• No results found

CELLULAR RESPIRATION. Chapter 19 & 20. Biochemistry by Campbell and Farell (7 th Edition) By Prof M A Mogale


Academic year: 2021

Share "CELLULAR RESPIRATION. Chapter 19 & 20. Biochemistry by Campbell and Farell (7 th Edition) By Prof M A Mogale"


Loading.... (view fulltext now)

Full text



Chapter 19 & 20

Biochemistry by Campbell and Farell





1. Cellular respiration (energy capture)

 The enzymatic breakdown of food stuffs in the presence of oxygen to produce cellular energy (ATP)



 Conversion of energy-rich food stuffs into acetyl CoA.

 Different for carbohydrates (glycolysis), proteins and lipids (β-oxidation)

 Very little or no ATP is formed during this stage  Energy released is stored as NADH and FADH2


STAGE 2 (Citric acid cycle)

• Aerobic oxidation of acetyl CoA to CO2. in mitochondria (8 steps) • Common for carbohydrates, amino acids and fatty acids

• Acetyl CoA (2C) combine with oxaloacetate (4C) to form citric acid (6C) • Citric acid is gradually oxidized to regenerate oxaloacetate

• Energy released is stored as NADH and FADH2,


 The individual reactions of the citric acid cycle

 Phase 1 : Introduction and loss of two carbon atoms (Steps 1–4) Step 1: Condensation of OA and Acetyl CoA to form citrate and CoA

 The reaction is catalysed by the enzyme citrate synthase (condensing enzyme)

 A synthase is an enzyme that make a new covalent bond during the

reaction but does not require the direct input of ATP

 The reaction is an exergonic reaction ΔGo = -32.8 kJ/mol (energy


Step 2: Isomerization of Citrate to isocitrate

 Catalysed by the enzyme aconitase that utilize an Fe2+-- sulphur

cluster as a cofactor

 In this reaction a symmetrical (achiral) compound is converted to a

chiral compound (a secondary alcohol) yielding several possible isomers

 The reaction proceed via an enzyme


 Aconitase is the target site for the toxic action of fluoroacetate, a plant product that has been used as a pesticide

 Fluoroacetate is a suicide enzyme (trojan horse) inhibitor which is converted in to the toxic inhibitor fluorocitrate in the active side of the enzyme by the enzymes acetyl CoA synthetase and citrate synthase

 The action of fluoroacetate is similar to that of the legendary trojan horse


Step 3: Oxidative decarboxylation of isocitrate to alpha-ketoglutarate (First oxidation)

• The reaction is the first of the

two oxidative decarboxylation of the citric acid cycle

• The reaction which proceed in

two steps is catalysed by the enzyme isocitrate


• One molecule of NADH and one

molecule of CO2 are produced during this stage

• One mole of NADH will

eventually produce approxi-mately 2,5 mole of ATP when it donates its electron to O2

during oxidative phosphorylation


Step 4: Formation of Succinyl-CoA and CO2 from alpha-ketoglutarate (Second oxidation step)

• The reaction proceed in several steps and is catalysed by a

multienzyme system known as alpha-ketoglutarate dehydrogenase complex

• The alpha-ketoglutarate dehydrogenase enzyme complex utilizes thiamine pyrophosphate(TPP), FAD, lipoic acid and Mg2+ as enzyme


Step 4: Formation of Succinyl-CoA and CO2 from alpha-ketoglutarate (Second oxidation step)


• The reaction is similar to the one catalysed my pyruvate dehydrogenase complex that convert pyruvate into acetyl CoA.

