21.8 The Citric Acid Cycle

Copyright © 2010 Pearson 

Education, Inc. 

21.8 The Citric Acid Cycle 

• The carbon atoms from the first two stages of 

catabolism are carried into the third stage as 

acetyl groups bonded to coenzyme A. 

• Like the phosphoryl groups in ATP molecules, the 

acetyl groups in acetyl-SCoA molecules are 

readily removed in an energy-releasing hydrolysis 

reaction. 

Chapter Twenty One 1

• Oxidation of two carbons to give CO 2 and transfer 

of energy to reduced coenzymes occurs in the 

citric acid cycle, also known as the tricarboxylic 

acid cycle (TCA) or Krebs cycle (after Hans Krebs, 

who unraveled its complexities in 1937). 

► The citric acid cycle is a closed 

loop of reactions in which the 

product of the final step which has 

four carbon atoms, is the reactant 

in the first step. 




• The net result of the citric acid cycle is: 

– Production of four reduced coenzyme molecules, 

3 NADH and 1 FADH 2 

– Conversion of an acetyl group to two CO 2 

molecules 

– Production of one energy-rich molecule (GTP) 

• ADP acts as an allosteric activator for the 

enzyme for Step 3. NADH acts as an inhibitor 

of the enzyme for Step 3. 

• By such feedback mechanisms, the cycle is 

activated when energy is needed and 

inhibited when energy is in good supply. 

• The eight steps of the citric acid cycle are 

shown in greater detail on the next slide. 







21.9 The Electron-Transport Chain and 



ATP Production 

• At the conclusion of the citric acid cycle, the 

reduced coenzymes formed in the cycle are ready 

to donate their energy to making additional ATP 

• Hydrogen and electrons from NADH and FADH 2 

enter the electron-transport chain at enzyme 

complexes I and II, respectively. 

• The enzyme for Step 6 of the citric acid cycle is 

part of complex II. FADH 2 produced there does 

not leave complex II. Instead it is immediately 

oxidized there by reaction with coenzyme Q. 

• Following formation of the mobile coenzyme Q, 

reductions occur when electrons are transferred. 


► Coenzyme Q is also known as ubiquinone because of its widespread occurrence 

and because its ring structure with the two ketone groups is a quinone. 

• Electrons are passed from weaker to 

increasingly stronger oxidizing agents, with 

energy released at each transfer. 




Other important electron acceptors are various 

cytochromes, which are proteins that contain 

heme groups in which the iron cycles between 

Fe +2 and Fe +3 and proteins with iron–sulfur 

groups in which the iron also cycles between Fe +2 

and Fe +3 . 




H + ions are released for transport through the inner 

membrane at complexes I, III, and IV. Some of these 

ions come from the reduced coenzymes. 




• The H + concentration difference creates a potential 

energy difference across the two sides of the inner 

membrane (like the energy difference between water 

at the top and bottom of the waterfall). 

• The maintenance of this concentration gradient 

across the membrane is crucial—it is the mechanism 

by which energy for ATP formation is made available. 

• ATP synthase: The enzyme complex in the inner 

mitochondrial membrane at which H + ions cross the 

membrane and ATP is synthesized from ADP. 




• Oxidative phosphorylation: 

The synthesis of ATP from ADP 

using energy released in the 

electron transport chain. 

• Each of the enzyme complexes 

I–IV contains several electron 

carriers. 

• In the last step of the chain, 

electrons combine with 

oxygen that we breathe and 

H + ions from the surroundings 

to produce water. 




21.10 Harmful Oxygen By-Products 



and Antioxidant Vitamins 

• More than 90% of the oxygen we breathe is used in 

electron transport– ATP synthesis reactions. 

• In these and other oxygen-consuming redox 

reactions, the product may not be water, but one or 

more of three highly reactive species. 

• The superoxide ion, ·O 2 - , and the hydroxyl free 

radical, ·OH, can grab an electron from a bond in 

another molecule, which results in breaking that 

bond. The third oxygen by-product is hydrogen 

peroxide, H 2O 2 , a relatively strong oxidizing agent. 


Conditions that can enhance production of these 

three reactive oxygen species are represented in the 

drawing below. Some causes are environmental, such 

as exposure to smog or radiation. Others are 

physiological, including aging and inflammation. 




• Our protection against harmful oxygen species is 

provided by superoxide dismutase and catalase, 

which are among the fastest-acting enzymes. 

• These and other enzymes are active inside cells 

where oxygen by-products are constantly generated. 

It is estimated that 1 in 50 of the harmful oxygen 

species escapes destruction inside a cell. 




• Protection is also provided by the antioxidant 

vitamins E, C, and A , all of which disarm free radicals 

by bonding with them. 

• Vitamin E is fat-soluble, and its major function is to 

protect cell membranes from potential damage.

21.8 The Citric Acid Cycle

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