THE CITRIC ACID CYCLE. aka Krebs cycle or Tricarboxylic acid (TCA) cycle

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1 THE CITRIC ACID CYCLE aka Krebs cycle or Tricarboxylic acid (TCA) cycle

2 Overview of Citric Acid Cycle Feed to cycle is the acetyl group note that this is already partially oxidized In this cycle, we will complete its oxidation into two molecules of CO 2. The electrons lost by carbon will be captured by NAD + and FAD and fed to oxidative phosphorylation

3 Overview of Cellular Respiration

4 The Citric Acid Cycle Start here

5

6 Stage 1 Oxidize 2-C Units Condense acetyl with oxaloacetate Animal poison fluoroacetate stops cycle 6-C molecule 4-C molecule +

7 Why is Thioester bond so energetic? kj/mole kj/mole

8 Citrate Synthesis Mechanism

9 Origin of Oxaloacetate Can be derived from pyruvate by carboxylation Pyruvate + CO 2 + ATP + H 2 O Oxaloacetate + ADP + P i + 2H +

10 Citrate Synthesis Undesirable side reaction is to hydrolyze the acetyl group off of acetyl CoA before the acetyl is attached to the oxaloacetate Catalyzed by Citrate Synthase Open form Closed form

11 Citrate Synthesis Oxaloacetate binds first and creates binding sites for acetyl CoA so oxaloacetate is immediately ready for reaction The oxaloacetate then reacts with acetyl CoA forming the unstable compound, citryl CoA The formation of citryl CoA causes the enzyme to completely close and brings enzyme residues in close contact so that water can hydrolyze off the CoA After desorbing CoA and citrate, the enzyme returns to its open position Thus, induced fit minimizes side reactions such as hydrolysis of acetyl CoA

12 Citrate Isomerized to Isocitrate The isomerization is accomplished by dehydrating the citrate then hydrating the intermediate, cis-aconitate Why the reaction: easier to oxidize a 2 o ROH than a 3 o ROH (next step) At equilibrium molecules in this reaction mainly exist as citrate However, reaction is pulled to right as the exergonic oxidation following this step depletes the reaction below of isocitrate thus shifting it to the right 2 o ROH 3 o ROH At equilib: 90% 4% 6%

13 Alcohols 1 o, 2 o, 3 o (Reminder)

14 Citrate to Isocitrate mechanism

15 Isocitrate is Oxidized and De- Carboxylated (1 st Redox) Isocitrate + NAD + -ketogluterate + CO 2 + NADH Carboxylate Lewis resonance structures

16 Detailed chemistry

17 Second Oxidative De-carboxylation Similar to the formation of acetyl CoA by de-carboxylating pyruvate, we have the same tri-enzyme type of complex for -ketoglutarate (both an -ketoacid) -ketoglutarate + CoA + NAD + Succinyl CoA + CO 2 + NADH -ketogluterate dehydrogenase complex

18 Similar to Pyruvate Conversion to Acetyl CoA NAD + NADH + H+ R R = CH 2 -CH 2 -COO - ketoglutarate and succinyl CoA R = CH 3 for pyruvate and acetyl CoA

19 Detailed Chemistry Analogous to TPP :B :B H:B + :B :B Flexible amide arm With disulfide bridge B:H + Uses FAD to reduce back to sulfide and NAD + to oxidize back to FAD

20 Stage 1-4 Steps to oxidize C to CO 2 So far we ve had a 2-C moiety enter (acetyl) and we ve converted that into 2 moles of CO 2 Generated 2 moles of NADH (high transfer potential electron carrier) Stage 2 4 Steps produce energy and regenerate oxaloacetate In regenerating oxaloacetate pull more energy out of cycle. In doing so, take 1 step to preserve the thioester bond energy by phosphorylating GDP to GTP Then spend three steps in converting a methylene group back to a carbonyl carbon in oxaloacetate

21 Taking Advantage of the Energetic Thioester Bond ΔG o for hydrolysis of thioester bond in succinyl CoA is kj/mole Compare to ATP phosphate hydrolysis at -30 kj/mole We preserve that energy by making GTP This reaction utilizes a swinging histidine side chain to transfer the PO 2-4 group from succinyl phosphate to GDP

