The process of cellular oxidation is essentially an energy transformation process; i which the energy stored in the food molecules am transformed into chemical energy of A (Adenosine triphosphate). Molecular oxygen is taken into the cell, the food molecules are, oxidized and energy, carbon dioxide and water are released.
The energy that is released durin the oxidation of the food is used to couple phosphoric acid and ADP (Adenosine diphosphate) to the energy rich ATP. All the three types of food, such as carbohydrates, proteins and lipids are oxidized inside the cell in essentially the same manner. The pathways of these three types of molecules converge on a point known as Krebs cycle.
The carbon dioxide produced inside cells diffuses out into blood and finally eliminated through the respiratory system. From the foregoing discussion, it is clear that ATP is the energy currency of the cell. However, it should be noted that ATP does not store energy, but rather transfers it to points in a cell requiring it. The process of cellular oxidation is represented by the following reactions:
1 ADP + H3P04 +ENERGY (7 Kcal) -*---------------- ► ATP + H20
Similarly, ATP is hydrolyzed to yield ADP, H3P04 and energy
2 ATP + H90 ------------------- ► ADP + H3P04 + ENERGY (7 Kcal)
In the two above mentioned reactions, note that 7 Kcal of energy is released or utilized per 1 mole, i.e. (6.023 X 1023) molecules of ATP synthesized or hydrolyzed and not just one molecule.
Cellular oxidation takes place in three steps: (1) Glycolysis, (2) Krebs / TCA / Citric acid cycle and (3) Electron transport system and oxidative phosphorylation.
Glycol sis (glycols', sugar; lyses, breakdown) :
It refers to the anaerobic (without oxygen) breakdown of monosaccharides, especially glucose to two molecules of pyruvic acid (pyruvate) with a concominant release of a relatively small amount of energy in the form of ATP. The complete glycolytic pathway was worked out by Gustav Embden, Otto Meyerhoff, Carl Nituberg and Jacob Parnas in 1940.
The glycolytic pathway is also known as Embden-Meyerhoff-Parnas (EMP) pathway in the honour of its discoverers. In this pathway, glucose breaks down into two molecules of a three carbon compound, pyruvate. It takes place in ten steps. The first five steps constitute the preparatory phase, while the next five the pay-off phase.
(a) Preparatory phase:
1-. Glucose is phosphorylated at C-6 to Glucose 6-phosphate.
2. Glucose 6-phosphate is isomerized to Fructose 6-phosphate.
3. Fructose 6-phosphate is phosphorylated at C-1 to Fructosel, 6 diphosphate
4. Fructosel, 6 diphosphate is split into a molecule each of Glyceraldehyde 3- phosphate and Dihydroxy ace tone phosphate.
5. Dihydroxyacetone phosphate is isomerized to Glyceraldehyde 3-phosphate.
(b) Pay-off phase:
1. Each molecule of Glyceraldehyde 3-phosphate is oxidized and phosphorylated by inorganic phosphate to 1, 3 diphosphoglycerate.
2. 1, 3 diphosphoglycerate is dephosphorylated to 3-phosphoglycerate. The phosphate group removed is tagged to ADP to result in one molecule of ATP.
3. 3-phosphoglycerate is isomerized to 2-phosphoglycerate.
4. 2-phosphoglycerate undergoes dehydration, by the removal of a molecule of water, to phosphoenolpyruvate.
5. Phosphoenolpyruvate is dephosporylated to pyruvate. The phosphate group removed is tagged to ADP to result in another molecule of ATP.
Two phases of glycolysis the steps are enumerated. The enzymes catalyzing the steps are: 1. Hexokinase; 2. Phosphohexose isomerase; 3. Phosphofructokinase; 4. Aldolase; 5. Triosephosphate isomerase; 6. Glyceraldehyde 3-phosphate dehydrogenase; 7. Phosphoglycerate kinase; 8. Phosphoglycerate mutase; 9. Enolase and 10. Pyruvate kinase.
Note that the steps 1 and 3, each use one molecule of ATP to form respect® phosphorylated intermediates. In steps 7 and 10,4 molecules of ATP (2 for each step) a I formed. Therefore, the net gain of ATP molecules in glycolysis is 4 - 2 = 2. This formatii of ATP at the level of glycolysis is known as substrate level phosphorylation.
