1、第九章 脂代谢 (Lipid Metabolism),一.脂肪的消化与吸收二.脂肪的分解三.脂肪酸的氧化(分解代谢)四.酮体的合成与分解,三酰甘油是好的能量储存载体,油脂是动物体主要的储能物质；也是冬眠动物、候鸟唯一的能源；但不是植物体的主要能源。三酰甘油的高还原状态使其氧化时比碳水化合物和蛋白质释放更多的能（ 38 kJ/g vs 18 kJ/g ）高疏水性使其不会增加细胞额外的渗透压和额外的重量；这同时使其在被消化和运输之前必须先被乳化。高化学稳定性使其不易和其它化合物发生反应；这由使脂肪酸在代谢时必须被乳化。动物细胞中的脂肪酸可来源于食物、脂肪组织中储存的小油滴，也可以由过量的糖或蛋白质
2、转化而来。,一、脂肪的消化和吸收,食物中的脂肪在小肠中与胆汁混合后乳化，接着在脂酶（lipase）作用下发生水解生成甘油和脂肪酸被肠黏膜吸收。在肠细胞中转化为三酰甘油并与胆固醇酯、脂蛋白等结合形成乳糜微粒（ chylomicrons ）乳糜微粒通过淋巴系统和血液进入组织。在毛细血管中三酰甘油被重新水解成甘油和脂肪酸,The emulsification, absorptionand transport of diet triacylglycerol,二、脂肪的分解,The hormones epinephrine（肾上腺素） and glucagon（胰高血糖素）, signaling a l
3、ack of glucose in the blood, will bind to receptors on adipocyte surface and activate the triacylglycerol lipase via phosphorylation.Fatty acids released are carried to energy-demanding tissues (e.g., skeletal muscle, heart, and renal cortex) via the 62 kD monomeric serum albumin (each binding about
4、 10 fatty acids).,脂肪组织贮存的甘油三酯的动员,甘油的代谢,The glycerol molecule generated can be converted to glycolytic intermediate glyceraldehyde 3-phosphate by the successive action of glycerol kinase, glycerol 3-phosphate dehydrogenase, and triose phosphate isomerase.,三、脂肪酸的氧化,Early labeling experiments (1904): f
5、atty acids are degraded by sequential removal of two-carbon units When dogs were fed with odd-numbered fatty acids attached to a phenyl group, benzoate(苯甲酸) was excreted; and when fed with even-numbered, phenylacetate（苯乙酸） was excreted.Hypothesis: the b-carbon is oxidized,with two-carbon units relea
6、sed by each round of oxidation.These experiments are a landmark in biochemistry, in using synthetic label (the phenyl group here) to elucidate reaction mechanism, and was done long before radioisotopes was used in biochemistry!,Franz Knoops labelingExperiments (1904): fatty acids are degraded by oxi
7、dation at the b carbon, i.e., b oxidation.,脂肪酸在线粒体外被活化,Fatty acids are converted to fatty acyl-CoA (a high energy compound) via a fatty-acyl-adenylate intermediate (enzyme-bound, mixed anhydride) by the action of fatty acyl-CoA synthetases (also called fatty acid thiokinase).Pyrophosphate is hydrolyzed by inorganic pyrophosphatase, thus two anhydride bonds of ATP are consumed to form one high-energy thioester（硫酯） bond, thus pulling the reaction forward: a common phenomena in biosynthetic reactions.Acyl-adenylates are often formed when COOH groups are activated in biochemistry.,
