The following points highlight the four major parts of respiration of starch or sugar. The parts are: 1. Glycolysis 2. Krebs or Citric Acid Cycle 3. Anaerobic Oxidation of Pyruvic Acid 4. Fate of Pyruvate under Aerobic Conditions.
Part # 1. Glycolysis:
The reactions of the EMP pathway of glycolysis are given in Fig. 17-2A, B; 17-3. The process may be divided into two steps: first the conversion of glucose into fructose-1, 6-bisphosphate; second, the splitting of the latter compound into two three-carbon compounds which are converted to pyruvic acid. As will be observed three reactions occur in the conversion of glucose to fructose-1, 6-bisphosphate.
Process of glycolysis has two main steps:
In this step glucose is phosphorylated with ATP in the presence of the enzyme hexokinase to produce glucose 6-phosphate and ADP. Then glucose 6-phosphate is converted to its isomer fructose 6-phosphate in the presence of phosphoglucoisomerase enzyme. Fructose 6-phosphate in the presence of ATP molecule and the enzyme phosphohexokinase forms fructose 1, 6-bisphosphate and ATP.
In this step splitting of fructose 1, 6-bisphosphate into two three-carbon compounds, 3-phosphoglyceraldehyde and dihydroxyacetone phosphate in the presence of the enzyme aldolase occurs. These two are interconverted through the enzyme phosphotrioseisomerase.
The fate of these three C-compounds is as follows: 3-phosphoglyceraldehyde, in the presence of NAD+, which acts as a hydrogen acceptor, and inorganic phosphate as well as phosphoglyceraldehyde dehydrogenase, is converted into 1, 3-bisphospho-glyceric acid and NAD+ is reduced to NADH + H+.
Then, 1-3-bisphosphogly- eerie acid is transformed into 3-phosphoglyceric acid when one phosphate group is lost to ADP to form ATP. Enzyme phosphoglyceryl kinase mediates this transformation. Thus one ATP is produced.
The resulting 3-phosphoglyceric acid (PGA) is transformed to 2-phosphoglyceric acid in the presence of the enzyme phosphoglycerylmutase. Then 2-phosphoglyceric acid is converted into 2-phosphoenolpyruvic acid and water is released in the presence of the enzyme enolase.
Dihydroxyacetone phosphate and glyceraldehyde-3-phosphate (the two trioses) are interconvertible and exist in equilibrium through the action of phosphotrioseisomerase.
2-phosphoenolpyruvic acid loses a phosphate group to ADP. Thus ATP and pyruvic acid are formed in the presence of the enzyme pyruvic kinase (Fig. 17-2, 17-2B).
It is possible to calculate the total number of ATP molecules produced when a molecule of glucose is metabolized through glycolysis (Fig. 17-3):
ATP produced directly = 4 molecules
ATP consumed = 4 molecules
Net gain = 2 molecules
From hydrogen produced and sent down the hydrogen produced and sent down the hydrogen transport system = 6 molecules
Glycolysis can take place both under anaerobic or aerobic conditions. In the following some of the pathways of glycolysis are tabulated (Table 17-1):
Glycolysis according to EMP pathway can be divided into two phases. These are preparatory and oxidative. From figure 17-2A, B, the account given previously in glycolysis can be divided into series of steps as well.
The preparatory phase has 4 while the oxidative phase has 5 steps. It may be worth mentioning that during the preparatory phase, glucose is broken down and low energy phosphorylation occurs. The energy is expanded during this phase. In the oxidative phase, high energy phosphate bonds are formed and thus energy is stored.
In the following a brief account of the specific steps is given:
I. Phosphorylation of Sugar:
II. Formation of Glyceraldehyde 3-phosphate (GAP):
III. Oxidation of Glyceraldehyde 3-phosphate (GAP):
IV. Formation of Pyruvic Acid:
In the following a brief account of the specific steps is given:
Essential features of glycolysis are summarized below:
(i) The initial compound e.g. glucose is a 6-carbon compound.
