Chapter 16 The Citric Acid Cycle 1
Catabolic Fates of Pyruvate Fig. 14.3 2
Overview: The Three Stages of Cellular Respiration Stage 1: Acetyl-CoA [ 乙醯輔酶 A] Production from Glucose Fatty Acids Amino Acids Stage 2: Acetyl-CoA Oxidation (TCA cycle) Stage 3: Electron Transfer & Oxidative Phosphorylation Fig. 16-1
From Pyruvate to Acetyl-CoA Fig. 16-2 Conversion from a 3-carbon unit to a 2- carbon unit is achieved by Oxidative decarboxylation of pyruvate by The pyruvate dehydrogenase complex, A cluster of 3 enzymes that Requires 5 cofactors (4 of which are vitamin-derived)
Which of the below is not required for the oxidative decarboxylation of pyruvate to form acetyl-coa? (A). ATP (B). CoA-SH (C). FAD (D). Lipoic acid (E). NAD+ 5
Which combination of cofactors is involved in the conversion of pyruvate to acetyl-coa? (A). Biotin, FAD, and TPP (B). Biotin, NAD+, and FAD (C). NAD+, biotin, and TPP (D). Pyridoxal phosphate, FAD, and lipoic acid (E). TPP, lipoic acid, and NAD+ 6
Satellite View of the TCA Cycle You simply have to learn: The eight steps and their characteristics Reactants and products Enzyme names Places where Oxidation occurs CO 2 is released ATP is produced NADH is produced FADH 2 is produced Fig. 16-7
Malonate is a competitive inhibitor of succinate dehydrogenase. If malonate is added to a mitochondrial preparation that is oxidizing pyruvate as a substrate, which of the following compounds would you expect to decrease in concentration? (A). Citrate (B). Fumarate (C). Isocitrate (D). Pyruvate (E). Succinate 8
Step 1: The Cycle Begins OAA 丁酮二酸 檸檬酸 A 2 + 4 carbon condensation exploiting an induced fit conformational change That ensures productive regeneration of CoA-SH Driven by a large negative G, essential because The cellular concentration of OAA is M or less
Step 2: An Isomerization A clever way to make an isomer! Though energetically unfavorable, removal of product (in step 3) pulls the reaction along
Step 3: Oxidative Decarboxylation! Loss of a carbon and of electrons Generates reducing power In an irreversible step of the cycle
Step 4: Here comes another one, just like the other one Enzyme and reaction are virtually identical to the pyruvate dehydrogenase complex With same set of five coenzymes And is likewise irreversible
Step 5: One high-energy compound yields another 丁二酸 To carry out substrate-level phosphorylation Via a phosphorylated histidine, located At the interface with the nucleotide-binding subunit (see Fig. 16-10)
Step 6: There s a whole lot of oxidizing going on at the only membranebound enzyme of the TCA cycle, which has 3 iron-sulfur centers that bring electrons from FADH 2 to the chain of electron carriers in the membrane This ultimately yields 1.5 ATPs/electron pair The reaction can be strongly inhibited by the succinate analog malonate 延胡索酸
Step 7: Stereospecific Hydration 蘋果酸 And that s about all there is to say about this reaction
Step 8: How Can It Go? 丁酮二酸 This apparently unfavorable step proceeds only because of the extremely low cellular concentrations of OAA Which is continually being removed by citrate synthase (step 1)
You should know, where the electrons and phosphoryl groups go Fig. 16-13
Energetics of Glycolysis vs TCA Cycle Glycolysis per glucose 2 ATP 2 NADH Anything missing here? TCA Cycle per 2 acetates 2 GTP (ATP) 6 NADH 2 FADH 2 Overall, about 65% of the total 2840 kj/mole of glucose becomes available to the cell
TCA Cycle Regulation The PDH complex is controlled by both allosteric and covalent mechanisms The 3 TCA cycle exergonic steps are also sites of regulation Citrate synthase Isocitrate dehydrogenase -ketoglutarate dehydrogenase Note: Inhibition by ATP, NADH, succinyl-coa Activation by ADP and Ca ++ Fig. 16-18
Citric Acid Cycle Components Are Important Biosynthetic Intermediates In aerobic organisms, the citric acid cycle is an amphibolic Pathway ( 雙向反應 ), one that serves in both catabolic ( 合成 ) and anabolic ( 分解 ) processes. Besides its role in the oxidative catabolism of carbohydrates, fatty acids, and amino acids, the cycle provides precursors for many biosynthetic pathways (Fig. 16 15), through reactions that served the same purpose in anaerobic ancestors. alpha Ketoglutarate and oxaloacetate can, for example, serve as precursors of the amino acids aspartate and glutamate by simple transamination (Chapter 22). 21
The TCA Cycle is Amphibolic Fig. 16-15
Anaplerotic ( 添補 ) Reactions Replenish ( 補充 ) Citric Acid Cycle Intermediates As intermediates of the citric acid cycle are removed to serve as biosynthetic precursors, they are replenished by anaplerotic reactions (Fig. 16 15; Table 16 2). Under normal circumstances, the reactions by which cycle intermediates are siphoned off into other pathways and those by which they are replenished are in dynamic balance, so that the concentrations of the citric acid cycle intermediates remain almost constant. Table 16 2 shows the most common anaplerotic reactions, all of which, in various tissues and organisms, convert either pyruvate or phosphoenolpyruvate to oxaloacetate or malate. The most important anaplerotic reaction in mammalian liver and kidney is the reversible carboxylation of pyruvate by CO2 to form oxaloacetate, catalyzed by pyruvate carboxylase. When the citric acid cycle is deficient in oxaloacetate or any other intermediates, pyruvate is carboxylated to produce more oxaloacetate. 23
Important Anaplerotic Reactions Pyruvate carboxylase a key enzyme Requires biotin (attached to lysine!) Positively activated by acetyl-coa PEP carboxylase Activated by fructose 1,6 bisphosphate