Making energy available — cellular respiration

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There are several sets of reactions capable of breaking down glucose to store energy in ATP molecules. The three main ones are glycolysis, the Krebs cycle (or citric-acid cycle) and oxidative phosphorylation (the electron transport chain). Each set is called a pathway. Taken in sequence, they together are referred to as cellular respiration. This step-by-step process has two functions:

  1. Energy is released in small, controlled quantities; and
  2. the activation energy necessary for each step is small enough to be adequately lowered by an appropriate enzyme.
Cellular respiration, from Openstax College

The steps of cellular respiration, from Openstax College

There is also a pathway which can function for a short time without oxygen, anaerobic glycolysis, which explains why we can live for a very short period without breathing.

Glycolysis

The first pathway, glycolysis, is a series of reactions which take place in the cytoplasm of the cell and convert one glucose molecule into two pyruvate molecules. The overall equation is:1)Coenzyme electron carriers NAD and FAD were discussed in the last chapter.

glucose + 2 ATP + 2 NAD+ + 4 ADP + 2 Pi → 2 pyruvate + 4 ATP + 2 NADH + 2 H+

Each step of this pathway (and all the others) is brought about by an enzyme. As an illustration, here is a table indicating the steps of glycolysis in some detail.

Step

Input

Output

Enzyme

1

glucose + ATP

glucose-6-phosphate

hexokinase (or glucokinase, if in the liver)

2

glucose-6-phosphate

fructose-6-phosphate

glucose-6-phosphate polymerase

3

fructose-6-phosphate + ATP

fructose-1,6-biphosphate

phosphofructokinase

4

fructose-1,6-biphosphate (split)

glyceraldehyde-3-phosphate + dihydroxyacetone phosphate

aldelase

5

dihydroxyacetone phosphate

glyceraldehyde-3-phosphate

triosephosphate isomerase

6

2 glyceraldehyde-3-phosphate + 2 Pi + 2 NAD+

2 1,3-biphosphoglycerate + 2 NADH

glyceraldehyde-3-phosphate dehydrogenase

7

2 1,3-biphosphoglycerate + 2 ADP (dephosphorylation)

2 3-phosphoglycerate + 2 ATP

phosphoglycerate kinase

8

2 3-phosphoglycerate

2 2- phosphoglycerate

phosphoglycerate mutase

9

2 2- phosphoglycerate

2 phosphoenolpyruvate

enolase

10

2 phosphoenolpyruvate + 2 ADP + 2H+ (dephosphorylation)

2 pyruvate + 2 ATP

pyruvate kinase

The glycolysis pathway is used by almost every organism on Earth and so must have evolved very early in the history of life.

Notice that glycolysis does not require oxygen in order to proceed; it is an example of anaerobic respiration.

Ignoring those complex chemical names, the table shows that each step is chaperoned, so to speak, by a different enzyme. It also shows that although the overall process is to convert glucose to pyruvate, it requires two ATP to get started (input column) but produces four (output column), for a net gain of two ATP. This is (only) somewhat simpler in a diagram.

Glycolysis overview, from Openstax College

Glycolysis overview, from Openstax College

The Krebs cycle

Recall the overall view of cellular respiration:

C6H12O6 + 6O2 → 6H2O + 6CO2 + energy

i.e.,

glucose + oxygen → water + carbon dioxide + energy

The pyruvate molecules formed by glycolysis in the cytoplasm of the cell pass into the mitochondrial matrix, where the next step takes place. Pyruvate does not enter directly into the Krebs cycle. It is first converted by the enzyme pyruvate dehydrogenase as follows:

  • one C and two O atoms are removed as CO2 (decarboxylation);
  • the pyruvate is oxidized and its electrons serve to reduce NAD+ to NADH + H+;
  • finally, coenzyme-A is added to produce acetyl-CoA, which enters the Krebs cycle.

This is shown at the top of the following figure. This step has been referred to variously as the linking step, the grooming step or the bridging step, but I prefer the preparation step – or prep step.

Recall that fats from lipid digestion are converted to acetyl-CoA and this enters directly into the Krebs cycle, bypassing the glycolysis stage necessary for glucose.

