Aerobic Respiration

Aerobic respiration

It uses oxygen and completely oxidises the organic food mainly carbohydrate (Sugars) to carbon dioxide and water. It therefore, releases the entire energy available in glucose.

$C_6{H}_{12}{O}_6+6O_2\stackrel{\text{enzymes}}{\to }6C{O}_2+6H_{2}O+\text{energy}$(686 Kcal)

It is divided into two phases : Glycolysis, Aerobic oxidation of pyruvic acid.

Glycolysis / EMP pathway

(1) Discovery : It was given by Embden, Meyerhof and Parnas in 1930. It is the first stage of breakdown of glucose in the cell.

(2) Definition : Glycolysis ( Gr. glykys= sweet, sugar lysis= breaking) is a stepped process by which one molecule of glucose (6c) breaks into two molecules of pyruvic acid (3c).

(3) Site of occurrence : Glycolysis takes place in the cytoplasm and does not use oxygen. Thus, it is an anaerobic pathway. In fact, it occurs in both aerobic and anaerobic respiration.

(4) Inter conversions of sugars : Different forms of carbohydrate before entering in glycolysis get converted into simplest form like glucose, glucose 6-phosphate or fructose 6-phosphate. Then these sugars are metabolized into the glycolysis.

(5) Special features of glycolysis : The special features of glycolysis can be summarised as follows :

(i) Each molecule of glucose produces 2 molecules of pyruvic acid at the end of the glycolysis.

(ii) The net gain of ATP in this process is two ATP molecules (four ATPs are formed in glycolysis but two of them are used up in the reaction).

(iii) During the conversion of 1, 3-diphosphoglyceraldehyde into 1, 3-diphosphoglyceric acid one molecule of NADH₂ is formed. As each molecule of glucose yields two molecules of 1,3-diphosphoglyceric acid, hence each molecule of glucose forms 2 molecules of NADH₂.

(iv) During aerobic respiration (when oxygen is available) each NADH₂ forms 3 ATP and H₂O through electron transport system of mitochondria. In this process ½ O₂  molecule is utilized for the synthesis of each water molecule.

In this way during aerobic respiration there is additional gain of 6 ATP in glycolysis

\mathop {2ATP}\limits_{({\text{net}}\,{\text{gain}})}#xA0+ \mathop {6ATP}\limits_{{\text{(addition}}\,{\text{gain)}}}#xA0\to \mathop {8ATP}\limits_{{\text{(total}}\,{\text{net}}\,{\text{gain)}}}

(v) Reaction of glycolysis do not require oxygen and there is no output of CO₂.

(vi) Formation of 1, 3- diphosphoglyceraldehyde called non enzymatic phosphorylation.

(vii) Overall reaction of glycolysis represented by following reaction :

{C_6}{H_{12}}{O_6} \to \mathop {2{C_3}{H_4}{O_3}}\limits_{{\text{Pyruvate}}}#xA0+ 4H

Aerobic oxidation of pyruvic acid

(1) Oxidative decarboxylation of pyruvic acid : If sufficient O₂ is available, each 3-carbon pyruvate molecule (CH₃COCOOH) enters the mitochondrial matrix where its oxidation is completed by aerobic means. It is called gateway step or link reaction between glycolysis and Kreb’s cycle.

Decarboxylation and dehydration :

\mathop {C{H_3}CO.COOH}\limits_{{\text{(Pyruvic acid)}}}#xA0+ \mathop {CoA.SH + NAD}\limits_{{\text{(CoA)}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \mathop {\xrightarrow{{\begin{subarray}{l} #xA0{\text{Pyruvic}}\,{\text{dehydrogenase}} \\ #xA0{\text{multienzyme}}\,{\text{complex}} \end{subarray}} }}\limits_{\begin{subarray}{l} #xA0**TPP \\ #xA0**LAA \end{subarray}}#xA0\mathop {C{H_3}.CO.S.CoA}\limits_{{\text{(acetyl - S - CoA)}}}#xA0+ NAD.2H + C{O_2}$\underset{\text{(acetyl - S - CoA)}}{C{H}_3.CO.S.CoA}+NAD.2H+C{O}_2$ **TPP=Thiamine pyrophosphate **LAA=Lipoic acid amideAcetyl CoA is a common intermediate of carbohydrate and fat metabolism. Latter this acetyl CoA from both the sources enters Kreb’s cycle. This reaction is not a part of Kreb’s cycle. (2) Kreb’s cycle / TCA cycle / Citric acid cycle Discovery : This cycle has been named after the German biochemist in England Sir Hans Krebswho discovered it in 1937. He won Noble Prize for this work in 1953. Krebs cycle is also called the citric acid cycle after one of the participating compounds. It takes place in the mitochondrial matrix.