• It was initially believed that the two carbon atoms lost as CO2 in step 3 and 4 of the CA cycle were the acetyl CoA carbons, however current experimental evidence (isotope tracing) show that this carbon atoms come from oxaloacetate

• The conversion of alpha-ketoglutarate to succinyl CoA is highly exergonic ( Go = -33.4 kJ/mol = -8.0 kcal/mol)


 Phase 2: Regeneration of oxaloacetate

 Two carbons have entered the CA cycle as acetyl CoA, and at this stage two have been lost as CO2

 In the remaining reactions, the four carbon intermediate ,

succinyl-CoA is converted to oxaloacetate in four steps (steps 5 -8), two of them involving dehydrogenation reactions


Step 5: Formation of Succinate (Substrate level phosphorylation)

 The reaction is catalysed by the enzyme succinyl-CoA synthetase

 A synthetase is an enzyme that creates a new covalent bond and requires a direct input of energy from a compound with a high phosphate transfer potential

 The free energy of hydrolysis of succinly-CoA to produce succinate is -33.4 kJ/mol


 Thus this reaction, cannot be empowered by hydrolysis of ATP to produce ADP + Pi

 The energy required for this reaction is provided by the

hydrolysis of the thioester bond of succiinyl-CoA to produce succinate and CoA-SH

 In this reaction the energy resulting from thiolysis (forward reaction) is also used to form GTP from GDP

 Note that the name of the enzyme describe the reverse

reaction in which GTP is hydrolysed to produce GDP thereby releasing energy to form the thoester bond

 This reaction step is referred to as substrate level

phosphorylation to distinguish it from formation of ATP coupled to oxidative phosphorylation


 In mammals, the GTP produced in the succinate synthetase reaction can exchange its terminal phosphoryl group with ADP to yield ATP via a reaction catalysed by the enzyme


Step 6: Flavin-Dependent Dehydrogenation

 Completion of the cycle involves conversion of the four-carbon

succinate to the four-carbon oxaloacetate

 The first of the remaining reactions is the FAD-dependent

dehydrogenation of two saturated carbon atoms to produced a

double bond catalysed by succinate dehydrogenase

 Succinate dehydrogenase is an inner mitochondrial

membrane-bound enzyme that is part of the electron transport chain involved in oxidative phosphorylation


 The reaction (the oxidation of an alkane to an alkene) is not sufficiently exergonic to reduce NAD, but it does yield

enough energy to reduce FAD

 In general, FAD is a better oxidising agent than NAD+ and

NADH is a better reducing agent than FADH2

 The action of succinate dehydrogenase is stereo selective, forming only the trans isomer, fumarate


Step 7: Hydration of the carbon-carbon double bond

 Fumarate is converted to L-malate by stereospecific addition of components of a water molecule across the double bond by the enzyme fumarate hydratase (fumarase)

 This reaction is highly exergonic in the foward direction and has an equilibrium constant of about 4


Step 8: Conversion of malate to oxaloacetate

 This is the final step of the citric acid cycle catalysed by the malate dehydrogenase enzyme

 The oxidation of alcohols to ketone or aldehyde groups are more energetically favourable and provide sufficient energy


 Stoichiometry of the citric acid cycle

• The cycle started when a two carbon acetyl-CoA combined

with a four carbon oxaloacetate to produce citrate

• Two carbon atoms were removed as carbon dioxide as citrate was further metabolized

• Four oxidation reactions occurred during the cycle, with NAD+

serving as an electron acceptor in three and FAD for the fourth • One high energy phosphate was generated by the reaction

catalysed by succinyl CoA synthetase

Acetyl-CoA + 3NAD


+ FAD + GDP + P


+ 2H





+ 3NADH +3H




+ + GTP + CoASH


 Energetics of the citric acid cycle

• Although some individual steps in the citric acid cycle may be endergonic the overall reaction of the cycle is exergonic (Table 19.2, C & F, Page 547) ΔGo = - 40 kJ/mol

• In terms of ATP production a total of 9 ATP molecules are produced per one turn of cycle

Pathway Substrate-Level Phosphorylation Oxidative Phosphorylation Total ATP

Krebs Cycle 1 ATP 3 NADH = 6 ATP

1 FADH2 = 2 ATP 9


 Regulation of the citric acid cycle

• Regulation of the citric acid cycle occurs both at the level of entry

of fuel in to the cycle and by control of key reactions within the cycle

• The most important factor controlling the citric acid cycle is the

relative intra-mitochondrial concentration of NAD+ and NADH

• Key sites for allosteric regulation are the reactions catalysed by


 Anaplerosis and Cateplerosis

• Most citric acid intermediates are used as biosynthetic intermediates and hence may be depleted

