19.TCA

  • basic
    • 图
    • 图
    • TCA gives -2H2O +GTP +FADH2 +2CO2 +3NADH + 3H+

      • 第0步 +CO2 +NADH +H+
    • Combining glycolysis and the TCA cycle gives

      • 注意一个glycose进行一次glycolysis和两次TCA
    • 理论上 1 * glycose → 38 * ATP
      • All six carbons of glucose are released as CO2
      • 4 molecules of ATP are formed in substrate-level phosphorylation
      • glycolysis + TCA 产生了 10 * NADH 与 2 * FADH2
      • in electron transport and oxidative phosphorylation
        • NADH → 3 ATP
        • FADH2 → 2 ATP
      • 但是细胞质基质产生的NADH不一定进入电子传递链
  • steps
    • step0 - Oxidative Decarboxylation of Pyruvate → Acetyl-CoA (+CO2 +NADH +H+)
      • pyruvate + CoASH + NAD+ → Acetyl-CoA + NADH + CO2 + H+
      • CoA 是 Coenzyme A 辅酶A
      • pyruvate must first enter the mitochondria to enter the TCA cycle
      • 酶 - pyruvate dehydrogenase
        • a multienzyme complex
        • The pyruvate dehydrogenase complex (PDC) is a noncovalent assembly of three different enzymes
        • 由三种不同酶构成的非共价组装体
        • operating to catalyze successive steps in the conversion of pyruvate to acetyl-CoA
        • 它们协同作用，催化丙酮酸转化为乙酰辅酶 A 过程中的连续反应步骤
        • The active sites of all three enzymes are not far removed from one another
        • 这三种酶的活性中心彼此相距不远
        • the product of the first enzyme is passed directly to the second enzyme and so on, without diffusion of substrates and products through the solution
        • 第一种酶的产物会直接传递给第二种酶，依此类推，无需底物和产物在溶液中扩散
    • step1 - oxaloacetate + Acetyl-CoA → citrate (-H2O) 调控步骤
      • oxaloacetate + Acetyl-CoA ⇄ Citryl-CoA + H2O ⇄ citrate + CoA
      • The overall ∆G°is -31.4 kJ/mol, and the reaction is irreversible
      • Although the mitochondrial concentration of oxaloacetate is very low (much less than 1 µM), the strong negative ∆ G°drives the reaction forward.
      • 酶 - Citrate Synthase
        • a dimer of 49-kD subunits
        • On each subunit, oxaloacetate and acetyl-CoA bind to the active site
        • Binding of oxaloacetate induces a conformational change that facilitates the binding of acetyl-CoA and closes the active site
        • In the monomer of citrate synthase, citrate is shown in green, and CoA is red
        • regulation
          • a highly regulated enzyme
          • allosteric inhibitor
            • NADH , a product of the TCA cycle
            • succinyl-CoA, the product of the following step in the cycle
    • step2 - Citrate ⇄ Isocitrate
      • The Isomerization of Citrate
      • 柠檬酸的OH在中间,进一步氧化比较困难, 所以把OH变到旁边
      • 酶 - Aconitase 顺乌头酸酶
    • step3 - isocitrate → α-ketoglutarate (+CO2 +NADH +H+) 调控步骤
      The First Oxidation in the Cycle
      • isocitrate + NAD+ → NADH + H+ +oxalosuccinate → CO2 + α-Ketoglutarate
      • decarboxylate 脱羧
      • The reaction has a net ∆G° of -8.4 kJ/mol, and it is sufficiently exergonic to pull the aconitase reaction forward
      • 酶 - Isocitrate Dehydrogenase
        • 链接电子传递链与TCA
        • regulation
          • allosteric inhibitors
            • NADH
            • ATP
          • allosteric activator
            • ADP
              • lowering the Km for isocitrate by a factor of 10
              • The enzyme is virtually inactive in the absence of ADP
    • step4 - α-ketoglutarate → succinyl-CoA (+CO2 +NADH +H+) 调控步骤
      A Second Decarboxylation
      • α-Ketoglutarate + NAD+ +CoA → NADH + H+ +CO2 + succinyl-CoA
      • free energy changes for these reactions are-29 to-33.5 kJ/mol
      • 酶 - α-Ketoglutarate Dehydrogenase
        • a multienzyme complex
    • step5 - succinyl-CoA → succinate (+GTP)
      • Succinyl-CoA + GDP + P → succinate + GTP +CoA
      • 酶 - Succinyl-CoA Synthetase 琥珀酰辅酶A合成酶
      • drive the phosphorylation of GDP to GTP (in mammals) or ADP to ATP (in plants and bacteria)
