1. Lipid Biosynthesis
  • 发生地点
    • Eukaryotic Lipids are Made at a Variety of Sites
    • Occurs mainly in liver and adipocytes (mammals)
    • Most lipid biosynthesis in eukaryotic cells occurs in the ER.
    • Enzymes of lipid synthesisare membrane-bound with active sites facing the cytosol.
    • Other lipid synthesis locations include - plasma membrane, mitochondria, lysosomes and peroxisomes
  • fatty acid synthesis vs degradation
    合成与消耗反应的区别
      1. Intermediates in fatty acid synthesisare linked covalently to the sulfhydryl 巯基 groups of special proteins, the acyl carrier proteins (ACPs). Incontrast, fatty acid breakdown intermediates are bound to the-SH group of coenzyme A.
      1. Fatty acid synthesis occurs in the cytosol, whereas fatty acid degradation takes place in mitochondria.
      1. In animals, the enzymes of fatty acid synthesis are components of one long polypeptide chain, the fatty acid synthase, whereas no similar association exists for the degradative enzymes.
      1. The coenzyme for the oxidation–reduction reactions of fatty acid synthesis is NADP+/ NADPH, whereas degradation involves the NAD+/ NADH couple.
  • 原料&能量
    • Acetyl-CoA
        1. Amino acid degradation produces cytosolic acetyl-CoA.
          normally insufficient for fatty acidbiosynthesis
        1. Fatty acid oxidation produces mitochondrial acetyl-CoA.
          cannot cross the mitochondrial membrane
        1. Glycolysis yields cytosolic pyruvate, which is converted to acetyl-CoA by pyruvate dehydrogenase
          cannot cross the mitochondrial membrane
      • 2/3来源的acetyl-CoA不能直接跨线粒体膜, 因此需要 acetyl-CoA + oxaloacetate →(citrate synthase) citrate → 跨膜 → citrate →(ATP-citrate lyase) acetyl-CoA + oxaloacetate
    • Reducing Power - NADPH
      • NADPH can be produced in the pentose phosphate pathway as well as by malic enzyme.
      • Reducing equivalents (electrons) derived from glycolysis in the form of NADH can be transformed into NADPH by the combined action of malate dehydrogenase and malic enzyme.
        NADH不能用于fatty acid合成, 借助合成再分解malate变成NADPH
    • 蓝线代表carbon, 红线代表electrons
  • Acetyl-CoA → Malonyl-CoA
    • The carboxylation of acetyl-CoA to form malonyl-CoA is essentially irreversible and is the committed step in the synthesis of fatty acids.
      a “committed step” is an effectively irreversible enzymatic reaction that occurs early in a metabolic pathway, once this step happens, the molecule is “committed” to continuing down that specific pathway to the end product
    • The reaction is catalyzed by acetyl-CoA carboxylase,which contains a biotin维生素B7 prosthetic group辅基
    • This carboxylase is the only enzyme of fatty acid synthesis in animals that is not part of the multienzyme complex called fatty acid synthase
    • The acetyl-CoA carboxylase reaction produces malonyl-CoA for fatty acid synthesis
  • Acyl Carrier Proteins (ACP) Carry the Intermediates in Fatty Acid Synthesis
    • The basic building blocks of fatty acid synthesis are acetyl and malonyl groups, but they are first passed to Acyl Carrier Proteins (ACP)
    • ACP consists (in E. coli) of a single polypeptide chain of 77 residues to which is attached (on a serine residue) a phosphopantetheine group磷酸泛酰巯基乙胺基团, the same group that forms the “business end” of coenzyme A.
      fatty acid 就是通过蓝色这个p巴拉巴拉集团连接到CoA/ACP上的
    • Thus, ACP is a somewhat larger version of coenzyme A, specialized for use in fatty acid biosynthesis.
