7: Cellular Respiration

Question 1

What is chemiosmosis?

Answer 1

The process in which there is a production of adenosine triphosphate (ATP) in cellular metabolism by the involvement of a proton gradient across a membrane.

Question 2

What is dephosphorylation?

Answer 2

Removal of a phosphate group from a molecule.

Question 3

What is oxidative phosphorylation?

Answer 3

Production of ATP using the process of chemiosmosis and oxygen.

Question 4

What is phosphorylation?

Answer 4

Addition of a high-energy phosphate to a compound, usually a metabolic intermediate, a protein, or ADP.

Question 5

What is a redox reaction?

Answer 5

Chemical reaction that consists of the coupling of an oxidation reaction and a reduction reaction.

Question 6

What is substrate-level phosphorylation?

Answer 6

Production of ATP from ADP using the excess energy from a chemical reaction and a phosphate group from a reactant.

Question 7

How do redox reactions affect energy?

Answer 7

An oxidation reaction removes an electron from an atom in a compound, and a reduction reaction adds the electron to another compound. Oxidation results in a decrease in potential energy in the oxidized compound, and reduction results in an increase in the potential energy of the reduced compound. Most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons.

Question 8

What is an electron carrier?

Answer 8

Compounds which bind and carry high-energy electrons between compounds in pathways. These compounds can be easily reduced or oxidized.

Question 9

Where is NAD derived from?

Answer 9

Nicotinamide adenine dinucleotide (NAD) is derived from vitamin B₃ (niacin).

Question 10

What is the difference between NAD⁺ and NADH?

Answer 10

NAD⁺ is the oxidized form and NADH is the reduced form. NADH has accepted two electrons and a proton.

Question 11

What is the general equation for converting NAD⁺ to NADH?

Answer 11

NAD⁺ can accept electrons from an organic molecule according to the general equation:

RH (reducing agent) + NAD⁺ (oxidizing agent) -> NADH (reduced) + R⁺ (oxidized)

Question 12

Where is FAD derived from?

Answer 12

Flavin adenine dinucleotide (FAD⁺) is derived from B₂ (riboflavin). Its reduced form is FADH₂.

Question 13

What is NADP?

Answer 13

NADP contains an extra phosphate group.

Question 14

Where are NAD and FAD used?

Answer 14

NAD⁺ and FAD⁺ are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis.

Question 15

How do phosphate groups store energy?

Answer 15

The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and repel one another when arranged in series, such as in ADP and ATP. This makes them inherently unstable.

Question 16

Where does the energy to produce ATP come from?

Answer 16

The energy to produce ATP comes from the metabolism of glucose. ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.

Question 17

Where does chemiosmosis take place?

Answer 17

Chemiosmosis occurs in mitochondria of eukaryotic cells, and in the plasma membrane of prokaryotic cells.

Question 18

What is the purpose of chemiosmosis?

Answer 18

Chemiosmosis is used to generate 90% of the ATP made during glucose catabolism, and is the method used in the light reactions of photosynthesis to harness the energy of sunlight.

Question 19

In what ways can oxidative phosphorylation be impacted?

Answer 19

In type 2 diabetes, the oxidation efficiency of NADH is reduced, impacting oxidative phosphorylation.

Question 20

What is aerobic respiration?

Answer 20

The process by which organisms convert energy in the presence of oxygen.

Question 21

What is an anaerobic process?

Answer 21

A process that does not use oxygen.

Question 22

What is glycolysis?

Answer 22

The process of breaking glucose into two three-carbon molecules with the production of ATP and NADH.

Question 23

What is an isomerase?

Answer 23

An enzyme that converts a molecule into its isomer.

Question 24

What is pyruvate?

Answer 24

A three-carbon sugar that can be decarboxylated and oxidized to make acetyl CoA, which enters the citric acid cycle under aerobic conditions; it is the end product of glycolysis.

Question 25

What is the purpose of glycolysis?

Answer 25

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. Nearly all living organisms carry out glycolysis as part of their metabolism.

Question 26

Is glycolysis aerobic or anaerobic?

Answer 26

This process is anaerobic.

Question 27

Where does glycolysis take place?

Answer 27

It takes place in the cytoplasm of both prokaryotic and eukaryotic cells.

Question 28

How does glucose enter heterotrophic cells?

Answer 28

Glucose enters heterotrophic cells in two ways. One is through secondary active transport which takes place against the glucose concentration gradient. The other uses a group of integral proteins called GLUT proteins, or glucose transporter proteins, which assist in the facilitated diffusion of glucose.