      • 是TCA中唯一一个用底物水解的能量磷酸化的反应 substrate-level phosphorylation
        不依赖电子传递链, 不利用质子梯度
      • nucleoside diphosphate kinase reaction
        • The GTP produced by mammals in this reaction can exchange its terminal phosphoryl group with ADP
    • step6 - succinate ⇄ fumarate (+FADH2)
      • succinate +FAD ⇄ fumarate + FADH2
      • 酶 - succinate dehydrogenase
        • succinate dehydrogenase is an integral membrane protein
          其他都是soluble proteins found in the mitochondrial matrix
        • tightly associated with the inner mitochondrial membrane
        • actually part of the electron transport chain
        • FAD-Dependent
    • step7 - fumarate → malate (-H2O)
      • fumarate + H2O → L-malate
      • Trans-Hydration of fumarate
      • 酶 - Fumarase 延胡索酸酶
    • step8 - malate ⇄ oxaloacetate (+NADH +H+)
      • malate + NAD+ ⇄ oxaloacetate + NADH + H+
      • very endergonic, with a △G°of +30 kJ/mol
      • the concentration of oxaloacetate in the mitochondrial matrix is usually quite low
      • pulled forward by the favorable citrate synthase reaction
      • 酶 - Malate Dehydrogenase 脱氢酶
  • TCA主线外的分支
    • The TCA Cycle Provides Intermediates for Biosynthetic Pathways
      • 图
      • four-, five-, and six-carbon species produced in the TCA cycle also fuel a variety of biosynthetic processes
      • α-Ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate are all precursors of important cellular species
      • In order to participate in eukaryotic biosynthetic processes, they must first be transported out of the mitochondria
        • ATP–citrate lyase
          • Citrate broken down to generate oxaloacetate and acetyl-CoA, a precursor of fatty acids
          • Oxaloacetate is recycled to malate or pyruvate, which re-enters the mitochondria
          • This cycle permits citrate to provide acetyl-CoA for biosynthetic processes
          • 图
    • The Anaplerotic补给
      • cell also feeds many intermediates back into the TCA cycle from other reactions
      • The catabolism of amino acids provides pyruvate, acetyl-CoA, oxaloacetate, Fumarate, α-ketoglutarate, and succinate
      • Pyruvate carboxylase, phosphoenolpyruvate (PEP) carboxylase, and malic enzyme catalyze anaplerotic reactions
      • Pyruvate carboxylase
        • catalyze the most important anaplerotic reactions
        • pyruvate 变成 acetyl-CoA进TCA跑一圈就没了, 通过这个反应可以补充中间的酸
        • exists in the mitochondria of animal cells but not in plants
        • has an absolute allosteric requirement for acetyl-CoA
          • Thus, when acetyl-CoA levels exceed the oxaloacetate supply, allosteric activation of pyruvate carboxylase by acetyl-CoA raises oxaloacetate levels, so that the excess acetyl-CoA can enter the TCA cycle
          • 这个酶需要acetyl-CoA的别构激活, 所以acetyl-CoA超量供应时候就会激活这个酶, 把 pyruvate 变成 oxaloacetate 平衡TCA第一步两个反应物的量
  • regulation
    • the sites of regulation would be characterized by large, negative ∆G values
      • citrate synthase
      • isocitrate dehydrogenase
      • α-ketoglutarate dehydrogenase
    • 图
    • The principal regulatory signals
      • NADH
        • 全都抑制
        • inhibits Pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase
      • cell energy
        • The TCA cycle is turned on when either the ADP/ATP or NAD/NADH ratio is high
      • acetyl-CoA
        • acts as a signal to the TCA cycle that glycolysis or fatty acid breakdown is producing two-carbon units
        • activates pyruvate carboxylase, the anaplerotic reaction that provides oxaloacetate, the acceptor for increased flux of acetyl-CoA into the TCA cycle
      • succinyl-CoA
        • an intracycle regulator
        • inhibiting citrate synthase and α-ketoglutarate dehydrogenase
  • 乙醛酸循环