    • ACP 相当于合成 fatty acid 的专用 CoA
  • In Some Organisms, Fatty Acid Synthesis Takes Place in Multienzyme Complexes
    • fatty acyl synthase I (FAS I)
      • FAS in mammals on homodimeric FAS I, each 270-kD polypeptide of which contains all reaction centers required to produce a fatty acid
      • FAS in lower eukaryotes, such as yeast and fungi, the enzymatic activities of FAS are distributed on two multifunctional peptide chains
      • 图
      • In both the fungal and the mammalian FAS structures, the close association of enzymic domains within one large complex permits efficient transfer of intermediates from one active site to the next.
    • fatty acyl synthase II (FAS II)
      • In plants, most bacteria, and parasites寄生虫
      • the enzymes of fatty acid synthesis are separated and independent
      • this collection of enzymes is referred to as FAS II
  • C16 fatty acid synthesis reaction 核心反应
    • 外面是第一圈, 中间是后续循环, 原理一样, 底物长度逐渐增加
    • 第一圈把 Acetyl-ACP / Malonyl-ACP 接一块, 后续在接一块的butyryl上面循环加2个C, 第一圈和后续循环的原理是一样的, 除了底物C每次加2
      1. Acetyl-CoA / Malonyl-CoA 上 ACP
        起手
      • initiate elongation cycle
      • 酶 - malonyl-CoA–acetyl-CoA-ACP transacylase (MAT)
      1. KS操作
        合体长链/acetyl 与 malonyl-ACP
      • The β-ketoacyl-ACP synthase (KS) catalyzes the decarboxylative condensation of the acyl group with malonyl-ACP to produce a beta-ketoacyl酮脂酰-ACP intermediate (acetoacetyl-ACP in the first cycle)
      • hydrolysis of ATP drove the carboxylation of acetyl-CoA to form malonyl-ACP, so indirectly ATP is responsible for the condensation reaction to form acetoacetyl-ACP, and Malonyl-CoA can be viewed as a form of stored energy for driving fatty acid synthesis
        能量流动 ATP → malonyl-ACP → fatty acid
      1. Reduction of the β-Carbonyl Group
        一系列操作删除C=O, 饱和碳链
      • ① reduction of the β-carbonylgroup by β-ketoacyl-ACP reductase (KR) to form a β-alcohol
        C=O → C-OH
      • ② dehydration by β-hydroxyacyl-ACP dehydratase (DH)
        C-OH → C=C
      • ③ reduction by 2,3-trans-enoyl-ACP reductase (ER)
        C=C → C-C
      • 与fatty acid degradation in reverse类似, 但有区别
          1. different enzymes are involved
          1. the alcohol formed in biosynthesis has the D-configuration rather than the L-form seen in catabolism
          1. the reducing coenzyme is NADPH, whereas NAD+ and FAD+ are the oxidants in the catabolic pathway
      1. end
      • This cycle continues with the net addition of a two carbon unitin each turn until the chain is 16 carbons long.
      • The KS cannot accommodate larger substrates, so the reaction cycle ends with a 16-carbon chain
  • C16 fatty acid elongation
    • Longer chains are made through special elongation reactions, which occur both in the mitochondria and at the surface of the ER.
    • The ER reactions
      • are similar to former - addition of two-carbon units at the carboxyl end of the chain by the oxidative decarboxylations of malonyl-CoA
      • this decarboxylation provides the thermodynamic driving force for the condensation reaction
    • The mitochondrial reactions
      • involve addition of acetyl units
      • These reactions are essentially a reversal of fatty acid oxidation, with the exception that NADPH is utilized in the saturation of the double bond, instead of FADH2
  • Regulation of fatty acid synthesis and oxidation
    • 产物对两个方向的负反馈调节
      • 图
      • Malonyl-CoA
        • inhibiting the carnitine acyltransferase
        • inhibits the uptake of fatty acylcarnitine 脂肪酰肉碱 (and thusfatty acid oxidation) by mitochondria.