The pyruvate produced in glycolysis can now proceed in two directions, subject toil availability of molecular oxygen. If oxygen is available (aerobic condition), pyruvate enters t! I Krebs cycle and is broken down into carbon dioxide and water. Conversely, in the absence I oxygen (anaerobic condition), pyruvate is converted into lactate in most cells and bacteria. TN hydrogen atoms derived from NADH and H+ are transferred to pyruvate to form L-lactate, h outlined in the reaction below. This step is known as lactate fermentation, which is acharacterisl feature of many anaerobic bacteria (lactic acid bacteria) and tissues deprived of oxygen, yeast, however, the anaerobic respiration ends up in ethanol and CO,. This is known as alcohol fermentation.
Pyruvate + NADH + H+ ________ L-Lactate + NAD+
When we sit with our arms or legs tightly folded, blood supply to the skeletal muscle is inhibited and hence, molecular oxygen. Under this circumstance, pyruval I forms L-Lactate in skeletal muscle cells. As soon as the bloods supply to the muscle i I restored, the accumulated lactate is degraded. This act causes a fatigue sensation, |
The cycle is named in the honour of Hans Krebs, who discovered varioi steps of the cycle. Krebs cycle is also known as Citric acid cycle or Tricarboxyllic ac I (TCA) cycle. In this cycle, pyruvate is completely oxidized to carbon dioxide by the removal p hydrogen atoms. The hydrogen atoms removed, reduce oxidized co-enzymes like FADai NAD+ into FADH2 and NADH, respectively. The process is catalyzed by several enzymes which are present in the mitochondrial matrix. Therefore, this process takes place INH mitochondrial matrix. Pyruvate undergoes a'series of simultaneous decarboxylation ai dehydrogenation (oxidation) reactions. Decarboxylation refers to the removal of carbon dioxii f and dehydrogenation to removal of hydrogen atoms from the substrates. The two reactionsa often combined together as oxidative decarboxylation. Most of the substrates of the cycle
Oxidized by oxidized coenzymes (FAD and NAD+) the oxidized coenzymes themselves are reduced to FADH2 and NADH and H+. These reduced coenzymes are important from the view point of ATP formation. In prokaryotic cells, having no mitochondria, the enzymes of this cycle are attached on the inner side of the plasma membrane.
The primary molecule entering the Krebs cycle is. a C-2 (two carbon compound), acetyl co-enzymeA. It transfers the acetyl group from the pyruvate to oxaloacetate (C-4 product of the Krebs cycle). Pyruvate-undergoes an oxidative decarboxylation with co-enzymeA (CoA- SH) to result in acetyl Co-enzymeA and CO, The hydrogen atoms removed from the pyruvate reduce NAD+ to NADH and H+. The enzyme catalyzing this reaction is a multienzyme complex known as pyruvate dehydrogenase. This reaction is the first step of the Krebs cycle. The entire cycle is depicted in.
Pyruvate + CoA-SH + NAD+ _______ ^ Acetyl CoA + NADH + H+
The other steps in the sequence are:
1. Acetyl CoA (C-2), formed in the previous step, reacts with oxaloacetate (C-4) and forms a tricarboxyllic acid, citrate (C-6). This is the first product of the Krebs cycle. Therefore, the cycle is also named as tricarboxyllic acid (TCA) cycle.
2. Citrate is isomerized to isocitrate (C-6).
3. Isocitrate undergoes an oxidative decarboxylation to form a-ketoglutarate (C-5) and COr A pair of hydrogen atoms is also removed, which reduce NAD+ to NADH and H+.
4. a-ketoglutarate undergoes another oxidative decarboxylation with a molecule of CoA-SH to result in succinyl co-enzymeA (C-4) and COr A pair of hydrogen atoms is removed, which reduce NAD+ to NADH and H+.
5. Succinyl co-enzymeA changes to succinate (C-4) with the regeneration of CoA- SH. A high energy triphosphate (GTP) is formed from GDP and inorganic phosphate. The energy of GTP is transferred to ATP.
GTP + ADP Mg2+ ATP + GDP
6. Succcinate is oxidized to fumarate (C-4). A pair of hydrogen atoms is remove which reduces the oxidized co-enzyme, FAD to FADHr
7. Fumarate is hydrated to L-malate (C-4).
8. L-malate is oxidized to oxaloacetate (C-4) by the the oxidized co-enzyme, NAD1 the oxidized co-enzyme itself is reduced to NADH and H+.
