The energy currency of life

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Biologists refer that 'adenosine triphosphate (ATP) is the energy currency of life.'1 The production of ATP is essential for the continual working of a cell, and the principle process which generates ATP is aerobic respiration. This process is a catabolic one, in which organic fuel (primarily glucose) is converted into ATP, using oxygen. The equation for aerobic respiration is as follows:

C6H12O6 + 6O2 à 38 ATP + 6CO2 + 6H2O. G=-2880 kJ/mol

From the equation above, one molecule of glucose provides 38 molecules of ATP. Generation of ATP is fundamental as many cellular processes require energy in this form. Such processes include muscular contraction, protein synthesis and various active transport systems. If a cell is more adapted for respiration, more ATP can be produced for these essential processes. Mitochondria are the main site of ATP production, and are most frequent in cells that have high ATP demands. For example cardiac muscle cells contain many mitochondria, as cardiac muscle is required to continually contract to pump blood around the body. Mitochondria have several adaptations for aerobic respiration, which make the process more effective, and in turn generating a higher yield of ATP. Aerobic respiration consists of five main stages: glycolysis, the link reaction, the Krebs cycle, oxidative phosphorylation and ATP production. Mitochondria are involved in four out of these five stages, and therefore their adaptations make a significant difference to the effectiveness of aerobic respiration.2

Glycolysis

The first step of aerobic respiration is glycolysis, which is the catabolism of glucose to form more reactive pyruvate. The process does not require oxygen, occurs in the cytoplasm of a cell and proceeds in a series of stages. Each step is controlled by a different enzyme, and the product of one step becomes the substrate for the enzyme controlling the next. Firstly, under the action of the enzyme hexokinase, glucose is phosphorylated to make glucose phosphate. The inorganic phosphate and energy required for this conversion are supplied by ATP hydrolysis. The phosphate molecule is charged, enabling the glucose to become more reactive. In turn, an isomeric change occurs, converting the glucose phosphate to fructose phosphate, catalyzed by the enzyme phosphoglucose isomerase. Fructose phosphate is then phosphorylated to form fructose bisphosphate, under the action of phosphofructokinase. Like the phosphorylation in the first stage, it requires ATP hydrolysis to provide inorganic phosphate and energy. Fructose bisphosphate now undergoes lysis, where the 6-carbon compound is reversibly split into 3-carbon glyceraldehyde-3-phosphate and 3-carbon dihydroxyacetone phosphate, catalyzed by the enzyme aldolase. These two molecules are isomers and can be reversibly converted into one other, catalyzed by isomerase. The subsequent stage of glycolysis however, only requires glyceraldehyde-3-phosphate and therefore the net products from lysis will be two 3-carbon glyceraldehyde-3-phosphate molecules. Therefore in glycolysis so far, the cell has gained two glyceraldehydes-3-phosphate molecules, and lost two ATP molecules.2

The enzyme triose phosphate dehydrogenase now works with the coenzyme nicotinamide adenine dinucleotide (NAD+) to remove two hydrogen atoms from each glyceraldehyde-3-phosphate molecule. Consequently NAD+ undergoes reduction to become NADH, and glyceraldehyde-3-phosphate is converted to a compound called 1,3-biphosphoglycerate. Energy is released in this stage, which is used to phosphorylate the product. Through a series of conversions, phosphoenal pyruvate is made from 1,3-bisphosphoglycerate, and two molecules of ADP are phosphorylated to form two molecules of ATP. In the final step of glycolysis, phosphoenal pyruvate is converted to pyruvate, catalyzed by pyruvate kinase. This conversion also involves the phosphorylation of ADP, which produces a further two molecules of ATP. In glycolysis, four ATP molecules are made, but two are used, and therefore the net gain of ATP is two molecules (see Figure 1).