Summary of Kreb’s cycle (i) All the enzymes, reactants, intermediates and products of TCA cycle also are found in aqueous solution in the matrix, except the succinate dehydrogenase (mitochondrial marker enzyme) which is located in the inner mitochondrial membrane. (ii) Oxidation of one mole of acetyl CoA uses 4 molecules of water and releases one molecule of water. (iii) Liberates 2 molecules of carbon dioxide. (iv) Gives off 4 pairs of hydrogen atoms.(v) Produces one GTP/ ATP molecule during the formation of succinate. (vi) One mole of acetyl CoA gives 12 ATP during oxidation in Krebs cycle. (vii) Regenerates oxaloacetate used in last cycle for reuse.The above summary is for one molecule of acetyl coenzyme A. There are two acetyl coenzyme A molecules formed from one molecule of glucose by glycolysis and oxidative decarboxylation of pyruvate. Product formed during aerobic respiration by Glycolysis and Kreb’s cycle. 22 ATP produced by oxidation of NADH₂ and FADH₂ in Kreb’s cycle and 6 ATP comes from oxidative decarboxylation of pyruvic acid.These ATPs are not included neither in glycolysis nor kreb’s cycle.
Energy storage and energy transfer : In respiration energy released takes in the form of chemical energy, stored in a form called ATP. Energy transfer of biological oxidation hinges on the formation of labile high energy phosphate bonds of ATP. Nicotinamide adenine dinucleotide phosphate (NAD), Flavin adenine dinucleotide(FAD), Guanosine triphosphate are also the product of respiration and converted to ATP by electron transport system. Adenosine triphosphate There are several compounds like NAD, FAD, GTP and ATP are known as energy yielding compounds. The best known, and probably the most important of these are adenosine triphosphate (ATP). It serves as the energy currency of the cells. Structure of ATP : Adenosine triphosphate is a nucleotide consisting of three main constituents (i) A nitrogen contain purine base (Adenine). (ii) A five carbon sugar ribose.(ii) Three inorganic phosphate groups. The bonds attaching the last two phosphate to the rest of the molecule are high energy bonds (~) contain more than twice the energy of an average chemical bond. ATP hydrolysis : The energy is usually released from ATP by hydrolysing the terminal phosphate groups. $\text{Adenosine}\text{triphosphate }\stackrel{\text{hydrolysis}}{\to }$$\text{Adenosine}\text{diphosphate}(ADP)+Pi+7.3Kcal.....$

$\text{Adenosine}\text{diphosphate}\stackrel{\text{hydrolysis}}{\to }$$\text{Adenosine}\text{monophosphate(AMP)}+Pi+7.3Kcal.$Phosphorylation : The ATP hydrolysis reactions are reversible because ATP are synthesized from ADP, Pi and energy (take up for the bond formation).

