October 24, 2007

Fatty Acid Oxidation, Krebs Cycle and Glycolysis

Posted in Cell Biology at 2:51 am by D. Borst

So my cell biology class is taking a very modular approach to Fatty Acid Oxidation, the Krebs Cycle and Glycolysis. Im sure that my biochemistry class will not be nearly so informal, however, for completeness in my discussion of the mitochondria and because I said I would, here is a quick overview of these three important cycles that lead to the cell getting energy from food.

There are two standard food inputs that the cell uses to make ATP–Fatty Acids and Glucose. Ultimately, Glucose is reduced to two molecules of pyruvate in the cytosol, which are then further reduced to two molecules of Acetyl-CoA in the matrix space. In the fatty acid cycle, fatty acids are reduced in length to make units of Acetyl-CoA. This supply of Acetyl-CoA is what drives the Citric acid cycle in the cell.

Let us first tackle glycolysis. Using one ATP, Glycolysis is phosphorylated by hexokinase (remember that kinases are enzymes involved in phosphorylation) and changed into fructose 6-P by phosphoglucose isomerase. Using 1 ATP, phosphofructokinase phosphorylates the fructose on its first carbon, making fructose 1,6-P. This product is then cut by aldolase into hydroxyacetone phosphate and glyceraldehyde 3-phosphate. The hydroxyacetone P is transformed into another glyceraldehyde by tirose phosphate isomerase, giving us two glyceraldehyde 3-P. By gaining a phosphate, each glyceraldehyde 3-P reduces a NAD+, yielding twp 1,3-biphosphoglycerate and two NADH. Each biophosphoglycerate interacts with a phosphoglycerate kinase to phosphorylate an ADP, yielding 2 ATP and a 3-phosphoglycerate. Phosphoglycerate mutase then moves the phosphate from the third carbon to the second, producing 2-phosphoglycerate, each of which is used to produce phosphoenolpyruvate through interaction with enolase. Each enolpyruvate is then used to phosphorylate an ATP via catalysis by pyruvate kinase, yielding pyruvate and ATP.

Each of the steps of glycolysis is reversible, except for those catalyzed by pyruvate kinase, phosphofructokinase and hexokinase, thus once you get a molecule of pyruvate, it cannot wend its way back along the path–it is stuck as a pyruvate (unless it finds another metabolic path to follow).

In this procedure, two molecules of ATP have been used to catalyze phosphorylation of the sugar, and four ATP’s were yielded, along with two molecules of NADH. This makes the net yield of glycolysis 2 ATP, 2 NADH and 2 Pyruvate for each molecule of Glucose. A schematic summary of this process is given in Alberts’ Molecular Biology and the Cell, availible here.

The products of glycolysis (pyruvate) are imported into the mitochondria, where they are decarboxylated and attached to the CoA enzyme. In the process of loosing CO2, pyruvate reduces a NAD+. The resulting Acetyl CoA then enters the Krebs cycle proper. Acetyl CoA is catalyzed by Citrate synthase to donate its Acetyl group to oxaloacetate (also involved in the malate-aspartate shuttle) to produce citrate. Citrate is transformed into isocitrate by aconitase, which is then used by isocitrate dehydrogenase to reduce an NAD+ producing α-keytoglutarate (also involved in malate aspartate shuttle). The keytoglutarate then interacts with the α-keytoglutarate dehydrogenase complex and the CoA enzyme to reduce a NAD+, and decarboxylate, yielding succinyl-CoA. It then is phosphorylated by succinyl CoA synthase to make succinate, phosphorylating a GDP in the process. Succinate then reacts with the membrane bound succinate dehydrogenase (Complex II in the electron transport chain), reducing FAD and producing Fumarate. Fumarate then reacts with fumerase and water to make malate which reacts with malate dehydrogenase to yield one final NADH and to restore oxaloacetate for use in another cycle.

