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

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

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

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)

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

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

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

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

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)

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.

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

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

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

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.

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。