Question 29

What are the inputs and outputs of glycolysis?

Answer 29

Glycolysis begins with the six carbon ring-shaped structure of a single glucose molecule, and ends with two molecules of a three-carbon sugar called pyruvate.

Question 30

What are the phases of glycolysis?

Answer 30

Glycolysis consists of two distinct phases. The first part of the pathway traps the glucose molecule in the cell and uses energy to modify it so that it can be split evenly into the two pyruvate molecules. The second part extracts energy from the molecules and stores it as ATP and NADH.

Question 31

What are the (energy-requiring) steps of the first half of glycolysis?

Answer 31

• The first step of glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates using ATP, producing glucose-6-phosphate, which is more reactive than glucose. This molecule will no longer interact with GLUT proteins, and cannot cross the membrane to leave the cell (because of the negative charge of the phosphate).
• In step 2, an isomerase converts glucose-6-phosphate into fructose-6-phosphate.
• In step 3, fructose-1,6-biphosphate is produced, catalyzed by phosphofructokinase using ATP. This enzyme is rate-limiting, more active when the concentration of ADP is high.
• In step 4, aldolase catabolizes this intermediate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate.
• In step 5, an isomerase transforms the dihydroxyacetone-phosphate into glyceraldehyde-3-phosphate.

Question 32

What are the (energy-releasing) steps of the second half of glycolysis?

Answer 32

• In step 6, the sugars are oxidized by NAD⁺ (producing NADH), and phosphorylated to produce 1,3-bisphosphoglycerate (which does not occur by ATP, rather an independent phosphate molecule). This is a limiting factor for this pathway, which depends on the availability of NAD⁺ (and hence NADH and an oxidizing agent such as oxygen).
• In step 7, phosphoglycerate kinase catalyzes 1,3-bisphosphoglycerate to donate a phosphate to ADP, forming ATP (substrate-level phosphorylation). A carbonyl group on the molecule is oxidized to a carboxyl group, forming 3-phosphoglycerate.
• In step 8, a mutase (which is a kind of isomerase) moves the phosphate from the third carbon to the second carbon, producing 2-phosphoglycerate.
• In step 9, enolase catalyzes a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP).
• The 10th and final step is catalyzed by pyruvate kinase, which results in the creation of an additional ATP molecule by substrate-level phosphorylation and the compound pyruvic acid (or its salt form, pyruvate).

Question 33

How many ATP and NADH molecules are produced during glycolysis?

Answer 33

A net gain of two ATP molecules and two NADH molecules are produced, with an investment of two ATP molecules.

Question 34

What is acetyl CoA?

Answer 34

Combination of an acetyl group derived from pyruvic acid and coenzyme A, which is made from pantothenic acid (a B-group vitamin).

Question 35

What is the citric acid cycle?

Answer 35

A series of enzyme-catalyzed chemical reactions of central importance in all living cells.

Question 36

What are some other names of the citric acid cycle?

Answer 36

The Krebs cycle and the TCA cycle.

Question 37

Why is the Krebs cycle so named?

Answer 37

The Krebs cycle is named after Hans Krebs who first identified the steps in the pathway in the 1930s in pigeon flight muscles.

Question 38

Why is the TCA cycle so named?

Answer 38

The TCA cycle is named after the group name for citric acid, tricarboxylic acid (TCA).

Question 39

What happens to the pyruvate produced by glycolysis?

Answer 39

At the end of glycolysis, pyruvate is transported into mitochondria, the sites of cellular respiration. There, pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called CoA.

Question 40

What is the purpose of acetyl CoA?

Answer 40

Acetyl CoA can be used in many ways, but its major function is to deliver the acetyl group derived from pyruvate to the next stage of the pathway in glucose catabolism.

Question 41

What are the steps of the breakdown of pyruvate?

Answer 41

• In the first step, a carboxyl group is removed from pyruvate, which releases a molecule of CO₂. This results in a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase.
• In step 2, the hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD⁺ -> NADH.
• In step 3, the enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA.

Question 42

How does acetyl CoA work?

Answer 42

In the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups.

Question 43

Where does the citric acid cycle take place?

Answer 43

The citric acid cycle takes place in the mitochondrial matrix. All but one of the enzymes are soluble (succinate dehydrogenase is found in the inner membrane of the mitochondrion).

Question 44

What are some characteristics of the citric acid cycle?

