9 Pathways That Harvest Chemical Energy. 9 Pathways That Harvest Chemical Energy 9.1 How Does Glucose Oxidation Release Chemical Energy? 9.2 What Are

9Pathways That Harvest

Chemical Energy

9 Pathways That Harvest Chemical Energy

9.1 How Does Glucose Oxidation Release Chemical Energy?

9.2 What Are the Aerobic Pathways of Glucose Metabolism?

9.3 How Does Oxidative Phosphorylation Form ATP?

9.4 How Is Energy Harvested from Glucose in the Absence of Oxygen?

9.5 How Are Metabolic Pathways Interrelated and Regulated?

9 Pathways that Harvest Chemical Energy

Human infants are born with a lot of “brown fat”—it has many mitochondria with iron-containing pigments. When brown fat is catabolized, its energy is released as heat, which helps keep the baby warm.

Opening Question:

Can brown fat in adults be a target for weight loss?


Fuels: molecules whose stored energy can be released for use.

In cells, energy from fuel molecules is used to make ATP.

Glucose is the most common fuel in cells.


Cells get energy from glucose by chemical oxidation in a series of metabolic pathways.

Five principles of metabolic pathways:

• Complex transformations occur in a series of separate reactions.

• Each reaction is catalyzed by a specific enzyme.


• Many metabolic pathways are similar in all organisms.

• In eukaryotes, metabolic pathways are compartmentalized in specific organelles.

• Key enzymes in each pathway can be inhibited or activated to alter the rate of the pathway.


Burning or metabolism of glucose:

C6H12O6 + 6 O2 →

6 CO2 + 6 H2O + free energy

An oxidation–reduction reaction: glucose loses electrons (becomes oxidized) and oxygen gains them (becomes reduced).


Glucose catabolism pathway stores the free energy in ATP:

ADP + Pi + free energy → ATP

The ATP can be used to do cellular work.


ΔG (change in free energy) from complete combustion of glucose is–686 kcal/mol.

Highly exergonic; it drives the endergonic formation of many ATP molecules.


Three catabolic processes harvest the energy from glucose:

• Glycolysis: glucose is converted to pyruvate; it is anaerobic.

• Cellular Respiration: pyruvate is converted to three molecules of CO2; it is aerobic.


• Fermentation: converts pyruvate into lactic acid or ethyl alcohol (anaerobic).

The breakdown of glucose is incomplete and lactic acid and ethyl alcohol still have a lot of energy.

Figure 9.1 Energy for Life


Oxidation–Reduction (Redox) reactions: one substance transfers electrons to another substance.

Reduction: gain of one or more electrons by an atom, ion, or molecule.

Oxidation: loss of one or more electrons.


Oxidation and reduction always occur together.

The compound that is reduced is the oxidizing agent.

The compound that is oxidized is the reducing agent.

In-Text Art, Ch. 9, p. 167 (1)


In glucose metabolism, glucose is the reducing agent, O2 is the oxidizing agent.

Transfer of electrons is often associated with the transfer of hydrogen ions

H = H+ + e–

When a molecule loses H atoms it becomes oxidized.


The more reduced a molecule is, the more energy it has.

In a redox reaction some energy is transferred from the reducing agent (glucose) to the reduced product.

Figure 9.2 Oxidation, Reduction, and Energy


Coenzyme NAD+ is a key electron carrier in redox reactions.

Figure 9.3 NAD+/NADH Is an Electron Carrier in Redox Reactions


Oxygen accepts electrons from NADH:

NADH + H+ + ½ O2 → NAD+ + H2O

The reaction is exergonic:

ΔG = –52.4 kcal/mol

Oxidizing agent is molecular oxygen (O2).


Eukaryotic and prokaryotic cells harvest energy from glucose using different combinations of metabolic pathways.

Some prokaryotes can harvest energy by anaerobic respiration.

The five metabolic pathways occur in different parts of the cell.

Figure 9.4 Energy-Yielding Metabolic Pathways

Table 9.1


Glycolysis

• Takes place in the cytosol

• Converts glucose into 2 molecules of pyruvate

• Produces 2 ATP and 2 NADH

• Occurs in 10 steps.


Steps 1–5 require ATP (energy-investing reactions).

Steps 6–10 yield NADH and ATP (energy-harvesting reactions).

Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 1)




Steps 6 and 7 of glycolysis:


Step 6 is an oxidation–reduction.

Exergonic; the energy is used to reduce NAD+ to NADH.

Step 7 is substrate-level phosphoryation.

Exergonic; the energy is used to transfer a phosphate to ADP and form ATP.


Pyruvate Oxidation:

• Occurs in the mitochondrial matrix

• Produces acetate and CO2

• Acetate binds to coenzyme A to form acetyl CoA

• Is a multistep reaction catalyzed by the pyruvate dehydrogenase complex.