The replenished oxaloacetate again reacts with an acetyl co-enzymeA to start anothe cycle. Three molecules of C02 are produced per molecule of pyruvate oxidized in the Krebs cycle. Since glucose breaks down into two molecules of pyruvate, there will be two cycles peri molecule of glucose oxidized. This means that 2x3 = 6 molecules of C02 are produced pera molecule of glucose oxidized.
3 molecules of NADH and 1 molecule of FADH are also formed in one cycle. Besides, another molecule of NADH during the transition from pyruvate to acetyl co-enzymeA and 1 molecule of GTP in the cycle are also formed. Therefore, the total number of NADH molecules per molecule of glucose oxidized equals to 2 x 3 + 2 x 1 = 8 and FADH, to 2 x 1 = 2. Note that 2x1=2 molecules of NADH are also formed during glycolysis. These 2 molecules are changed to FADH,, before they enter into the energy transformation process.
It brings the total number of NADH and FADH, molecules entering into the energy transformation process to 8 and 2 + 2 = 4, respectively.
All hydrogen atoms carried by the co-enzymes break into protons and electrons'. The protons are funneled into the inter-membranal space of the mitochondrion, while the electrons to an inner membrane-bound system known as electron transport system (ETS) / respiratory chain.
Electron Transport System (ETS) / Respiratory Chain
The reduced co-enzymes such as NADH and FADH2 formed in the Krebs cycle and glycolysis establish a link between the Krebs cycle and ETS. They transfer their energy to ATP through a seies of oxidation-reduction reactions, collectively known as oxidative phosphorylation. The substrates in these reactions are co-enzymes and metalloproteins known as cytochromes, having the potential to be reduced and oxidized alternately.
The cytochromes are copper (Cu) and / or iron (Fe) containing proteins. Cu and Fe have variable valencies with Cu+I / Cu+2 and Fe+2 / Fe+3. They enjoy an advantage of undergoing reduction and oxidation alternately. Another advantage of this system is that the components are arranged in the order of their decreasing free energy and increasing oxidation-reduction potential.
Therefore, when the electrons flow across the series of the co-enzymes and cytochromes in a downhill manner, they release thermal energy. This energy is trapped by the system to form ATP from ADP and inorganic phosphate.
All the components of the ETS are bound to the inner mitochondrial membrane and are grouped into four complexes: I, II, III and IV. Complex I have an enzyme known as NADH reductase. The reduced enzyme transfers a pair of hydrogen atoms to the Co-enzyme Q and it is oxidized. Complex II has FAD bound succinate dehydrogenase. It also
Transfers- a pair of hydrogen atoms to Co-enzyme Q. Co-enzyme Q is a mobile carrier and the converging point for NADH and FADHr At this point, the hydrogen atoms split into protons and electrons. The protons are pumped into the inter-membranal space, while the electrons flow downhill through the cytochromes, cytochromes, b and c, constitute complex III and a and a3 complex IV. Complex IV is also known as cytochrome oxidase. There is a cytochrome c between complex III and IV. It shuttles between the two complexes and feeds the complex IV with electrons. Finally, the complex IV transfers the electrons to molecular oxygen and reduces it to oxygen radical. The protons pumped into the inter-membranal space flow back into the matrix through particles, attached to the inner mitochondrial membrane. While the protons flow back, they release thermal energy.
This energy is utilized for coupling inorganic phosphate to ADP forming ATP. The F, particle has an enzyme, ATPase that catalyzes the coupling reaction.
Here, note that the thermal energy produced during the downhill flow of electrons is not utilized to form ATP, but rather to pump protons to the intermembranal space. The protons finally react with the oxygen radicals to form water molecules.
For the complete oxidation of a mole of reduced NADH, 2.5 moles of ATP and for that of FADH2, moles of ATP are produced.
The ETS is inhibited by several inhibitors. The most noteworthy among these are cyanides and carbon monoxide. Everybody must have been aware with Potassium cyanide poisoning.
This agent directly inhibits the last complex of the ETS, cytochrome oxidase. It stops the flow of electrons to molecular oxygen and thus ceases respiration within minutes of its consumption. Carbon dioxide has an identical but less severe effect in comparison to cyanide.