some cells obtain energy (ATP) by fermentation, breaking down glucose in the absence of oxygen. For most eukaryotic cells and many bacteria, which live under aerobic conditions and oxidize their organic fuels to carbon dioxide and water, glycolysis is but the first stage in the complete oxidation of glucose. Rather than being reduced to lactate, ethanol, or some other fermentation product, the pyruvate produced by glycolysis is further oxidized to H2O and CO2 .

This aerobic phase of catabolism is called respiration. In the broader physiological or macroscopic sense, respiration refers to a multicellular organism’s uptake of O2 and release of CO2 . Biochemists and cell biologists, however, use the term in a narrower sense to refer to the molecular processes by which cells consume O2 and produce CO2—processes more precisely termed cellular respiration.

 Cellular respiration occurs in three major stages. In the first, organic fuel molecules—glucose, fatty acids, and some amino acids—are oxidized to yield two-carbon fragments in the form of the acetyl group of acetyl-coenzyme A (acetyl-CoA). In the second stage, the acetyl groups are oxidized to CO2 in the citric acid cycle, and much of the energy of these oxidations is conserved in the reduced electron carriers NADH and FADH2 .

In the third stage of respiration, these reduced coenzymes are themselves oxidized, giving up protons (H+ ) and electrons. The electrons are transferred to O2 via a series of electron-carrying molecules known as the respiratory chain, resulting in the formation of water (H2O). In the course of electron transfer, much of the energy available from redox reactions is conserved in the form of ATP, by a process called oxidative phosphorylation. Respiration is more complex than glycolysis and is believed to have evolved much later, after the rise of oxygen levels in the earth’s atmosphere that resulted from the evolution of photosynthesis by cyanobacteria.

FIGURE DEPICTING :- Stages of cellular respiration


In aerobic organisms, glucose and other sugars, fatty acids, and most amino acids are ultimately oxidized to CO2 and H2O via the citric acid cycle and the respiratory chain. Before entering the citric acid cycle, the carbon skeletons of monosaccharides and fatty acids are degraded to the acetyl group of acetylCoA, the form in which the cycle accepts most of its fuel input. Many amino acid carbons also enter the cycle as acetate, although several amino acids are degraded to other cycle intermediates such as succinate and malate, which then enter the cycle.

The overall reaction catalyzed by the pyruvate dehydrogenase complex is an oxidative decarboxylation, an irreversible oxidation process in which the carboxyl group is removed from pyruvate as a molecule of CO2 and the two remaining carbons become the acetyl group of acetyl-CoA.

The NADH formed in this reaction gives up a hydride ion (:H− ) to the respiratory chain.


The combined dehydrogenation and decarboxylation of pyruvate to the acetyl group of acetyl-CoA. requires the sequential action of three different enzymes and five different coenzymes or prosthetic groups—

  • thiamine pyrophosphate (TPP)
  • flavin adenine dinucleotide (FAD)
  • coenzyme A (CoA, sometimes denoted CoA-SH, to emphasize the role of the —SH group)
  • nicotinamide adenine dinucleotide (NAD)
  • lipoate.

Four different vitamins required in human nutrition are vital components of this system: thiamine (in TPP), riboflavin (in FAD), niacin (in NAD), and pantothenate (in CoA).

Coenzyme A has a reactive thiol (—SH) group that is critical to the role of CoA as an acyl carrier in many metabolic reactions. Acyl groups are covalently linked to the thiol group, forming thioesters. Because of their relatively high standard free energies of hydrolysis,  thioesters have a high acyl group transfer potential—that is, donation of their acyl groups to a variety of acceptor molecules is a favorable reaction. The acyl group attached to coenzyme A may thus be thought of as “activated” for group transfer.

The fifth cofactor of the PDH complex, lipoate, has two thiol groups that can undergo reversible oxidation to a disulfide bond (—S—S—), similar to that between two Cys residues in a protein. Because of its capacity to undergo oxidation-reduction reactions, lipoate can serve both as an electron (hydrogen) carrier and as an acyl carrier.


The PDH complex contains three enzymes—pyruvate dehydrogenase (E1 ), dihydrolipoyl transacetylase (E2 ), and dihydrolipoyl dehydrogenase (E3 ) —each present in multiple copies.