Pyruvate oxidation:

Exergonic; one NAD+ is reduced to NADH.


Citric acid cycle

• Acetyl CoA is the starting point.

• The acetyl group is completely oxidized to 2 molecules of CO2.

• Energy released is captured by ADP, NAD+, FAD, and GDP.

Figure 9.6 The Citric Acid Cycle


Step 8 of the citric acid cycle:

This oxidation reaction is exergonic.


Overall, the oxidation of one glucose molecule yields:

• Six CO2

• Ten NADH

• Two FADH2

• Four ATP


Pyruvate oxidation and the citric acid cycle are regulated by concentrations of starting materials.

The starting molecules (acetyl CoA and oxidized electron carriers) must be replenished.

The electron carriers are reduced and they must be reoxidized.


If O2 is present, it accepts the electrons and H2O is formed.


Oxidative phosphorylation: ATP is synthesized by reoxidation of electron carriers in the presence of O2.

Two stages:

• Electron transport

• Chemiosmosis


Electron transport:

• Electrons from NADH and FADH2 pass through the respiratory chain of membrane-associated carriers.

• Electron flow results in a proton concentration gradient across the inner mitochondrial membrane.


Chemiosmosis:

• Electrons flow back across the membrane through a channel protein, ATP synthase, which couples the diffusion with ATP synthesis.


Why does the electron transport chain have so many steps?

Why not in one step?

2 NADH + 2 H+ + O2 → 2 NAD+ + 2 H2O


This reaction is extremely exergonic; too much free energy would be released all at once and could not be harvested by the cell.

In a series of reactions, each releases a small amount of energy that can be captured by an endergonic reaction.


The respiratory chain is located in the folded inner mitochondrial membrane.

Energy is released as electrons are passed between carriers.

Figure 9.7 The Oxidation of NADH and FADH2 in the Respiratory Chain


Protons are also actively transported.

Protons accumulate in the intermembrane space and create a concentration gradient and charge difference. This potential energy is called the proton-motive force.

Diffusion of protons back across the membrane is coupled to ATP synthesis (chemiosmosis).

Figure 9.8 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmotic Mechanism (Part 1)

Figure 9.8 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmotic Mechanism (Part 2)


ATP synthesis is reversible.

ATP synthase can also act as an ATPase, hydrolyzing ATP to ADP and Pi.

ATP ADP + Pi + free energy

Why is ATP synthesis favored?


• ATP leaves the mitochondrial matrix as soon as it is made, keeping ATP concentration in the matrix low, and driving the reaction toward the left.

• The H+ gradient is maintained by active transport.


The chemiosmosis hypothesis was a departure from the conventional scientific thinking of the time.

The first experimental evidence of chemiosmosis came from studies on chloroplast thylakoid membranes.

Figure 9.9 An Experiment Demonstrates the Chemiosmotic Mechanism (Part 1)

Figure 9.9 An Experiment Demonstrates the Chemiosmotic Mechanism (Part 2)

Working with Data 9.1: Experimental Demonstration of the Chemiosmotic Mechanism

Experiments to demonstrate the chemiosmosis hypothesis used isolated chloroplast membranes with embedded ATP synthase.

By moving the membranes from high to low pH (high to low H+ concentrations), they were able to drive ATP synthesis.


ATP formation was measured using:

• Luciferase, which catalyzes the formation of a luminescent molecule if ATP is present.

• Molybdate to measure phosphorylation directly.

Working with Data 9.1, Table 1


Question 1:

Which two experiments show that a proton gradient is necessary and sufficient to stimulate ATP formation?

Explain your reasoning.


Question 2:

Why was there less ATP production in the absence of Pi?


The structure and function of ATP synthase is the same in all living organisms.

It is a molecular motor with two parts:

• F0 unit—the transmembrane H+ channel

• F1 unit—projects into mitochondrial matrix; rotates to expose active sites for ATP synthesis

Figure 9.10 How ATP Is Made


These molecular motors can make up to 100 ATP molecules per second.

The mechanism was demonstrated by isolating the F1 unit and attaching fluorescently labeled microfilaments to it.

With no proton gradient, ATP was hydrolyzed, causing the motor to spin —easily visible in the spinning microfilaments.


Although O2 is an excellent electron acceptor, incomplete electron transfer can result in toxic intermediates.


Superoxide and hydroxyl radicals have unpaired electrons and react with other molecules to gain or loose electrons to become more stable.

This changes the structure and function of the other molecules.


Superoxide damage has been implicated in several human diseases and aging.

Enzymes can “scavenge” the oxidizers and convert them to water.

Antioxidant vitamins such as vitamin E act in a similar way.


Many bacteria and archaea have evolved pathways that allow them to exist where O2 is scarce or absent, by using other electron acceptors— anaerobic respiration.

