6.7: The Krebs/TCA/Citric Acid Cycle
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Glycolysis through fermentative reactions produces ATP anaerobically. The evolution of respiration (the aerobic use of oxygen to efficiently burn nutrient fuels) had to wait until photosynthesis created the oxygenic atmosphere we live in now. Read more about the source of our oxygenic atmosphere in Dismukes GC et al. [(2001) The origin of atmospheric oxygen on Earth: the innovation of oxygenic photosynthesis. Proc. Nat. Acad. Sci. USA 98:2170-2175].
The Krebs cycle is the first pathway of eukaryotic mitochondrial oxygenic respiration. The Krebs cycle no doubt evolved a few reactions at a time, perhaps at first as a means of protecting anaerobic cells from the “poisonous” effects of oxygen. Later, natural selection evolved the aerobic Krebs cycle, electron transport and oxidative phosphorylation metabolic pathways we see today. Whatever their initial utility, these reactions were an adaptive response to the increase in oxygen in the Earth’s atmosphere. As we’ve seen, respiration is a much more efficient pathway than glycolysis is for extracting chemical energy from nutrients. Animals rely on it, but even plants and photosynthetic algae use the respiratory pathway when sunlight is not available! Here we focus on oxidative reactions in the mitochondrion, beginning with pyruvate oxidation and continuing to the redox reactions of the Krebs cycle.
In mitochondria, pyruvate dehydrogenase catalyzes pyruvate oxidation to Acetyl-SCoenzyme A (Ac-S-CoA). The Krebs cycle then completely oxidizes Ac-S-CoA. These mitochondrial redox reactions generate \(\rm CO_2\) and a lot of reduced electron carriers (NADH, \(\rm FADH_2\)). The free energy released in these redox reactions is coupled to the synthesis of only one ATP per oxidized pyruvate (i.e., two per the glucose we started with). In fact, it is the NADH and \(\rm FADH_2\) molecules that have captured most of the free energy in the original glucose molecules.
The entry of pyruvate from glycolysis into the mitochondrion and its oxidation are summarized in Figure 6.21.
Pyruvate oxidation by pyruvate dehydrogenase converts a 3-C carbohydrate into a 2-C acetate molecule, releasing a molecule of \(\rm CO_2\). In this highly exergonic reaction, a reduced coenzyme A, CoA-SH, forms a high-energy thioester (-S-) linkage with the acetate in Ac-SCoA. Pyruvate oxidation results in the reduction of \(\rm NAD^{+}\) to NADH and the production of a molecule of \(\rm CO_2\), along with the production of Ac-S-CoA (Figure 6.22).
The Krebs cycle functions during respiration to oxidize Ac-S-CoA and to reduce \(\rm NAD^{+}\) and FAD to NADH and \(\rm FADH_2\), respectively. Overall, the cycle is exergonic. Intermediates of the Krebs cycle also function in amino acid metabolism and interconversions. All aerobic organisms alive today share the Krebs cycle. This is consistent with its spread early in the evolution of our oxygenic environment. Because of the central role of Krebs cycle intermediates in other biochemical pathways, parts of the pathway may even have predated the complete respiratory pathway. This centrality of the Krebs cycle to cellular metabolism is emphasized in the biochemical pathways chart shown at the top of this chapter, and it is shown as it occurs in animals in Figure 6.23 (below).
Then Ac-S
After the oxidation of pyruvate in mitochondria, the resulting Ac-S-CoA enters the Krebs cycle in the first reaction, condensing with oxaloacetate (OAA) to form citric acid, or citrate. Citrate is a tricarboxylic acid (TCA), for which the cycle was first named. There are four redox reactions in the Krebs cycle. As we discuss the cycle, look for the accumulation of reduced electron carriers (\(\rm FADH_2\), NADH) and a small amount of ATP synthesis by substratelevel phosphorylation. Also follow the carbons in pyruvate into \(\rm CO_2\). The following checklist will help you understand the events of the cycle:
- Find the two molecules of \(\rm CO_2\) produced in the Krebs cycle itself.
- Find GTP (which quickly transfers its phosphate to ADP to make ATP). Note that in bacteria, ATP is made directly at this step.
- Count all the reduced electron carriers (NADH, \(\rm FADH_2\)) in the biochemical pathway. Each will “carry” a pair of electrons into the mitochondria. If you include the electrons on each of the NADH molecules made in glycolysis, how many electrons have been removed from cytoplasmic glucose for during its complete oxidation in mitochondria?
Remember that glycolysis produces two pyruvates per glucose and thus two molecules of Ac-S-CoA. Thus, the Krebs cycle turns twice per glucose that enters the glycolytic pathway. The high-energy thioester linkages formed in the Krebs cycle fuel ATP synthesis as well as the condensation of oxaloacetate and acetate to form citrate in the first reaction. Each NADH carries about 50 Kcal of the 687 Kcal of free energy originally available in a mole of glucose; each \(\rm FADH_2\) carries about 45 Kcal of this free energy. This energy will fuel ATP production during electron transport and oxidative phosphorylation.
159-2 Highlights of the Krebs Cycle
While you’re counting stuff, figure out the \(\Delta Go\) for oxidizing a mole of glucose through the Krebs cycle. Is the cycle exergonic?
Finally, the story of the discovery of the Krebs cycle is as interesting as the cycle itself! Albert von Szent-Györgyi won a Nobel Prize in 1937 for discovering some organic acid oxidation reactions that were initially thought to be part of a linear pathway. Hans Krebs performed the elegant experiments showing that the reactions were part of a cyclic pathway. He proposed (correctly!) that the cycle would be a supercatalyst that would catalyze the oxidation of yet another organic acid. Some of the experiments are described by Krebs and his coworkers in their classic paper: Krebs HA, et al. [(1938) The formation of citric and αketoglutaric acids in the mammalian body. Biochem. J. 32: 113–117].
Hans Krebs and Fritz Lipmann shared the 1953 Nobel Prize in Physiology or Medicine. Krebs was recognized for his discovery of the TCA cycle, which more commonly carries his name. Lipmann was recognized for proposing ATP as the mediator between food (nutrient) energy and intracellular work energy and for discovering the reactions that oxidize pyruvate and synthesize Ac-S-CoA, bridging the Krebs cycle and oxidative phosphorylation (to be considered in the next chapter).
160 Discovery of the Kreb Cycle
You can read Krebs’ review of his own research in Krebs HA [(1970) The history of the tricarboxylic acid cycle. Perspect. Biol. Med. 14:154-170]. For a classic read on how Krebs described his supercatalyst suggestion, check out Citrate and α-Ketoglutarate Formation in Mammals. For more about the life of Lipmann, check the brief Nobel note on the Fritz Lipmann Biography.
Thinking about the endosymbiotic origins of mitochondria, where does the Krebs cycle occur in bacteria?