The boxes coloured red show the enzymes which catalyze specific stages. After glycolysis is complete, the cell has gained two reactive 3-carbon pyruvate molecule and two ATP molecules. Besides this, additional energy is stored in NADH, which will be used to produce ATP during oxidative phosphorylation.

The link reaction and the Krebs cycle

Pyruvate formed from glycolysis is moved by active transport, into the mitochondrial matrix; the site of the link reaction. The link reaction is the process that follows glycolysis, and involves the conversion of pyruvate into acetyl CoA. Pyruvate dehydrogenases work with the coenzyme NAD+, removing two hydrogen atoms from each pyruvate molecule. As a result of this, NAD+ is reduced to NADH. In addition, the pyruvate is decarboxylated by the enzyme pyruvate decarboxylase, thus forming a molecule of carbon dioxide. The coenzyme called Coenzyme A then binds and remains temporarily attached to the acetyl, forming acetyl CoA (see Figure 2). Coenzyme A helps to feed the acetyl into the next stage, the Krebs cycle for further oxidation. Mitochondria contain protein carrier molecules in their envelope in order to move pyruvate from the cytoplasm into their matrix. These carrier molecules are made up of globular protein and have a specific, complementary shape to pyruvate.

The Krebs cycle, also called the tricarboxylic acid cycle, is the oxidation of pyruvate. Occurring in the mitochondrial matrix, its main significance is to transfer the chemical energy of pyruvate to NAD+ and the coenzyme flavin adenine dinucleotide (FAD). In addition, a small amount of ATP is produced. Firstly, the two-carbon acetyl CoA from the link reaction, is fed into the cycle, and combines with four-carbon oxaloacetate, to produce six-carbon citrate. During the cycle, citrate is broken down by the processes of dehydrogenation and decarboxylation in order to recycle the oxaloacetate. The first step involves the conversion of citrate into its isomer, isocitrate, using the enzyme aconitase. The isocitrate generated is then oxidised by the enzyme isocitrate dehydrogenase to a five carbon compound alpha-ketoglutarate, which consequently reduces NAD+ and produces one molecule of carbon dioxide. Further oxidation then occurs, converting alpha-ketoglutarate to a molecule called succinyl CoA, catalyzed by alpha-ketoglutarate dehydrogenase. As a result of this, a further NADH molecule is formed, as well as a further molecule of carbon dioxide. In turn, succinyl CoA is converted into succinate, entailing the phosphorylation of ADP to generate ATP.3

The coenzyme FAD now oxidizes succinate forming the four-carbon molecule fumarate. By the addition of water, fumarate is subsequently converted into the four-carbon compound malate. Malate recycles the four-carbon oxaloacetate by undergoing oxidation. During this, NAD+ is reduced producing the third molecule of NADH. Overall, each Krebs cycle yields three NADH molecules, one FADH2 molecule, two carbon dioxide molecules, and one ATP molecule, shown in Figure 2 below:

The principal adaptation of mitochondria for these two processes is that their matrix contains all the required coenzymes and enzymes. In addition, mitochondria contain circular DNA and 70s ribosomes, enabling them to make their own proteins. The circular DNA molecule contains approximately '15,000-20,000 base pairs'3, and through the process of transcription, DNA is read to messenger RNA - mRNA. mRNA is translated on the surface of the 70s ribosomes, coding for proteins. Therefore mitochondria are able to make their own proteins, for example enzymes. Also, the inner mitochondrial membrane is abundant in globular proteins, which play a central role in the next stage of aerobic respiration: oxidative phosphorylation.