22 Persevering Energy Making GTP ;

23 What is GTP? Guanosine Triphosphate (GTP) ATP

24 Regenerating Oxaloacetate-1 st Oxidation Overall, in order to get back to oxaloacetate need to convert a methylene group CH 2 by oxidizing to a carbonyl C=O. In the first step FAD is reduced rather than using NAD +, since the free energy change is insufficient to reduce NAD + This enzyme is Complex II in the ETC Succinate dehydrogenase Carbon Oxid # -2, -2-1, -1 Total Valence e

25 Regenerating Oxaloacetate-1 st Oxidation + 2H+ + 2e- FAD + 2H+ + 2e- FADH 2

26 Detailed Chemistry Two Views

27 Succinate Dehydrogenase Embedded in the inner mitochondrial membrane So FADH 2 transfers its electrons directly to Coenzyme Q (CoQ) in the Electron Transport Chain (to be considered next chapter).

28 Regenerating Oxaloacetate-2 nd Oxidation Fumerate is then hydrolyzed Carbon oxid # fumarase

29 Regenerating Oxaloacetate-3 rd Oxidation Oxidation of the alcohol carbon gives oxaloacetate ΔG o = kj/mole (endergonic) ; driven by consumption of oxaloacetate by converting acetyl CoA Maleate dehydrogenase Oxidation number 0 +2 Total valence e

30 Oxidation of L-Malate

31 Energetics in Citric Acid Cycle

32 Citric Acid Cycle Energetics Including Glycolysis all 6 Glucose carbons are converted to CO 2 1 in pyruvate to acetyl CoA and two in Citric Acid Cycle And 4 moles of ATP Go through cycle twice for each glucose

33 Citric Acid Productivity The 12 reduced species from oxidizing 6 Glucose carbons will eventually make ATP in the ETC 2 NAD + from Glycolysis 2 NAD + from pyruvate to acetyl CoA 6 NAD + from TCA 2 FADH 2 from TCA Altogether, they will make 34 molecules of ATP Together with 4 ATP made in Glycolysis gives theoretical total of 38 ATP

34 Summary of Citric Acid Cycle For each Pyruvate (3-C molecule) go through PDH and citric acid cycle once or twice/glucose molecule Take in partially oxidized 2-C molecule acetyl From this we make in citric acid cycle 2 molecules of CO 2 Convert one molecule of GDP to GTP Reduce 3 molecules of NAD + to NADH Reduce 1 molecule of FAD to FADH 2 Enzymes in the cycle are closely association with one another Citric acid cycle is anaerobic since no O 2 is involved, however oxygen (ETC) needs to be present to re-oxidize NADH and FADH 2 back to their oxidized forms.

35 Overall Energy Summary Theoretical ATP Yield Process ATP/GTP Reduced Species ATP Glycolysis 2 2 NADH 2 x 3 =6 Preparation for TCA 2 NADH 2 x 3 = 6 TCA 2 6 NADH 6 x 3 = 18 2 FADH 2 2 x 2 = 4 Total 4 34 Total theoretical yield of ATP from glucose is 38 ATP

36 Citric Acid Control As we have seen, pyruvate dehydrogenase acts to control acetyl CoA entering the cycle. However acetyl CoA from fat metabolism enters the cycle directly Therefore, the cycle must also have its own means of control; these are at two points: Allosteric enzymes Isocitrate dehydrogenase -ketogluterate dehydrogenase (like PDH)

37

38 Citric Acid Control Citric acid cycle has its own control points. Why? Because other energy sources (fats and proteins) enter metabolism via acetyl-coa Isocitrate dehydrogenase Positively stimulated by ADP which means cell needs more ATP Inhibitory are high levels of NADH and ATP -ketogluterate dehydrogenase Inhibited by its products (succinyl CoA and NADH) Why these control points When isocitrate dehydrogenase is inhibited, citrate builds up; citrate then transfers to cytoplasm where it allosterically inhibits phosphofructokinase to prevent glucose processing -ketogluterate accumulation can be used for amino acids and purine bases

39 When Energy Needs are Met TCA used for Intermediate Generation

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