The addition of phosphate group to ADP and AMP called phosphorylation. Energy required for the bond formation is equal to the energy released in hydrolysis. The significant role of ATP as an intermediate energy transfer compound. Major functions of ATP : ATP molecules receive the energy, which released in exergonic reactions and make this energy available for various endergonic reactions. Some of the important process in which ATP is utilized are as follows : (i) Synthesis of carbohydrates, proteins, fats, etc. (ii) Translocation of organic food. (iii) Absorption of organic and inorganic food. (iv) Protoplasmic streaming. (v) Growth. Nicotinamide adenine dinucleotide phosphate/ Nicotinamide adenine dinucleotide (NADP/NAD) : It is called universal hydrogen acceptor, produced during aerobic respiration (glycolysis+ Kreb’s cycle) and also in anaerobic respiration, work as coenzyme in ATP generation Via electron transport system. NADP have one additional phosphate. NAD plays a crucial role in dehydrogenation processes. Some dehydrogenases do not work with NAD, but react with NADP (Nicotinamide adenine dinucleotide phosphate). Formerly called Coenzyme II or Triphosphopyridine nucleotide = TPN Nicotinamide is a vitamin of B group. First NAD and NADP both functions as hydrogen acceptors. Later H ions and electrons (e^_) from these are transported through a chain of carriers and after being released at the end of a chain react with O₂ and from H₂O (see Electron Transport chain). During the release of 2 electron from 2H⁺ atoms from NAD. 2H and their reaction with O₂ to form water, 3 ATP molecules are synthesized. (3) Electron transport system : The electron transmitter system is also called electron transport chain (ETC), or cytochrome system (CS), asfive out of these nine carriers are cytochrome. It is the major source of cells energy, in the respiratory breakdown of simple carbohydrates intermediates like phosphoglyceraldehyde, pyruvic acid, isocitric acid, $\alpha -$ketoglutaric acid, succinic acid and malic acid are oxidised. The oxidation in all these brought about by the removal of a pair of hydrogen atoms (2H) from each of them. This final stage of respiration is carried out in ETS, located in the inner membrane of mitochondria (in prokaryotes the ETS is located in mesosomes of plasma membrane). The system consists of series of precisely arranged nine electron carriers (coenzyme) in the inner membrane of the mitochondrion, including the folds or cristae of this membrane. These nine electron-carriers function in a specific sequence and are : Nicotinamide adenine dinucleotide (NAD), Flavin mononucleotide (FMN), Flavin adenine dinucleotide (FAD),Co-enzyme-Q or ubiquinone, Cytochrome-b, Cytochrome-c₁, Cytochrome-c, Cytochrome-a and Cytochrome-a₃, The first carrier in the chain is a flavoprotein which is reduced by NADH₂. Coenzyme passes these electron to the cytochromes arranged in the sequence of b-c₁-c-a-a₃, finally pass the electron to molecular oxygen. In this transport, the electrons tend to flow from electro-negative to electro-positive system, so there is a decrease in free energy and some energy is released so amount of energy with the electrons goes on decreasing. During electron-transfer, the electron-donor gets oxidised, while electron-acceptor gets reduced so these transfers involve redox-reaction and are catalysed by enzymes, called reductases. Oxidation and reduction are complimentary. This oxidation-reductiion reaction over the ETC is called biological oxidation. $\text{Electron}-\text{donor}\stackrel{{\text{e}}^{-}}{\to }\text{electron}-\text{acceptor}$ here, electron-donor and electron -acceptor form redox pair. During the electron transfers, the energy released at some steps is so high that ATP is formed by the phosphorylation of ADP in the presence of enzyme ATP synthetase present in the head of F₁-particles present on the mitochondrial crista. This process of ATP synthesis during oxidation of coenzyme is called oxidative phosphorylation, so ETS is also called oxidative phosphorylation pathways. $\text{ADP}+\text{Pi}\stackrel{\text{ATP Synthetase}}{\to }\text{ATP}$ From the cytochrome a₃, two electrons are received by oxygen atom which also receives two proton (H⁺) from the mitochondrial matrix to form water molecule. So the final acceptor electrons is oxygen.So the reaction $H_2+\frac{1}{2}{O}_2\to H_{2}O$ (called metabolic water) is made to occur in many steps through ETC, so the most of the energy can be derived into a storage and usable form. (i) Two route systems of ETC : The pairs of hydrogen atoms from respiratory intermediates are received either by NAD⁺ or FAD coenzymes which becomes reduced to NADH₂ and FADH₂. These reduced coenzyme pass the electrons on to ETC. Thus, regeneration of NAD⁺ or FAD takes place in ETC. There are two routes ETC : (a) Route 1 : NADH₂passes their electrons to Co-Q through FAD . In route 1 FAD is the first electron carrier. 3 ATP molecules are produced during the transfer of electron on following steps : NAD to FAD Cyt b to Cyt c₁ and Cyt a to Cyt a₃ (b) Route 2 : FADH₂ passes their electron directly to FAD. 2 ATP molecules are produced during the transfer of electron on following steps. Cyt b to Cyt c₁ and Cyt a to Cyt a₃ (ii) Structure of mitochondria in relation to oxidative function : On inner side of mitochondria elementary particles or F₀-F₁ complex of ATPase complex or elementary particle (oxysomes) are found. Previously it was considered that elementary particles contain all the enzyme of oxidative phosphorylation and electron transport chain. Component of electron transport chain are located in the inner membrane in the form of respiratory chain complexes. For complexes following theories are given : (a) Four complex theory : According to Devid green electron transport chain contains 4 complexes- Complex I : Comprises NADH dehydrogenase and its 6 Iron Sulphur centers (Fe-S). Complex II : Consists of Succinate dehydrogenase and its 3 Iron Sulphur centers. Complex III : Consists of cytochrome b and c, and a specific Iron-Sulphur centers. Complex IV : Comprises cytochromes a and a₃. (b) Five complex theory : According to Hatefi, (1976), Complex I to Complex IV are related to the electron transport.$\bullet$ Complex V related to mainly with ATP synthesis, so it is called ATPase /ATP syntheses complex.$\bullet$ The head piece (F₁) of the oxysome consists of 5 hydrophobic subunits ($\alpha ,\beta ,\gamma ,\delta ,\epsilon$), which are responsible for ATPase functioning.$\bullet$ The stalk (F₀) contain F₅ (oligomycin sensitivity conferring protein) i.e., CSCP and F₆. F₀ are related to the proton channel and embeded fully in thickness of inner mitochondrial membrane.$\bullet$ Five complex i.e., I, II, III, IV, V, have been isolated from mitochondrial membrane by chemical treatment.Complex I : NADH/NADPH : CoQ reductase Complex II : Succinate : CoQ reductase Complex III : Reduced CoQ (CoQH₂) : cytochrome C reductase Complex IV : Cytochrome C oxidase Complex V : ATPase Cytochrome C and Q are mobile components of the respiratory chain. (iii) Oxidative phosphorylation : The process of ATP synthesis during oxidation of reduced coenzymes in ETC is called oxidative phosphorylation. Peter Mitchell (1961) proposed the chemiosmotic mechanismof ATP synthesis (Noble prize in 1978) which states that ATP synthesis occurs due to H⁺- flow through a membrane. It involves two steps : (a) Development of proton gradient. At each step of ETC, the electron- acceptor has a higher electron -affinity than the electron-donor. The energy from electron-transport is used to move the proton (H⁺) from the mitochondrial matrix to inter-membranous or outer chamber. Three pairs of protons are pushed to outer chamber during the movement of electrons along route I while two pairs of protons are moved to outer chamber during the movement of electrons along route-II. This generates a pH-gradient across the inner mitochondrial membrane with protons(H⁺) concentration higher in the outer chamber than in the mitochondrial matrix. This difference in H⁺ concentration across the inner mitochondrial membrane is called proton-gradient($\Delta$pH). Due to proton gradient, an electrical potential ($\Delta$$\psi$) is developed across the inner mitochondrial membrane as the matrix is now electronegative with respect to the intermembranous (outer) chamber. The proton gradient and membrane electric potential collectively called proton motive force.

