生化統整 第三組 李秋霖0993B031 黃三珊0993B040

ㄧ、心智圖

二、重點預測 抑制劑與雙基質

抑制劑(Inhibitor) 干擾酵素作用降低其活性 其影響酵素有兩種方式: 1.reversible inhibitor:和酵素結合後再分開,使酵素回到原本狀態 　　　→依結合在active site 和 other site區分 　　　　• Competitive inhibition 　　　　• Noncompetitive inhibition 　　　　• Uncompetitive inhibition 2.irreversible inhibitor:與酵素結合後成為新的蛋白質,不具酵素活性

1.reversible inhibitor

Competitive Inhibitors： 和 s競爭相同的 binding site Vmax 不變, Km 變大(即受質和抑制劑競爭同一位置,S更難結合。且其實受質濃度不可能無限大,所以Vmax不變) 雙倒數圖Y截距不變

Noncompetitive Inhibition： S 和I 結合在酵素的 different sites (I 和 E 或 ES結合) Km 不變, Vmax 變小(即抑制劑不會干擾受質結合到活性部位,但結合的抑制劑仍存在,至反應速率受影響) 雙倒數圖 X 截距不變

Uncompetitive Inhibition: ㄧ種不是那麼極端的mixed inhibition (I 只能與ES結合) K2 變小,Vmax變小,Km也變小(因S濃度上升有利於I的抑制,但仍有I存在所以Vmax下降,k2也變小) 雙倒數圖平行線

2.irreversible inhibitor

1.Kinetically 和noncometitive inhibition很像， binding 在active site以外的其他部位，但是　　　　永遠地改變酵素構型 2.對酵素進行共價修飾，使其失去活性且無法恢復 3.如此使酵素“自殺”的I，我們稱之為suicide inhibitor。例如:Pencillin is an irreversible inhibitor of glycopeptide transpeptidase

雙質子反應（bimolecular kinetics） 雙基質反應仍可適用於 M-M 公式，但 兩種基質的 Km 要分別測定；測 S1 的時候，反應中的 S2 濃度要飽和 (使 S2 成為非主導因子)，反之亦然 1.Sequential (single-displacement)reactions （ㄧ個速率限制步驟） 　(1)Random (2)Ordered 先結合 leading substrate 類似 noncompetitive inhibition 2.Ping-pong (double-displacement)reactions 類似 uncompetitive inhibition

Sequential (single-displacement)reactions (1)Random A、B非競爭 AEB to PEQ is the Rate-Limiting Step If A has no influence on B binding purely random!

Sequential (single-displacement)reactions (2)Ordered A、B非競爭； A、P競爭 Reaction between A and B occurs in the ternary complex and is usually followed by an ordered release of the products, P and Q.

Ping-Pong reactions A 與 B 不競爭 A 與 Q 競爭 Formation of a covalently modified enzyme intermediate.(E’) The product of the enzyme’s reaction with A (called P in the above scheme) is released prior to reaction of the enzyme with the second substrate, B.

DoubleDisplacement Reaction

三、習題解說

What is the difference between competitive inhibition and noncompetitive inhibition? 競爭型抑制劑結合到活化部位，至受質不能。非競爭行一至計結合再活性部位以外的位置，受質仍可結合，但因為結合抑制劑仍存在，至酵素無法催化反應

1. Competitive Inhibitors: 許多臨床用來抗微生物感染、抗發炎及抗癌化療藥物都是酵素之可逆抑制劑（reversible inhibitor）。請問這些抑制劑的酵素活性抑制型態有那三種？如何運用酵素動力學（Michaelis-Menten kinetics）來決定抑制型態與抑制劑的結合常數KI呢？ 1. Competitive Inhibitors:

2. Nonompetitive Inhibitors :

3. Unompetitive Inhibitors :

Why is uncompetitive inhibition so rare Why is uncompetitive inhibition so rare? A possible explanation, with implications for the design of drugs and pesticides. A possible explanation may be that uncompetitive inhibition of an enzyme in a metabolic pathway can have enormously larger effects on the concentrations of metabolic intermediates than competitive inhibition, under circumstances where their effects on the kinetics of the isolated enzyme are very similar. The severely toxic effects that an uncompetitive inhibitor might be expected to have may have caused enzymes to have evolved in such a way that there has been selection against structures that might favour uncompetitive inhibition.

Enzyme inhibition in open systems. Superiority of uncompetitive agents Investigations of the open system behavior of reversible dead-end inhibitors were carried out by means of computer simulations and experimental studies. The results from both approaches indicate that substrate-competitive inhibition may often be an inappropriate basis for design of potential therapeutic agents. The use of uncompetitive (also called anticompetitive) inhibitors in this role is likely to be far more effective. Chemical analogs of pathogen-specific enzymic reaction products rather than analogs of substrates provide a promising basis for the systematic design of such uncompetitive inhibitors.