• Anaplerosis is a process whereby citric acid intermediates

used in biosynthe tic pathways are replenished

• Anaplerotic pathways may be classified into three groups, those replenishing oxaloacetate, those replenishing malate and those involving transamination of amino acids

• Cateplerosis is the oposite of anaplerosis i.e. pathways that

drain citric acid intermediates from the cycle for use in biosynthetic pathways


 Anaplerotic pathways that replenish oxaloacetate

• In mammals, the most important anaplerotic pathway for

generating oxaloacetate is the reversible ATP-dependent

carboxylation of pyruvate to give oxaloacetate

• This reaction is catalysed by the enzyme pyruvate


 Anaplerosis of malate

• The anaplerotic pathway for malate involves the malic

enzyme (malate dehydrogenase)

• This enzyme catalyses the reductive carboxylation of


 Role of citric acid cycle in lipogenesis and pyruvate and


 The Glyoxylate Cycle: An Anabolic Varient of the Citric

Acid Cycle

 One of the fundamental differences between plant and animal cells is that plants (and some microorganisms) can synthesize carbohydrates (glucose) from fat

 The conversion of fat into sugars is crucial in the development (germination) of seeds

 When seed germinate, triacylglycerols are broken down and converted into sugars, a process which provide the energy and raw materials for the growth of the plants

 Plants synthesize sugars from fats by means of the glyoxylate

cycle which is considered as an anabolic variant of the citric acid cycle


The Glyoxilate Cycle (Cont….)

• The glyoxylate cycle occurs in the glyoxysome, a specialized plant organelle that caries out both beta-cell oxidation of fatty acids to acetyl CoA and utilization of acetyl CoA in the

glyoxylate cycle

• In the glyoxylate cycle, acetyl CoA (provided by the beta- oxidation of fatty acids or by acetate thiokinase) reacts with oxaloacetate to give citrate, which is converted to isocitrate by the enzyme aconitase

• At this point, the glyoxylate cycle diverges from the citric acid cycle

• The next reaction is catalysed by isocitrate lyase, which

cleaves isocitrate to glyoxylate and succinate

• Glyoxylate then accepts acetate form another cellular acetyl-CoA to produce malate in a reaction catalysed by malate


 The Link between the Citric Acid cycle and molecular


• The citric acid cycle does not operate under anaerobic conditions

• This is because of the fact that the citric acid cycle is regulated among other things, by the concentration of NADH produced by the cycle

• After being produced by the citric acid cycle, NADH (and FADH2) donates its electrons to molecular oxygen through the respiratory chain

• Thus, in the absence of oxygen NADH will accumulate and inhibit the citric acid cycle


Related documents

Battery Management System (BMS) is an advantage to monitor and control for any battery charging technology especially in Electric Vehicles (EVs). A few factors

The structure then dissipates the excess energy in the form of either plastic deformation in case of ductile materials or through various damage mechanisms in the case of

Crotonyl- CoA Methionine cycle Serine One-carbon metabolism Lactate MGO UDP-GlcNAc Citrate Citrate Pyruvate Glucose Acetate Acetate Fumarate αKG Fatty acids Ketogenic amino acids

Study of the role of Alexandria Primary Health Care Study of the role of Alexandria Primary Health Care Program in the assessment of behaviour disorders of Program in the assessment

With Introductory pages, containg the First American Charter, the Fir^t American Pronunziamento, a Chart of the Official Symbols and Seals, Record Blanks of

Mackey brings the center a laparoscopic approach to liver and pancreas surgery not available at most area hospitals.. JOSHUA FORMAN, MD

An enrolled veteran may be eligible for some services that are not part of the Uniform Benefi ts package that includes: limited nursing home care; limited domiciliary care;