The steps of the Krebs cycle are the following:

  1. The citrate synthase enzyme (always enzymes) joins the 2-carbon acetyl-CoA with a 4-carbon molecule of oxaloacetate tor form a 6-carbon molecule, citrate (or citric acid, hence one of the names of the cycle). As it traverses the cycle, this molecule will lose carbon atoms to become once again a 4-carbon oxaloacetate molecule, which can start the cycle over again with the addition of some acetyl-CoA. But along the way, Good Things will be produced.
  2. Another enzyme, aconitase, converts citrate into isocitrate by a “simple” rearrangement of its bonds.
  3. A further enzyme, isocitrate dehydrogenase, oxidizes isocitrate into 5-carbon α‑ketoglutarate.2)Gulp! Carbon is released as CO2 and electrons serve to reduce NAD+ to NADH + H+, with a gain of one NADH + H+.
  4. Yet another enzyme α‑ketoglutarate dehydrogenase converts α‑ketoglutarate into 4-carbon succynal CoA. Once again, CO2 is released and oxidized electrons reduce NAD+ to NADH + H+, for a gain of one more NADH + H+.
  5. The enzyme succynal CoA dehydrogenase converts succynal CoA into succinate. This reaction is exergonic and the released energy serves to form GTP, guanosine triphosphate, similar to ATP, which in turn furnishes energy to convert ADP into ATP. A different enzyme would produce ATP directly. That makes a gain of one ATP.
  6. Succinate dehydrogenase converts succinate into fumarate. The oxidized electrons are passed to the electron carrier FAD and is then reduced to FADH2, so there is a gain of one FADH2.
  7. Fumarase catalyzes the addition of a water molecule to fumarate to form malate.
  8. Malate dehydrogenase oxidizes malate back to oxaloacetate (back to step 1). The electrons reduce NAD+ to NADH + H+, so one more NADH + H+ is gained.

Note that one glucose molecule makes two pyruvates and so brings about two “turns” of the Krebs cycle. The different oxidation steps result in a gain of three NADH + H+, one FADH2 and one ATP.

To recapitulate:

The Krebs cycle, from OPenstax College

The Krebs cycle, from Openstax College

Chemiosmotic theory of oxidative phosphorylation – electron transport chain

The Krebs cycle may be a great advantage over glycolysis in terms of efficiency of ATP production, but the electron transport chain (ETC), where oxidative phosphorylation takes place, has them both beat hollow.3)When I first read about it, I was giggling with joy. Oxidative phosphorylation takes the electrons carried by NADH and FADH2 produced in the Krebs cycle and uses them to produce more ATP – much more.

Recall that NAD and FAD are electron carriers, picking up electrons as they are reduced to NADH and FADH2 as follows:

NAD+ + 2e + 2H+ → NADH + H+

and

FAD + 2e + 2H+ →FADH2

and leaving them off as they are oxidized to NAD+ and FAD in the opposite reactions (just invert the arrow).

In the process of oxidative phosphorylation, the NADH and FADH2 produced in the Krebs cycle, with the help of two coenzymes, transport electrons to the first of a series of four enzyme complexes in the inner mitochondrion membrane. Each complex is reduced as it receives an electron and oxidized as it passes it on to the next complex, somewhat like a bucket brigade. The electron is passed along because each successive complex is more electronegative (electron loving) than the preceding one. (One also says it goes from a complex of higher reducing potential to a lower one.) The energy produced by these redox reactions serves to pump protons across the inner mitochondrial membrane into the inter-membrane matrix.

At the end, the electrons are passed to oxygen which uses them to combine with the residual hydrogen to produce water. The left-over oxidized NAD+ and FAD can return to glycolysis and the Krebs cycle to pick up more electrons.

Here is the amazing part! The gradually built-up proton gradient becomes sufficiently strong to power an incredible-seeming object in the inner mitochondrion membrane, an enzyme called the ATP synthase. The protons passing through it actually cause it to turn and to power the combination of ADP with phosphate radicals to produce ATP in large quantities.

In brief, the overall function of the ETC Is to use the energy of reduced NADH and FADH2 to pump H+ across the inner mitochondrial membrane into the inter-membrane matrix so that the built-up electrical potential can power the synthesis of ATP by ATP synthase.