Answer 44

The citric acid cycle is a closed loop, where the last step regenerates the compound used as input to the first. They are a series of redox, dehydration, hydration, and decarboxylation reactions that produce 2 CO₂, 1 GTP / ATP, and reduced forms of NADH and FADH₂. This pathway is aerobic, because the NADH and FADH₂ transfer their electrons to the next pathway in the system, which uses oxygen. The cycle produces very little ATP directly, and does not directly consume oxygen.

Question 45

What are the steps of the citric acid cycle?

Answer 45

• In step 1, the acetyl group of acetyl CoA combines with oxaloacetate to form citrate. The CoA is bound to a sulfhydryl group (-SH) and diffuses away to combine with another acetyl group. This step is irreversible because it is highly exergonic. This reaction is controlled by negative feedback of the presence of ATP (more ATP, less reaction).
• In step 2, citrate is converted into isocitrate by the loss and re-gain of one H₂O.
• In step 3, isocitrate is oxidized by NAD⁺ to produce alpha-ketoglutarate, CO₂, and two electrons, which reduce NAD⁺ to NADH. This step is also regulated by negative feedback from ATP and NADH, with a positive effect of ADP.
• In step 4, oxidation and decarboxylation also occurs, producing NADH and CO₂, and with an input of SH-CoA, forms succinyl CoA. The enzyme that catalyzes step 4 is regulated by feedback inhibition of ATP, succinyl CoA, and NADH.
• In step 5, a phosphate group is substituted for CoA, forming a high-energy bond which is used in substrate-level phosphorylation to form either GTP or ATP. Two forms of the enzyme (isoenzymes) are used depending on the type of animal tissue. One form is used in tissue which uses large amounts of ATP such as heart and skeletal muscle, the other in tissue that has a high number of anabolic pathways, such as liver, which uses GTP.
• In step 6, dehydration occurs, converting succinate to fumarate, transferring two hydrogen atoms to FAD ⇒ FADH₂. The energy contained in the electrons of these atoms is insufficient to reduce NAD⁺, but adequate to reduce FAD. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly. This is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion.
• In step 7, fumarate is hydrolyzed into malate.
• In the 8th and final step, malate is oxidized, producing NADH / H⁺ and oxaloacetate.

Question 46

What are the inputs and outputs of the citric acid cycle?

Answer 46

All six carbon atoms from glucose are eventually incorporated into the citric acid cycle. Each turn produces three NADH and one FADH₂. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules. One GTP or ATP is also made in each cycle. Several of these intermediates are precursors to the synthesis of non-essential amino acids. The cycle is amphibolic (both catabolic and anabolic). Amino acids may enter the citric acid cycle if there are excess amino acids in a cell, or if the body is in a state of starvation. Each amino acid must have its amino group removed prior to entry into the pathway. The lipids that are connected to the glucose pathways are cholesterol and triglycerides.

Question 47

What is (F1F0) ATP synthase?

Answer 47

A membrane-embedded protein complex that adds a phosphate to ADP with energy from protons diffusing through it.

Question 48

What is a prosthetic group?

Answer 48

A molecule bound to a protein that facilitates the function of the protein. They may be organic or inorganic, but are non-peptide.

Question 49

What is ubiquinone?

Answer 49

A soluble electron transporter in the electron transport chain that connects the first or second complex to the third.

Question 50

What is the electron transport chain?

Answer 50

A series of protein complexes that transfer electrons from electron donors to electron acceptors via redox reactions, and couples this electron transfer with the transfer of protons (H⁺ ions) across a membrane.

Question 51

How many complexes are in the electron transport chain?

Answer 51

There are four protein complexes in the electron transport chain.

Question 52

What are some characteristics of the complex I in the electron transport chain?

Answer 52

The first complex is composed of flavin mononucleotide (FMN) and an iron-sulfur (Fe-S)-containing protein. FMN, which is derived from vitamin B₂ / riboflavin, is one of several prosthetic groups / cofactors in the electron transport chain. The enzyme in complex I is NADH dehydrogenase, a very large protein containing 45 amino acid chains. Complex I pumps 4 hydrogen ions across the membrane into the mitochondrial intermembrane space, establishing a hydrogen ion gradient.

Question 53

What are some characteristics of the ubiquinone and complex II in the electron transport chain?

Answer 53

Complex II directly receives FADH₂, which does not pass through complex I. Ubiquinone (Q) connects the first and second complexes to the third. Q is lipid soluble and freely moves throughout the inner membrane. QH₂ (reduced Q) receives its electrons from NADH in complex I and from FADH₂, and delivers them to complex III. The FADH₂ electrons, by bypassing complex I, contribute to fewer ATP molecules produced.

