha(1,6) linkage) Function Cellulose Glycosaminoglycans Structural Chitin Alginates(Ca2+-binding polymers) Heparin Hyaluronates Recognition Agarose and agaropectin Cell surface polysaccharides

3 Polysaccharides ●醣由glycosidic bonds連接在一起 ●以命名來分:
(1)homopolysaccharide:只由一種單醣構成的聚醣 (2)heteropolysaccharide:由兩種↑單醣構成的聚醣 ●以功能來分: (1)儲存性: Starch 澱粉 和 glycogen 肝醣 (2)結構性: Chitin 幾丁質 和 cellulose 纖維素 (3)辨識性:細胞表面上的聚醣

4 Starch ●為植物儲存聚醣 ●分為amylose 和 amylopectin:
(1) Amylose:無分支，由alpha(1,4)鍵結而成，一個還原端，因整體還原端太少，為非還原醣。在水溶液裡會形成螺旋的構造，可容納碘，形成藍紫色，因此可被測定。 　(2) Amylopectin:有分支(12~30個葡萄糖)，無法形成螺旋狀，碘不會卡住，因此不呈藍紫色，不易被測定。 ●澱粉的分解(酵素): 　(1)alpha-amylase:隨機從中間切斷 　(2)beta-amylase:從非還原端一次切兩個醣下來 　(3) alpha– (1-6)glucosidase:可除分支(剩下所有的分支稱為Limit dextrin)

5 Glycogen and Dextrans ● Glycogen : (1)為動物儲存性的聚醣，在肝臟和肌肉中
(2)和澱粉最大不同於分支數目，肝醣比澱粉來得多(8~12個醣) (3)類似amylopectin，碘液加入不會形成藍紫色 ●Dextrans(糊精): (1)鍵結為alpha(1,6)鍵結，且(1,2) (1,3) (1,4)皆可為分支 (2)切下來為Isomaltose (3)主要為微生物儲存 (4)可作為生化上的應用

6 Cellulose(纖維素) ￭Cellulose is a linear homopolymer of D-glucose unit
￭The glucose units are linked by β(1→4)-glycosidic bond ￭Cellulose的結構為平面,且堆疊緊密,彼此之間產生氫鍵連結

7 Chitin(甲殼素) Exoskeletons of crustaceans, insects and cell walls of fungi similar to cellulose, but the –OH group on each C-2 is replaced by-NHCOCH3,so the repeating units are N-acetyl-D-glucosamines in β(1→4)linkage The chain of chitin are arranged in parallel or antiparallel

8 Alginate、agarose and agaropectin
Alginate is binding by鈣離子 Agarose and agaropectin are galactose polymers The favored conformation of agarose in water is a double helix with a threefold screw axis

9 Glycosaminoglycans(GAG)
在amino sugar且單醣裡至少有一個 negatively charged sulfate or carboxylate group Heparin(肝素)- with a very high negative charge, is a natural anticoagulant Hyaluronates玻尿酸(the lubricant fluid of the body’s joints)

10 Bacterial Cell Walls Bacteria are classified as either Gram-positive or Gram-negative. Most bacterial cell walls have a peptide-polysaccharide layer called peptidoglycan. The differences are Gram-positive is one bilayer and thick peptidoglycan outer shell and Gram-negative is two bilayers with thin peptidoglycan shell in between.

11 The structure of peptidoglycan
The backbone is a β(1→4)linked polymere of N-acetylglucosamine and N-acetylmuramic acid units. The structure is similar to chitin but joined to a tetrapeptide,usually L-Ala. D-Glu. L-Lys. D-Ala There are four usually things in tetrapeptide 1.L-Ala與醣鍵結 2.D-Glu使用側鏈的酸根與下一個amino acid結合,故稱為Isoglutamate 3.L-Lys測鏈與其他amino acid結合 4.D-Ala為D form

12 Bacterial Cell Walls Gram positive bacteria:The cross-link in Gram-positive cell walls is a pentaglycine bridge Gram negative bacteria: The linkage between the tetrapeptides of adjacent carbohydrate chains in peptidoglycan involves a direct amide bond

13 Cell walls of Gram-negative bacteria
The outer membrane of Gram-negative bacteria is coated with a highly complex lipopolysaccharide(LPS) The lipid portion of the LPS is embedded in the outer membrane and is linked to a complex polysaccharide. LPS 可做為免疫促進劑,由於只有細菌擁有LPS,而人類體內並無LPS

14 習題 Q1:Why are certain gram-negative bacteria more resistant than gram-positive bacteria to antibiotics that attack cytoplasmic targets? In Gram-negative bacteria the cell wall is surrounded by an extra layer made of polysaccharides, proteins, and phospholipids. This layer, though easily washed away by alcohol-acetone mixtures, blocks many antibiotics from reaching the peptidoglycan cell wall. Since b-lactam antibiotics like penicillin work by attacking the cell wall, this outer layer makes Gram-negative bacteria resistant to such antibiotics.

15 Q2:LPS與誘發發炎反應的關係? LPS 是所有革蘭氏陰性細菌在其外膜上 (outer membrane) 共有的醣酯質 (glycolipid) 成分，主要可以分做兩個區塊：親水性區塊，當中含有核心 (the core) 和O-antigen 酯多醣區域；親脂性的lipid A 區塊。前者中的O-antigen鏈是構造變化較多之處，隨著菌種的不同此塊區域的組成就會產生變異,根據研究顯示親脂性的lipid A 區塊為造成LPS 所誘發之生物性反應的主因，而親水性區域處則與這些反應無關。