  • E1 catalyzes first the decarboxylation of pyruvate, producing hydroxyethylTPP, and then the oxidation of the hydroxyethyl group to an acetyl group. The electrons from this oxidation reduce the disulfide of lipoate bound to E2 , and the acetyl group is transferred into thioester linkage with one –SH group of reduced lipoate.
  • E2 catalyzes the transfer of the acetyl group to coenzyme A, forming acetyl-CoA.
  • E3 catalyzes the regeneration of the disulfide (oxidized) form of lipoate; electrons pass first to FAD, then to NAD+ .
  • The long lipoyllysyl arm swings from the active site of E1 to E2 to E3 , tethering the intermediates to the enzyme complex to allow substrate channeling
FIGURE DEPICTING Oxidative decarboxylation of pyruvate to acetyl-CoA
FIGURE DEPICTING :- Oxidative decarboxylation of pyruvate to acetyl-CoA


In examining the eight successive reaction steps of the citric acid cycle, we place special emphasis on the chemical transformations taking place as citrate formed from acetyl-CoA and oxaloacetate is oxidized to yield CO2 and the energy of this oxidation is conserved in the reduced coenzymes NADH and FADH2 .

FIGURE DEPICTING Reaction involve in citric acid cycle
FIGURE DEPICTING :- Reaction involve in citric acid cycle

1. Formation of Citrate :- The first reaction of the cycle is the condensation of acetyl-CoA with oxaloacetate to form citrate, catalyzed by citrate synthase. In this reaction, the methyl carbon of the acetyl group is joined to the carbonyl group (C-2) of oxaloacetate. Citroyl-CoA is a transient intermediate formed on the active site of the enzyme. It rapidly undergoes hydrolysis to free CoA and citrate, which are released from the active site.

Reaction involve in citric acid cycle

2. Formation of Isocitrate via cis-Aconitate The enzyme aconitase (more formally, aconitate hydratase) catalyzes the reversible transformation of citrate to isocitrate, through the intermediary formation of the tricarboxylic acid cis-aconitate, which normally does not dissociate from the active site. Aconitase can promote the reversible addition of H2O to the double bond of enzyme-bound cis-aconitate in two different ways, one leading to citrate and the other to isocitrate.

Reaction involve in citric acid cycle

3. Oxidation of Isocitrate to α-Ketoglutarate and CO2 In the next step, isocitrate dehydrogenase catalyzes oxidative decarboxylation of isocitrate to form α-ketoglutarate. Mn 2+ in the active site interacts with the carbonyl group of the intermediate oxalosuccinate, which is formed transiently but does not leave the binding site until decarboxylation converts it to α-ketoglutarate. Mn 2+ also stabilizes the enol formed transiently by decarboxylation. There are two different forms of isocitrate dehydrogenase in all cells, one requiring NAD+ as electron acceptor and the other requiring NADP + .

The overall reactions are otherwise identical. In eukaryotic cells, the NADdependent enzyme occurs in the mitochondrial matrix and serves in the citric acid cycle. The main function of the NADP-dependent enzyme, found in both the mitochondrial matrix and the cytosol, may be the generation of NADPH, which is essential for reductive anabolic pathways such as fatty acid and sterol synthesis.

Reaction involve in citric acid cycle
FIGURE DEPICTING :- Iron-sulfur center in aconitase

4. Oxidation of α-Ketoglutarate to Succinyl-CoA and CO2 The next step is another oxidative decarboxylation, in which α-ketoglutarate is converted to succinyl-CoA and CO2 by the action of the α-ketoglutarate dehydrogenase complex; NAD+ serves as electron acceptor and CoA as the carrier of the succinyl group. The energy of oxidation of α-ketoglutarate is conserved in the formation of the thioester bond of succinyl-CoA

Reaction involve in citric acid cycle

5. Conversion of Succinyl-CoA to Succinate Succinyl-CoA, like acetylCoA, has a thioester bond with a strongly negative standard free energy of hydrolysis (ΔG′° ≈ −36 kJ/mol). In the next step of the citric acid cycle, energy released in the breakage of this bond is used to drive the synthesis of a phosphoanhydride bond in either GTP or ATP, with a net ΔG′° of only −2.9 kJ/mol. Succinate is formed in the process.