Question 54

What are some characteristics of the complex III in the electron transport chain?

Answer 54

Complex III is called cytochrome oxidoreductase, and is composed of cytochrome b, an Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins. They have a prosthetic group of heme (similar to the heme in hemoglobin, but which carries electrons rather than oxygen). The iron at its core is thus reduced and oxidized as it passes electrons, fluctuating between Fe⁺⁺ and Fe⁺⁺⁺. Cytochrome c carries electrons to complex IV, and accepts electrons from Q. Cytochrome c can only accept one electron at a time, in contrast to the two that Q can accept.

Question 55

What are some characteristics of the complex IV in the electron transport chain?

Answer 55

Complex IV is composed of cytochrome proteins c, a, and a₃. It contains two heme groups (one in each of in a and a₃), and three copper ions (a pair of CuA and one CuB in a₃). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. Then it bonds to hydrogen to form water from the surrounding medium. The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis.

Question 56

How does chemiosmosis work?

Answer 56

Chemiosmosis uses ATP synthase (a membrane protein / ion channel) to allow hydrogen ions down its concentration gradient back into the matrix, and acts as a generator, turning and facilitating the addition of an inorganic phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.

Question 57

How many hydrogen ions can be pumped across membranes by the electron transport chain?

Answer 57

The number of hydrogen ions pumped through the membrane by the electron transport chain varies between species.

Question 58

How many electrons can FAD shuttle compared to NAD and where is it used?

Answer 58

FAD⁺ can shuttle fewer electrons than NAD⁺, and so generates fewer ATP molecules. FAD⁺ is used more heavily than NAD⁺ in the brain.

Question 59

How are the intermediates used in the glucose catabolism pathways?

Answer 59

Intermediates in these pathways are used for other purposes, and sugars other than glucose may be fed in the glycolytic pathway. The five-carbon sugars that form nucleic acids, some nonessential amino acids, lipids, cholesterol, and triglycerides are made from these intermediates in these pathways. Amino acids and triglycerides are also broken down for energy by these pathways. In living systems, these pathways extract about 34% of the energy contained in glucose.

Question 60

Where is NAD+ used?

Answer 60

NAD⁺ is used more heavily than FAD⁺ in the liver.

Question 61

How much energy is lost by electrons in the electron transport chain?

Answer 61

Electrons lose energy as they pass through the electron transport chain, from about 60kcal/mol of NADH or 45kcal/mol of FADH₂ to about 0kcal/mol of water.

Question 62

What is fermentation?

Answer 62

Process of regenerating NAD⁺ with either an inorganic or organic compound serving as the final electron acceptor; occurs in the absence of oxygen.

Question 63

Where does lactic acid fermentation occur?

Answer 63

Lactic acid fermentation is a fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells. In muscles, lactic acid accumulation must be removed by the blood circulation and the lactate brought to the liver for further metabolism. Once the lactic acid has been circulated to the liver, it can be reconverted into pyruvic acid and further catabolized for energy.

Question 64

Which compounds are involved in lactic acid fermentation?

Answer 64

The chemical reactants of lactic acid are: pyruvic acid + NADH + H⁺ ⇒ lactic acid + NAD⁺. The enzyme used in this reaction is lactate dehydrogenase (LDH).

Question 65

How does ethanol fermentation work?

Answer 65

The first chemical reaction is: pyruvic acid ⇒ CO₂ + acetaldehyde + NADH ⇒ ethanol + NAD⁺. The first reaction is catalyzed by pyruvate decarboxylase, a cytoplasmic enzyme, with a coenzyme of thiamine pyrophosphate (TPP, derived from vitamin B1/thiamine). A carboxyl group is removed from pyruvic acid, releasing CO₂, and producing acetaldehyde. The second reaction is catalyzed by alcohol dehydrogenase to oxidize NADH to NAD⁺ and reduce acetaldehyde to ethanol.

Question 66

What does fermentation of pyruvic acid produce?

Answer 66

The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages. Ethanol tolerance of yeast is variable, ranging from about 5 percent to 21 percent, depending on the yeast strain and environmental conditions.

Question 67

Are prokaryotes aerobic or anaerobic?

Answer 67

Some prokaryotes are facultatively anaerobic, and so can switch between aerobic and anaerobic respiration depending on the availability of oxygen. Other prokaryotes are obligate anaerobes, and the presence of oxygen is lethally toxic to them.

Question 68

What does fermentation produce?