Table 9.2


Without O2, some ATP can be produced by glycolysis and fermentation.

Fermentation occurs in the cytosol. NAD+ is regenerated to keep glycolysis going.

There are many types of fermentation; the best understood are lactic acid and alcohol fermentation.


Lactic acid fermentation:

• Pyruvate is the electron acceptor and lactate is the product.

• Occurs in microorganisms and some muscle cells.

Figure 9.11 Fermentation (Part 1)


During active exercise, O2 cannot be delivered fast enough for aerobic respiration.

Muscle cells then break down glycogen and carry out lactic acid fermentation.

When lactate builds up, the increase in H+ lowers pH and causes muscle pain.


Alcoholic fermentation:

• Yeasts and some plant cells

• Requires two enzymes to metabolize pyruvate to ethanol

• Used to produce alcoholic beverages

Figure 9.11 Fermentation (Part 2)


Cellular respiration yields more energy than fermentation.

• Glycolysis plus fermentation = 2 ATP

• Glycolysis plus cellular respiration = 32 ATP

Glucose is only partially oxidized in fermentation, more energy remains in the products than in CO2.

Figure 9.12 Cellular Respiration Yields More Energy Than Fermentation


Two key events in the evolution of multicellular organisms were the increase in atmospheric O2 levels and the development of metabolic pathways to use that O2.


In some eukaryotes, ATP must be used to transport NADH into the mitochondrial matrix.

NADH shuttle systems transfer electrons captured by glycolysis onto substrates that can move across mitochondrial membranes.


Metabolic pathways do not operate in isolation.

Many pathways share intermediate molecules.

Pathways are regulated by enzyme inhibitors.

Figure 9.13 Relationships among the Major Metabolic Pathways of the Cell


Catabolic interconversions:

Polysaccharides are hydrolyzed to glucose, which enters glycolysis.

Lipids are broken down to

• glycerol → DHAP → glycolysis

• fatty acids → acetyl CoA → citric acid cycle


Proteins are hydrolyzed to amino acids, which feed into glycolysis or the citric acid cycle.


Anabolic interconversions

Most catabolic reactions are reversible.

Gluconeogenesis: glucose formed from citric acid cycle and glycolysis intermediates.

Acetyl CoA can be used to form fatty acids.


How do cells “decide” which pathways to use?

Levels of substances in the metabolic pool are quite constant.

Organisms regulate enzymes to maintain balance between catabolism and anabolism.


When jogging, energy is needed by leg muscles and heart muscles.

Glucose is catabolized to provide energy, and in the liver, more glucose is made by anabolism from amino acids and pyruvate.

How does the liver “know” that it should be making glucose rather than catabolizing it or storing it?

Figure 9.14 Interactions of Catabolism and Anabolism during Exercise


Systems biology seeks to understand how biochemical pathways interact.

The pathways are regulated so that levels of molecules such as blood glucose remain constant.

Key enzymes are subject to allosteric regulation.


Negative and positive feedback

A high concentration of a metabolic product inhibits action of an enzyme in the pathway.

Excess product of one pathway can activate an enzyme in another pathway, diverting raw materials away from synthesis of the first product.

Figure 9.15 Regulation by Negative and Positive Feedback

Figure 9.16 Allosteric Regulation of Glycolysis and the Citric Acid Cycle (Part 1)

Figure 9.16 Allosteric Regulation of Glycolysis and the Citric Acid Cycle (Part 2)


The main control point in glycolysis is phosphofructokinase (step 3), which is inhibited by ATP.

In fermentation, phosphofructokinase operates at a high rate to produce ATP.

If O2 is present, more ATP is produced, which inhibits the enzyme and slows glycolysis.


The main control point in the citric acid cycle is isocitrate dehydrogenase (step 3).

It is inhibited by NADH and ATP; if too much of either accumulates, the citric acid cycle shuts down.


Acetyl CoA is another control point:

If ATP levels are high and the citric acid cycle shuts down, accumulation of citrate activates fatty acid synthesis from acetyl CoA, diverting it to storage.

Fatty acids may be metabolized later to produce more acetyl CoA.

9 Answer to Opening Question

Brown fat cells make UCP1, a protein that inserts in the mitochondrial inner membrane and makes it permeable to protons.

Thus the H+ concentration gradient is not established, and ATP is not synthesized. The energy released during electron transport goes to heat instead.

9 Answer to Opening Question

Adult humans have mostly white fat, but researchers have bred mice that make more UCP1 as adults.

These mice stay thinner than normal mice because more of their food energy goes to heat.

UCP1 may be useful in the fight against obesity.

Documents

9 Pathways That Harvest Chemical Energy. 9 Pathways That Harvest Chemical Energy 9.1 How Does Glucose Oxidation Release Chemical Energy? 9.2 What Are