        • 在生产fatty acid过程中不让线粒体进行β-oxidation
      • citrate
        • levels rising (which reflect anabundance of acetyl-CoA) signal the initiation of fattyacid synthesis
        • an important allosteric activator of acetyl-CoA carboxylase
      • fatty acyl-CoA
        • when levels rise, fatty acid synthesisis inhibited and fatty acid oxidation activity increases
        • an important allosteric inhibitor of acetyl-CoA carboxylase
        • The degree of inhibition is proportional to the chain length of the fatty acyl-CoA
          碳链越长, 别构抑制作用越强
    • Hormonal Signals
      • 图
      • hormone
        • glucagon 胰高血糖素
          • G protein → cAMP → protein kinase A → phosphate ACC / phosphate lipase
        • insulin 胰岛素
          • receptor → cAMP ↓ 抵制胰高血糖素
        • epinephrine 肾上腺素
          • activate lipase
      • ACC
        • 结构图, Phosphorylation at Ser1200 is primarily responsible for decreasing the affinity for citrate
        • 2种状态
          • 1200没P
            • 不会降低对citrate的affinity
            • 高浓度脂肪才会抑制
            • 这种状态利于合成脂肪
          • 1200有P
            • 会降低对citrate的affinity
            • 低浓度脂肪就会抑制
            • 这种状态不利于合成脂肪
          • 图, 由于ACC单体串成一串, 示意图中每个P代表不同单体1200位置
        • glucagon可以将它切换到有P状态, 抑制脂肪合成
        • PP2A可以将它切换到无P状态, 促进脂肪合成
          • 这是五碳糖途径促进脂肪合成的一部分体现, 五碳糖途径可以促进脂肪合成 → Regulation of fatty acid synthesis and oxidation
      • lipase
        • glucagon / epinephrine 都可以激活它
        • lipase 将 triacylglycerol 分解为 fatty acid 与 glycerol
        • 分解出来的 fatty acid 进入线粒体进行 β-oxidation, 释放能量
        • 这个酶促进脂肪分解
  • lipid transport
    • When most lipids circulate in the body, they do so in the form of lipoprotein complexes.
    • 当大多数脂质在体内循环时，它们是以脂蛋白复合物的形式进行的
    • Simple, unesterified fatty acids are only bound to serum albumin and other proteins in blood plasma, but phospholipids, triacylglycerols, cholesterol, and cholesterol esters are all transported in the form of lipoproteins.
    • 简单的未酯化脂肪酸仅与血浆中的血清白蛋白和其他蛋白质结合, 而磷脂、甘油三酯、胆固醇和胆固醇酯则全部以脂蛋白的形式进行运输。
    • At various sites in the body, lipoproteins interact with specific receptors and enzymes that transfer or modify their lipid cargoes.
    • 在身体的不同部位，脂蛋白会与特定的受体和酶发生相互作用，从而转移或修饰它们所携带的脂质载荷
    • Lipoproteins were classified according to their densities.
    • 脂蛋白是根据它们的密度进行分类的
    • The densities are related to the relative amounts of lipid and protein in the complexes.
    • 密度与复合物中脂质和蛋白质的相对含量有关
    • Because most proteins have densities of about 1.3 to 1.4 g/mL, and lipid aggregates usually possess densities of about 0.8 g/mL, the more protein and the less lipid in a complex, the denser the lipoprotein.
    • 由于大多数蛋白质的密度约为 1.3 至 1.4 g/mL，而脂质聚集体的密度通常约为 0.8 g/mL，因此复合物中蛋白质越多、脂质越少，脂蛋白的密度就越大
    • there are high-density lipoproteins (HDLs), low-density lipoproteins(LDLs), intermediate-density lipoproteins (IDLs), very-low-density lipoproteins(VLDLs), and also chylomicrons 乳糜微粒
    • Pro-Atherogenic 促动脉粥样硬化 lipoproteins, Anti-atherogneic 抗动脉粥样硬化 lipoproteins
    • Chylomicrons
      • the largest lipoproteins.
      • They deliver TGs甘油三酯 from the intestine (via lymph and blood) to tissues (muscle for energy, adipose for storage).
      • They are present in blood only after feeding.
      • Cholesterol-rich chylomicron remnants 残留物deliver cholesterol to the liver.
        富含胆固醇的乳糜微粒残留物(chylomicron remnants)将胆固醇运送到肝脏