(b) Proton flow : Due to proton-gradient, the protons returns to the matrix while passing through proton channel of F₀-F₁ ATPase. This proton gradient activates the enzyme ATP synthetase or F₀ - F₁ ATPase.

ATP synthetase controls the formation of ATP from ADP and inorganic phosphate in the presence of energy.

(iv) Role of shuttle system in energy production : Glycolysis occurs in the cytoplasm outside the mitochondrion in which 2NADH₂ molecules are produced but ETC is located along inner mitochondrial membrane, so NADH₂ of glycolysis must enter inside the mitochondrion to release energy. But the inner mitochondrial membrane is impermeable to NADH₂. In mitochondrial membrane, there are 2 shuttle-system, each formed of carrier-molecule.

These shuttle systems are :

(a) Malate-Aspartate shuttle : When this electron shuttle occurs, transfer of electrons from NADPH₂ in cytoplasm occurs to NAD inside the mitochondria. This is more efficient and result in production of 38 ATP molecules. 

(b) Glycerol-Phosphate shuttle : In this shuttle transfer of electrons from NADH₂ in cytoplasm occurs to FAD inside mitochondria and it results in production of 36 ATP molecules. It is less efficient and results in the reduction of FAD inside the mitochondrion.

Which shuttle predominates depends on the particular species and tissues envolved, for example : 38 ATP are formed in kidney, heart and liver cell while 36 ATP molecules are formed in muscle cells and nerve cells. In these cells glycerol-phosphate shuttle is predominant and 2 ATP formed from NADH_2.

Other pathways of glucose oxidation 

(1) Entner-Doudoroff pathway

Discovery : Entner-Doudoroff path discovered by Entner & Doudoroff. This pathway is also called glycolysis of bacteria.

Certain bacteria such as Pseudomonas sacchorophila, P. fluorescens, P. lindeneri and P. averoginosa lack phosphofructokinase enzyme. They can not degrade glucose by glycolytic process.

(2) Pentose phosphate pathway

(i) Discovery : It is also called as Hexose monophosphate (HMP) shunt or Warburg Dickens pathway or direct oxidation pathway. It provides as alternative pathway for breakdown of glucose which is independent of EMP pathway (glycolysis) and Krebs cycle. Its existence was suggested for the first time by Warburg et al. (1935) and Dickens (1938). Most of the reaction of this cycle were described by Horecker et al. (1951) and Racker (1954).

(ii) Occurrence : Pentose phosphate pathway that exists in many organisms. This pathway takes place in the cytoplasmਊnd requires oxygen for its entire operation.

(iii) Description : There are two types of evidences is support of the existence of such an alternative pathway-works on the inhibiting action of malonic acid on the Krebs cycle and studies with the radioactive (C¹⁴).

Twelve molecules of NADH₂ formed in the reaction can be oxidised back to 12 NADP with the help of the cytochrome system and oxygen of the air.

$12\text{ NADP}{\text{H}}_{\text{2}}+6O_2\underset{\text{System}}{\stackrel{\text{Cytochrome}}{\to }}\text{12}{\text{H}}_{\text{2}}\text{O}+\text{12NADP}$

In this electron transfer process, 36 molecules of ATP are synthesized.

(iv) Significance of PPP

(a) It is the only pathway of carbohydrate oxidation that gives NADPH₂, Which is needed for synthetic action like synthesis of fatty acid (in adipose tissues) and amino acids (in liver).

(b) It synthesizes 3C-glyceraldehyde-3-P, 3C-dihydroxy acetone phosphate, 4C-erythrose-4-P, 5C-ribulose phosphate, 5C-xylulose phosphate, 5C-ribose phosphate, 6 C-Fructose 6-phosphate, 7C-sedoheptulose-7-phosphate.

(c) It is the major pathway by which necessary ribose and deoxyribose are supplied in the biosynthesis of nucleotidesਊnd nucleic acid.

(d) Erythrose 4 phosphate for the synthesis of lignin, oxine, anthocyanine and aromatic amino acid (phenylalanine, tyrosine, and tryptophan).

(e) Young growing tissues appears to use to the Krebs cycle as the predominant pathway for glucose oxidation, while aerial parts of the plants and other tissues seem to utilise the PPP as well as the Krebs cycle.

(f) It gives 6 CO₂, required for photosynthesis.

(g) Ribulose five phosphate is used in photosynthesis to produce RuBP which act as primary CO₂ acceptor in C₃ cycle.

(3) Cyanide resistant pathway : Cyanide-resistant respiration seems to be widespread in higher plant tissues. Cyanide prevents flow of electron from Cyt a₃ to oxygen, so called ETC inhibitor. In these plant tissues resistance is due to, a branch point in the ETS preceeding the highly cyanide-sensitive cytochromes. The tissues lacking this branch point, or alternate pathway and blockage of cytochromes by cyanide, inhibits the electron flow.

Significance

(i) The role of alternative pathway is that it may provide a means for the continued oxidation of NADH and operation of the tricarboxylic acid cycle, even through ATP may not be sufficiently drained off.

(ii) It is significant in respiratory climateric of ripening fruits and leads to the production of hydrogen peroxide and super oxide, which in turn enhances the oxidation and breakdown of membranes.

(iii) Necessary activities in the ripening process because peroxides are necessary for ethylene biosynthesis.