(ii) The end product is pyruvic acid (PA)-two molecules of 3-carbon compounds.
(iii) Under anaerobic conditions, PA is converted into lactic acid or ethyl alcohol.
(iv) PA is broken down into CO2 and H2O.
(v) There is a total of 4-phosphorylations during glycolysis.
(vi) During anaerobic conditions of glycolysis, 4 ATP molecules are formed while 2 are used.
(vii) Chemical bond energy from glucose is stored in ATP and NADH.
(viii) Glycolysis is in real practice dehydrogenation, comprising oxidation without the involvement of oxygen.
V. Oxidation of pyruvic acid:
Pyruvic acid (PA) produced through glycolysis does not enter the citric acid cycle directly. This 3-carbon compound is changed to acetic acid (2-carbon). One of the carbon is released as CO2 and the process is called decarboxylation. Acetic acid thus produced enters the mitochondria and combines with coenzyme A (CoA) to form acetyl-CoA.
This is an active (Fig. 17-4) acetate. The reactions are catalysed by a complex enzyme called pyruvate dehydrogenase (Fig. 17-4A). During the reaction, two hydrogen atoms are also released and these are accepted by NAD+ to form NADH + H+.
Two hydrogen atoms are passed down the hydrogen transport system and thus 3 ATP molecules are produced. Thus from two PA molecules, 6 ATP molecules are formed during oxidation.
Part # 2. Krebs or Citric Acid Cycle:
The fate of pyruvic acid depends upon the presence or absence of oxygen. Under aerobic conditions, PA is oxidized to acetyl-CoA which then undergoes a series of reactions. This is called Krebs cycle or citric acid or tricarboxylic acid cycle (TCA cycle). Each cycle of reactions produces one molecule of acetic acid i.e. acetyl-CoA into CO2 and water (Fig. 17-5 A).
Pyruvic acid can be used in several metabolic pathways in a cell. Some of these are given below:
Part # 3. Anaerobic Oxidation of Pyruvic Acid:
When oxygen is absent to act as ultimate electron (H) acceptor, the reactions of the electron transmitter system stop and pyruvic acid is not converted into CoA. Under such situations, pyruvic acid acts as hydrogen acceptor and it accepts H+ from NADH2 to produce lactic acid and NAD in animal muscles in the presence of lactic acid dehydrogenase (LDH). NAD is again ready to accept hydrogen from 3 phosphoglyceraldehyde.
In the yeast and higher plants, pyruvic acid is converted to CH3. CHO in the presence of pyruvic decarboxylase removing a CO2 molecule. The CH3CHO then accepts H2 from NADH2, alcohol dehydrogenase mediates the reaction and NAD and C2 H5 OH are produced.
During the conversion of pyruvic acid to lactic acid/alcohol no ATP molecules are produced.
Part # 4. Fate of Pyruvate under Aerobic Conditions:
(i) Formation of Acetyl CoA:
Pyruvic acid is decarboxylatedoxidatively and combines with coenzyme A (CoA.SH) to produce acetyl-CoA.
An enzyme complex called pyruvic dehydrogenase carries on the reaction:
Pyruvic dehydrogenase complex comprises the following enzymes decarboxylase transacetylase:
(ii) Condensation of Acetyl-CoA and Oxaloacetate:
Acetyl CoA is a two carbon compound and connects glycolytic pathway with Krebs cycle. As evidenced from the above equation, here acetyl-CoA combines with oxaloacetate in the presence of water and the enzyme citrate synthetase. Thus a six carbon compound called citric acid is produced and the reduced coenzyme is released in the process.
(iii) Conversion of Citric Acid into Isocitric Acid:
In this step citric acid is transformed to its isomer called isocitric acid through aconitase. This enzyme has iron as activator. There is also formation of an intermediate product called cis-aconitic acid.
