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5.4.2 cAMP信号途径 cAMP的产生与信号放大 cAMP的产生 cAMP的信号放大
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G-蛋白偶联受体与cAMP的产生
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信号放大: 蛋白激酶A的激活 ●结构： The inactive form of protein kinase A consists of two regulatory (R) and two catalytic (C) subunits. ●激活: Cyclic AMP binds to the regulatory subunits, leading to their dissociation from the catalytic subunits. The free catalytic subunits are then enzymatically active and able to phosphorylate serine residues on their target proteins. ?
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蛋白激酶A的激活
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蛋白激酶A的作用机理 被激活的蛋白激酶A可以以两种方式起作用:
Protein kinase A phosphorylates key target enzymes ; In many animal cells, increases in cAMP activate the transcription of specific target genes that contain a regulatory sequence called the cAMP response element, or CRE . ?
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cAMP及其信号放大 ?
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The free catalytic subunit of protein kinase A translocates to the nucleus and phosphorylates the transcription factor CREB (CRE-binding protein), leading to expression of cAMP-inducible genes.
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cAMP浓度变化引起的细胞内反应
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5.4.3 cAMP途径的信号 解除和抑制 ? 信号解除 信号抑制 ●通过磷酸二酯酶将cAMP降解,形成5'-AMP；
●通过抑制型的信号作用于Ri, 然后通过Gi起作用。
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cAMP的合成与分解
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cAMP信号的抑制
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毒素对cAMP信号途径的影响 ◆霍乱毒素(cholera toxin)
能把NAD+上的NAD-核糖转移到Gs蛋白的α亚基上，使G蛋白核糖化(ADP-ribosylation),这样将抑制GTP的水解……。 ◆百日咳毒素(whooping couch toxin) 作用机理同霍乱毒素相同，但是使Gi蛋白进行ADP核糖化,其结果也是使cAMP的浓度增加。
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核糖化作用
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5.5 磷脂肌醇信号途径 (Phosphatidylinositol Signaling)
路也称IP3、DAG、Ca2+信号通路,或称为 PKC(Protein kinase C)系统。
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磷脂肌醇信号途径 ?
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5.5.1 信号分子与系统组成 信号分子 刺激肌醇酯(inositol lipids)的信号分子有各种激素、神经递质类和一些局部介质。这些信号分子通过与受体的结合，并进一步激活磷酸酯酶C，产生两个第二信使。
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5.5.1 信号分子与系统组成 系统组成和特点 ◆组成 ●受体 ●Gq-蛋白
●Phosphatidylinositol-specific phospholipase C- (PI-PLC)
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磷脂肌醇信号途径
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5.5.2 第二信使的产生 ◆磷酸酯酶C-能够将质膜上的PIP2水解成两个第二信使: Inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG)
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DAG和IP3的产生
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5.5.3 信号的放大作用 蛋白激酶C(Protein kinase C,PKC)
◆PKC是钙和磷脂依赖性酶，在未受刺激的细胞中,PKC主要分布在细胞质中,呈非活性构象。它的激活需有Ca2+、DAG和脂的存在。
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IP3动员细胞Ca2+ 释放 ◆IP3 molecules formed at the membrane diffuse into the cytosol and bind to a specific IP3 receptor located at the surface of the smooth endoplasmic reticulum . ◆The IP3 receptor does more than bind a ligand; it is also a tetrameric Ca2+ channel. Binding of IP3 opens the channel, allowing Ca2+ ions to diffuse into the cytoplasm . ◆Calcium ions can also be considered as intracellular messengers because they bind to various target molecules, triggering specific responses.
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IP3动员的钙释放
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蛋白激酶C的激活 ◆PIP2水解释放出的DAG是水不溶的(非极性的)，一直停留在质膜上。
◆一旦IP3动员释放了Ca2+, DAG在 Ca2+和磷脂酰丝氨酸(phosphatidylserine,PS)的存在下使PKC结合到质膜上并使之激活。
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5.5.3 信号放大作用 PKC的作用机理 ◆Protein kinase C is a multifunctional serine and threonine kinase that phosphorylates a wide variety of proteins. ◆Protein kinase C has a number of important roles in cellular growth and differentiation, cellular metabolism, and transcriptional activation.
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蛋白激酶C的激活与基因调控 ?
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Ca2+的第二信使作用 ◆Calcium ions play a significant role in a remarkable variety of cellular activities, including muscle contraction, cell division, secretion, endocytosis, fertilization, synaptic transmission, metabolism, and cell movement. ◆钙调蛋白(calmodulin) Each molecule of calmodulin contains four binding sites for calcium. Calmodulin does not have sufficient affinity for Ca2+ to bind the ion in a non-stimulated cell. If, however, the Ca2+ concentration rises in response to a stimulus, the ions bind to calmodulin, changing the conformation of the protein and increasing its affinity for a variety of effectors. ?
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钙调蛋白的结构
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钙调蛋白的激活作用
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5.5.4 信号的终止 DAG信号的解除 DAG只是由PIP2水解得到的暂时性产物, 寿命只有几秒钟, 靠两种方式进行降解:
◆被DAG磷酸激酶磷酸化,生成磷脂酸(PA),PA被转化为CMP-磷脂酸,再与肌醇作用合成磷脂肌醇(PI)。 ◆DAG 被DAG 酯酶水解生成单脂酰甘油, 再进一步水解成自由的多不饱和脂肪酸和花生四烯酸甘油。
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5.5.4 信号的终止 IP3作用的终止 ◆在胞浆的肌醇磷酸脂3-激酶的作用下IP3被磷酸化成I(1,3,4,5)P4。 ◆IP3的水解
一步水解成肌醇。5’磷酸酶是一种膜结合的酶。 ◆在胞浆的肌醇磷酸脂3-激酶的作用下IP3被磷酸化成I(1,3,4,5)P4。
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Ca2+信号解除 ? ●IP4参与打开细胞质膜上的Ca2+ 通道, 使细胞质中的Ca2+较为持久地增高。
●胞内Ca2+浓度持久地升高, 可激活Ca2+-ATP酶（质膜、内质网膜的钙泵），从而降低胞质中的 Ca2+，使胞质中的Ca2+迅速恢复到基态水平(10-7 M),并使活性CaM-酶复合物解离,从而酶失去活性,细胞反应终止。
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