Oxidative phosphorylation and ATP production

The link reaction and Krebs cycle produce a total of eight NADH molecules and two FADH2 molecules, per glucose respired. Oxidative phosphorylation is the stage in which electrons are passed from these molecules to oxygen via the electron transport system. The electron transport system is a series of molecules, mostly globular proteins, situated within the inner mitochondrial membrane. The initial step entails dehydrogenase enzymes removing the hydrogen atoms carried by the reduced NAD and FAD molecules. These hydrogen atoms are then split into their protons and electrons. The electrons are passed down the electron transport system, through the electron carriers. The first of these carriers is flavoprotein, which is tightly bound to the prosthetic group flavin mononucleotide (FMN). Flavoprotein gains electrons from NADH, and as electrons are donated to an iron-suphur protein, it will become oxidised and the iron-sulphur protein will become reduced. In turn, electrons are transferred to a compound called ubiquinone. At this stage, electrons are also transferred from FADH2 to the electron transport chain. Ubiquinone is the only molecule of the electron transport that is not a protein. The electrons are then passed down a series of cytochromes, before being accepted by oxygen. The prosthetic groups of cytochromes are iron-containing heme groups, which are able to accept and donate electrons.2, 4

The electron transport chain does not directly make ATP; however it is coupled with a process called chemiosmosis, which does synthesise ATP. Chemiosmosis transfers the chemical energy from the electrons via proton pumping, in order to phosphorylate ADP to generate ATP. As electrons are passed from one carrier to the next, some of their energy is transferred to pump protons from the mitochondrial matrix into the intermembranal space, against their concentration gradient. This movement positively charges the intermembranal space, creating electrical potential energy.

ATP synthase molecules are embedded into the inner mitochondrial membrane, which allow the diffusion of protons back into the mitochondrial matrix. Protons diffuse down their electrical and concentration gradient, through the inner mitochondrial membrane. This movement transfers energy in order to phosphorylate ADP and in turn, produce ATP. Electrons that have passed down the electron transport chain are transferred to oxygen with protons which reduces the oxygen to produce the waste water of respiration. (See equation in figure 3 below:).

Mitochondria are adapted to this process for three main reasons. Firstly, the inner membrane is highly folded with infoldings called cristae. It is 'approximately five times longer than the outer mitochondrial membrane'3, increasing the surface area of the electron transport system. Thus a higher flow of electrons can occur; meaning more energy will be transferred to pump more protons into the intermembranal space. Consequently, a higher number of protons diffuse back into the mitochondrial matrix, increasing ATP generation. Secondly, ATP synthases exist within the inner membrane, allowing diffusion of protons back into the mitochondrial membrane and the phosphorylation of ATP. Thirdly the inner mitochondrial membrane contains a large proportion of protein, '75% protein by weight'3. The roles of intrinsic proteins are to act as electron carriers and proton pumps, and therefore they play an essential role in oxidative phosphorylation.

In conclusion, aerobic respiration is a complex process, requiring several stages in order to occur. Summarising the process, glucose is converted into pyruvate during glycolysis, in which ATP and NADH are also produced. Acetyl CoA is then formed from pyruvate during the link reaction in the mitochondrial matrix. As this occurs, hydrogen atoms are removed and carbon dioxide is formed. In turn, the Krebs cycle combines acetyl CoA with oxaloacetate, forming citrate. As oxaloacetate is recycled, NADH, FADH2, ATP and carbon dioxide are all formed. Finally, oxidative phosphorylation uses the chemical energy stored in NADH and FADH2 to generate ATP. Mitochondria are highly adapted to these processes, enabling aerobic respiration to be more effective, hence increasing the yield of ATP generated. As previously quoted, 'ATP is the energy currency of life'1; therefore, the process of aerobic respiration and the adaptations of mitochondria are indispensable for the survival of an individual.

References:

  1. http://hyperphysics.phy-astr.gsu.edu/HBASE/Biology/atp.html
  2. Campbell and Reece - Biology
  3. Becker - The world of a cell
  4. Norman Cohen - Cell structure, function and metabolism

Figures:

  1. http://library.thinkquest.org/C004535/media/kreb_cycle.gif
  2. http://library.thinkquest.org/C004535/media/kreb_cycle.gif
  3. http://stevebambas.com/images/AP%20220%20ox%20phosph.jpg

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