The net products of this reaction chain are 4 molecules of NADH, one molecule of GTP and one molecule of FADH2, which means for each molecule of Glucose, this has yielded 4 NADH, 2 GTP and 2 FADH2. The carbons added by pyruvate to the oxaloacetate are lost in steps 3 and 4 of the chain (the reactions from isocitrate to succinyl CoA, though it should be noted that the carbons lost are not the ones actually added on by the pyruvate, but rather those present from another cycle in the citric acid cycle. The energy released from pyruvate comes from the successive oxidation and decarboxylation of the molecule. A diagram of the reaction, from Alberts’ text, is available here

Fatty Acid oxidation also provides a source of Acetyl CoA. Fatty Acids are imported into the Mitochondria after they have been activated for oxidation in the cytosol. Once in the mitochondria fatty acid goes through a cycle of four reactions mediated by four enzymes. The first reaction is an oxidation mediated by acyl-CoA dehydrogenase (AD). AD has a prosthetic FAD group that transfers the electrons that it gains from the oxidation of the fatty acid to ETF, which passes them on to Coenzyme Q via ubiquinone oxioreductase. The oxidized fatty acid, a trans-Δ2-Enoyl-CoA, is then hydrated by enoyl-CoA Hydratase. Once hydrated, the compound is reduced once more, and the electrons lost in this oxidation are passed on to a NADH via 3-L-hydroxyacyl-CoA dehydrogenase. The resulting compound is then undergoes thiolysis. The thiolysis is acomplished via β-Keytoacyl-CoA thiolase. The thiolysis breaks the bond betwen the β carbonyl carbon and the α carbon, yielding Acetyl CoA and another Fatty acyl-CoA that is two atoms shorter than when the sequence of reactions started. A schematic of this from Stryers’ text can be found here.

Thus Fatty Acid Oxidation provides a FADH2, a NADH and a Acetyl-CoA per cycle, until the fatty acid chain has been fully oxidized.

On a per molecule basis, it becomes clear that Fats are a much better source of energy to make ATP than are glucose molecules. Fat molecules are arranged in triglycerides–fully reduced molecule with three fatty acid chains, each of which may contain 18 carbons. Thus a single triglyceride has the potential to drive (18/2)*3=27 rounds of the citric acid cycle—as compared to two per molecule of glucose.

To sum up,

  • Glycolysis breaks down glucose molecules into two molecules of pyruvate in 10 enzymatically mediated reactions.
  • Glycolysis occurs in the cytosol and its product, pyruvate, is imported into the mitochondria.
  • Glycolysis produces a net gain of 2 NADH and 2 ATP
  • Pyruvate is transformed into Acetyl CoA inside the mitochondria, yielding one more NADH.
  • Fatty Acids are activated in the cytosol, and transported inside the mitochondria for oxidation.
  • One round of oxidation of a fatty acid yields one molecule of Acetyl CoA, one NADH and one FADH2
  • Fatty Acid Oxidation is a 4 step process involving two oxidation steps, a hydration and a thiolysis, all of which are enzymatically mediated.
  • Acetyl CoA is the substrate for the TCA cycle.
  • The Citric Acid Cycle is an eight step enzymatically mediated process.
  • For each round of the TCA cycle, 3 NADH, 1 FADH2 and 1 GTP are produced.
  • Fatty acids, because of their long reduced carbon chains, are able to power many more cycles of the citric acid cycle than Glucose, on a per carbon basis.
  • The information for this post came from Alberts’ Molecular Biology and the Cell, as well as Voet and Voet’s Biochemistry.

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    7 Comments »

    1. tamo said,

      quite interesting, but not enough

    2. izi elekele said,

      i think you should tabulate your enzymes

    3. qudsia rais said,

      give in the form of cycle with strcture

    4. david said,

      uhhh acetyl-coa from fatty acid degradation cannot be used in the cac in animals…

    5. Muriel said,

      what i really wanted to know is how to calculate ATPs in order to answer a question like ‘how many ATp would be generated on the complete oxidation of 1-oleyol-2palmitoyl-3stearoyl-glycerol’
      thank you.

    6. カシオ エディフィス

    7. personal said,

      I think the admin of this site is actually working hard in favor of his web page, as here every data is
      quality based stuff.


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