(iv) Formation of α-ketoglutaric Acid:
In this step isocitric acid is oxidized or dehydrogenated to oxalosuccinic acid with the help of isocitrate dehydrogenase and Mn2+. Subsequently oxalo—succinic acid is decarboxylated to α-ketoglutaric acid. In fact it is an important intermediate compound of Krebs cycle and is also involved in the synthesis of amino acids.
(v) Oxidation of a-ketoglutaric Acid:
Through series of reactions α-ketoglutaric acid is decarboxylated and dehydrogenated by a complex enzyme called α-ketoglutaric dehydrogenase. This enzyme needs lipoic acid and TPP and also CoA is used. There is formation of succinyl CoA. The latter substance is acted upon by succinyl kinase and hence succinic acid is produced. This reaction involves formation of ATP and the reaction is exothermic.
(vi) Regeneration of Oxaloacetic Acid:
Succinic acid undergoes dehydrogenation to fumaric acid with the help of FAD. Fumaric acid gains a molecule of water and produces malic acid. Malic acid is then converted to oxaloacetic acid through dehydrogenation by NAD+.
Oxaloacetic acid produced can accept another molecule of acetyl-CoA and the whole cycle is repeated. As will be observed that in the oxidation of pyruvic acid, five reduced coenzymes and a molecule of ATP are produced.
There are six oxidation steps, one in the oxidation of PGAL to PGA in glycolysis, one during the formation of acetyl CoA and 4 in the Krebs cycle. In these steps, 6 pairs of H+ and 6 pairs of electrons are removed from the intermediates of the cycle. Six pairs of electrons and H+ reduce on NADP, 4 NAD and FAD molecules. They require 6 atoms of oxygen to be oxidized to water.
Three H2Omolecules are consumed as under:
—Condensation of oxaloacetic acid and acetyl-CoA
—Oxidation of α-ketoglutaric acid
—Hydration of fumaric acid to malic acid.
Three CO2 molecules were evolved from:
—The formation of acetyl oxalo-succinic acid
—Decarboxylation of oxalo-succinic acid
—Dehydrogenation of α-ketoglutaric acid
To sum up:
6 H2 + 3 CO2 + 6 O → 6 H2 O + 3 CO2
After deducting 3 molecules of water consumed in the Krebs cycle one gets the following result:
⅟2 glucose + 3O2 → 3 H2 O + 3 CO2
Doubling the equation we get the equation for the aerobic oxidation of glucose molecule:
C6 H12 O6 + 6 O2 6 H2 O → 6CO2 + 12 H2 O + Energy
It may be noted that 12 electron pairs (H+) arise in glucose oxidation. This molecule does not contain 24 hydrogen atoms. The additional twelve electrons needed arise from the hydrogen atoms of water which are consumed at various steps in the Krebs cycle (Fig. 17-6).
The important features of the citric acid cycle are summarized below:
(i) For the citric acid cycle Acetyl-CoA is the starting point.
(ii) Acetyl-CoA is produced from the metabolism of carbohydrates (pyruvic acid), fats (fatty acids) and proteins (certain amino acids).
(iii) Oxaloacetic acid is normally present in the cell and it reacts with the acetyl-CoA and H2O to form citric acid.
(iv) Citric acid undergoes series of break down steps to form oxaloacetic acid and the latter reenters the cycle.
(v) In each cycle two carbon atoms are released as CO2 and most of the CO2 is lost as a waste product.
(vi) Four dehydrogenation reactions occur in each cycle, thus, releasing four pairs of hydrogen atoms. Of these three pairs are accepted by NAD+ to form NADH+H+. Each pair of hydrogen generates 3 ATP molecules as it is transferred through the hydrogen transport system.
Thus 9 ATP molecules are generated by hydrogen accepted by NAD+. One pair of hydrogen is accepted by FAD+ to form FADH2. These hydrogens yield 2 ATP molecules. It may be mentioned that in each citric acid cycle 11 molecules of ATP are generated through the hydrogen transport system.
(vii) One ATP molecule is generated directly at the substrate level in each cycle.