Look again at cellular respiration, but with the accounting data added as in the following figure The accounting is clear. Oxidative phosphorylation can produce up to 34 ATP molecules per glucose molecule entering the glycolysis pathway, for a total of 36 ATP in cellular respiration. This is where we get most of our energy.

ATP production in cellular respiration, from Openstax College

ATP production in cellular respiration, from Openstax College

Now for some more details on oxidative phosphorylation, one of the most amazing results of evolution — at least, for me. The whole process can be broken down into the electron transport chain (ETC), the four complexes on the left in the following figure, and ATP Synthase, the red complex on the right. The whole system exists in one form or another in all eukaryotes. It serves also in photosynthesis in plants to make the energy necessary for making glucose, but that is for a later chapter.

Oxidative phosphorylation (electron transport chain), author's own work

Oxidative phosphorylation (electron transport chain), author’s own work

Consider the electrons transported to the ETC by one NADH. Note that only complexes 1, 3 and 4 pump protons across the membrane.

  1. One NADH is oxidized by complex 1, NADH dehydrogenase, to give up 2 e and 2 H+. For each electron, one H+ is pumped across the inner membrane.
  2. Both e are transferred to ubiquinone (or coenzyme Q), a mobile transfer molecule. CoQ is hydrophobic and so can move about within the membrane, by which means it delivers the e to complex C3, cytochrome b-c1, bypassing complex 2. The e are passed one at a time to cytochrome c (another mobile transfer molecule). For each e accepted by cytochrome c, another H+ is pumped across the inner membrane.
  3. Cytochrome c moves back and forth along the inner side of the inner membrane and so delivers the e one at a time to complex C4, cytochrome c oxidase. When C4 has received four e, it can use them to reduce an oxygen molecule, which is therefore the last electron acceptor in the chain: The four e, four H+ and the O2 form two water molecules, while the remaining four H+ are pumped across the membrane into the inter-membrane space..
  4. As shown in the figure, FADH2 delivers its e directly to complex C2, which does not pump H+. The e are passed to CoQ and then follow the same path as the other e-.
  5. The electric potential brought about by the H+ inside the cell membrane drives them across the membrane through the ATP Synthase, as the membrane itself is impervious to H+. Three H+ are necessary to make one molecule of ATP.

Many such chains exist in every mitochondrion’s inner membrane, so multiple H+ pumps are running all the time, converting glucose and oxygen into energy stored in ATP.

Anaerobic respiration

As its name implies, oxidative phosphorylation is an aerobic pathway, requiring oxygen. The electron transport chain requires oxygen for the penultimate step of adding H+ ions to O2 to make water. In the absence of oxygen, this cannot take place. If oxygen is lacking, energy must be obtained differently.

Glycolysis requires no oxygen other than what is present in the initial reactants and therefore is a form of anaerobic respiration. In case of rapid, intense energy consumption by muscles, the need may be fulfilled by glycolysis, which is much faster than the Krebs cycle, but can furnish energy for only about 15 seconds. Then another energy source must be found.

There is another, much less efficient means of creating energy from pyruvate by anaerobic respiration – fermentation. One sort of fermentation exists within the body; the other, in yeasts (in bread or beer, for instance).

When oxygen is limited, lactic acid fermentation can take place.. Enzymes use electrons from NADH to reduce the pyruvate to lactic acid plus a small amount of energy. In particular, muscle cells can can do this in order to get fast (but relatively little) energy, releasing the lactic acid into the muscles. The oxidized NADH (now NAD+) can now be recycled to glycolysis.

In bread or beer, ethanol fermentation takes place. Enzymes bring about the decarboxylation of pyruvate to acetaldehyde, releasing CO2 which makes the bubbles in beer or bread. Then enzymes use electrons from NADH to reduce acetaldehyde to ethanol. The ethanol evaporates when cooking bread, but is carefully conserved in the making of beer or wine.

To see how this energy is used, proceed to the chapter on muscles.

Notes   [ + ]

1. Coenzyme electron carriers NAD and FAD were discussed in the last chapter.
2. Gulp!
3. When I first read about it, I was giggling with joy.