7 糖代谢的其他途径 0课件.pptVIP

cAMP结构 第二信使(second?messenger)： 　　响应外部信号（第一信使），例如激素在细胞内合成的效应分子，例如cAMP、肌醇三磷酸或二酰基甘油等。第二信使再去调节靶酶，引起细胞内各种效应。  激素通过cAMP促进磷酸化作用，使磷酸化酶成为a型（有活性），合成酶变成b型（无活性）。合成酶由蛋白激酶磷酸化。 血糖浓度一般在80-120mg/100ml，称为葡萄糖耐量。肾糖阈为160-180，血糖过多则从尿排出。血糖低于70或过度兴奋可刺激延脑第四脑室“糖中枢”，引起肝糖原分解。下丘脑可分泌皮质释放因子，作用于肾上腺皮质，升高血糖。影响糖代谢的激素有： 1.胰岛素：由胰岛β细胞分泌，促进糖原合成酶活性，诱导葡萄糖激酶合成，加强磷酸果糖激酶作用。降低血糖效应。 2.肾上腺素和胰高血糖素：通过cAMP激活糖原磷酸化酶，诱导肝中磷酸烯醇式丙酮酸羧化激酶和果糖二磷酸酶的合成，促进异生，升高血糖。 以上激素都是水溶性激素，通过cAMP起作用. Regulation by covalent modification (phosphorylation): The hormones glucagon and epinephrine activate G-protein coupled receptors to trigger cAMP cascades. Both hormones are produced in response to low blood sugar. Glucagon, which is synthesized by a-cells of the pancreas, activates cAMP formation in liver. Epinephrine activates cAMP formation in muscle. The cAMP cascade results in phosphorylation of a serine hydroxyl of Glycogen Phosphorylase, which promotes transition to the active (relaxed) state. The phosphorylated enzyme is less sensitive to allosteric inhibitors. Thus, even if cellular ATP glucose-6-phosphate are high, Phosphorylase will be active. The glucose-1-phosphate produced from glycogen in liver may be converted to free glucose for release to the blood. With this hormone-activated regulation, the needs of the organism take precedence over needs of the cell. Commonly used terminology: a is the form of the enzyme that tends to be active, and independent of allosteric regulators (in the case of Glycogen Phosphorylase, when phosphorylated). b is the form of the enzyme that is dependent on local allosteric controls (in the case of Glycogen Phosphorylase when dephosphorylated). Insulin, produced in response to high blood glucose, triggers a separate signal cascade that leads to activation of Phosphoprotein Phosphatase. Thus insulin antagonizes effects of the cAMP cascade induced by glucagon epinephrine. The cAMP cascade induced in liver by glucagon or epinephrine has the opposite effect on glycogen s