Reaction involve in citric acid cycle

6. Oxidation of Succinate to Fumarate The succinate formed from succinyl-CoA is oxidized to fumarate by the flavoprotein succinate dehydrogenase

Reaction involve in citric acid cycle

7. Hydration of Fumarate to Malate The reversible hydration of fumarate to L-malate is catalyzed by fumarase (formally, fumarate hydratase). The transition state in this reaction is a carbanion

Reaction involve in citric acid cycle

8 Oxidation of Malate to Oxaloacetate In the last reaction of the citric acid cycle, L-malate dehydrogenase catalyzes the oxidation of L-malate to oxaloacetate, coupled to the reduction of NAD+ to NADH.

Reaction involve in citric acid cycle


  • Although the citric acid cycle directly generates only one ATP per turn (in the conversion of succinyl-CoA to succinate), the four oxidation steps in the cycle provide a large flow of electrons into the respiratory chain via NADH and FADH2 and thus lead to formation of a large number of ATP molecules during oxidative phosphorylation.
  •  Production of two molecules of pyruvate from one molecule of glucose in glycolysis yields 2 ATP and 2 NADH.
  •  In oxidative phosphorylation, passage of two electrons from NADH to O2 drives the formation of about 2.5 ATP, and passage of two electrons from FADH2 to O2 yields about 1.5 ATP.
  • This stoichiometry allows us to calculate the overall yield of ATP from the complete oxidation of glucose. When both pyruvate molecules are oxidized to 6 CO2 via the pyruvate dehydrogenase complex and the citric acid cycle, and the electrons are transferred to O2 via oxidative phosphorylation, as many as 32 ATP are obtained per glucose. In round numbers, this represents the conservation of 32 × 30.5 kJ/mol = 976 kJ/mol, or 34% of the theoretical maximum of about 2,840 kJ/mol available from the complete oxidation of glucose. These calculations employ the standard free-energy changes; when corrected for the actual free energy required to form ATP within cells
FIGURE DEPICTING Products of one turn of citric acid cycle
FIGURE DEPICTING :- Products of one turn of citric acid cycle


  • The citric acid cycle (Krebs cycle, TCA cycle) is a nearly universal central catabolic pathway in which compounds derived from the breakdown of carbohydrates, fats, and proteins are oxidized to CO2 , with most of the energy of oxidation temporarily held in the electron carriers FADH2 and NADH. During aerobic metabolism, these electrons are transferred to O2 and the energy of electron flow is trapped as ATP.
  • Acetyl-CoA enters the citric acid cycle (in the mitochondria of eukaryotes, the cytosol of bacteria) as citrate synthase catalyzes its condensation with oxaloacetate to form citrate.
  • In seven sequential reactions, including two decarboxylations, the citric acid cycle converts citrate to oxaloacetate and releases two CO2 . The pathway is cyclic in that the intermediates of the cycle are not used up; for each oxaloacetate consumed in the path, one is produced.
  • For each acetyl-CoA oxidized by the citric acid cycle, the energy gain consists of three molecules of NADH, one FADH2 , and one nucleoside triphosphate (either ATP or GTP).
  • Besides acetyl-CoA, any compound that gives rise to a four- or five-carbon intermediate of the citric acid cycle—for example, the breakdown products of many amino acids—can be oxidized by the cycle.
  • The citric acid cycle is amphibolic, serving in both catabolism and anabolism; cycle intermediates can be drawn off and used as the starting material for a variety of biosynthetic products.
  • Vertebrates cannot synthesize glucose from acetate or from the fatty acids that give rise to acetyl-CoA.
  • When intermediates are shunted from the citric acid cycle to other pathways, they are replenished by several anaplerotic reactions, which produce four-carbon intermediates by carboxylation of three-carbon compounds; these reactions are catalyzed by pyruvate carboxylase, PEP carboxykinase, PEP carboxylase, and malic enzyme.
  • Enzymes that catalyze carboxylations commonly employ biotin to activate CO2 and to carry it to acceptors such as pyruvate or phosphoenolpyruvate.


  • Lehninger  principles of biochemistry seventh edition By  David L. Nelson and Michael M. Cox
  • voets and voets biochemistry 4th edition
  • Life sciences  fundamental and practices sixth edition, pathfinder publication By Pranav Kumar and Usha Mina
  • Essential cell biology (fourth edition) by ALBERTS, BRAY, HOPKIN, JOHNSON, LEWIS, RAFF, ROBERTS, WALTER

:- Article Written By Zahra Madraswala


Leave a Comment

Your email address will not be published.