Answer 68

All forms of fermentation except lactic acid produce gas. The production of particular types of gas is used as an indicator of the fermentation of specific carbohydrates, which plays a role in the laboratory identification of the bacteria. Various methods of fermentation are used by assorted organisms to ensure an adequate supply of NAD+ for the sixth step in glycolysis. Fermentation frequently accounts for any increased acidity in a cell, however the products of fermentation do not typically accumulate in cells.

Question 69

How is glycogen produced and consumed by animals?

Answer 69

When ATP is available, excess glucose is stored as glycogen. It is made and stored in both liver and muscle. This allows ATP to be produced for a longer period of time during exercise. Glycogen is broken down into glucose-1-phosphate and glucose-6-phosphate in both muscle and liver cells, and this product enters the glycolytic pathway.

Question 70

What is sucrose?

Answer 70

A disaccharide with a molecule of glucose and a molecule of fructose bonded together with a glycosidic linkage.

Question 71

What is fructose?

Answer 71

One of the three dietary monosaccharides, along with glucose and galactose, which is absorbed directly into the bloodstream during digestion. The catabolism of fructose produces the same number of ATP molecules as glucose.

Question 72

What is galactose?

Answer 72

One of the monosaccharides in the disaccharide lactose. The catabolism of galactose produces the same number of ATP molecules as glucose.

Question 73

How is urea produced?

Answer 73

In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule. The ammonia molecules may come from amino acids, on removal prior to entry into the citric acid cycle. Urea is the principal waste product in mammals produced from the nitrogen originating in amino acids, and it leaves the body in urine.

Question 74

What is cholesterol?

Answer 74

A lipid that contributes to cell membrane flexibility and is a precursor of steroid hormones.

Question 75

Is the synthesis of cholesterol a cycle?

Answer 75

The synthesis of cholesterol starts with acetyl groups and proceeds in only one direction. The process cannot be reversed.

Question 76

What is a triglyceride?

Answer 76

A form of long-term storage in animals. They are made of glycerol and three fatty acids.

Question 77

How are triglycerides used in the glucose catabolism pathways?

Answer 77

Triglycerides can be both anabolized and catabolized in certain steps of the glucose catabolism pathways.

Question 78

How are fatty acids incorporated into the citric acid cycle?

Answer 78

Animals can make most of the fatty acids they need. Fatty acids are catabolized in a process called beta-oxidation that takes place in the mitochondrial matrix and converts their fatty acid chains into two carbon units of acetyl groups. The acetyl groups are picked up by CoA to form acetyl CoA that enters the citric acid cycle.

Question 79

How does glycerol enter glycolysis?

Answer 79

Glycerol can be phosphorylated to glycerol-3-phosphate, which may enter glycolysis.

Question 80

How did the evolution of photosynthesis influence glycolysis?

Answer 80

An early form of photosynthesis developed that did not use water as a source of hydrogen ions, but rather other molecules, like hydrogen sulfide, and subsequently produced sulfur rather than oxygen. It is thought that glycolysis developed at this time and could take advantage of the simple sugars being produced, but these reactions were unable to fully extract the energy stored in the carbohydrates. A later form of photosynthesis used water as a source of electrons and hydrogen, and generated free oxygen.

Question 81

When did glycolysis likely evolve?

Answer 81

The development of glycolysis probably predated the evolution of photosynthesis, as it was well-suited to extract energy from materials spontaneously accumulating in the “primeval soup”.

Question 82

What is a GLUT protein?

Answer 82

An integral membrane protein that transports glucose.

Question 83

What is an example of a GLUT protein?

Answer 83

Different forms of the GLUT protein control passage of glucose into the cells of specific tissues. GLUT4, for example, is a glucose transporter that is stored in vesicles. A cascade of events that occurs upon insulin binding to a receptor in the plasma membrane causes GLUT4-containing vesicles to fuse with the plasma membrane so that glucose may be transported into the cell.

Question 84

How is cellular respiration regulated?

Answer 84

Control of a cell’s metabolism prevents metabolic reactions from reaching a state of equilibrium in order to provide balanced amounts of energy in the form of ATP and generating the correct amount of intermediates used in the metabolism of macromolecules. Some reversible reactions are catalyzed by two different enzymes, one for each direction, which allows greater control of the rate of reaction, preventing equilibrium. A form of feedback activation or inhibition is by binding to the allosteric site of several of the enzymes used in the glucose pathways, in order to increase or decrease enzyme activity. The most commonly used molecules for this are ATP, ADP, AMP, NAD⁺, and NADH.

Question 85

What is hexokinase?

Answer 85

An enzyme that phosphorylates hexoses, forming hexose phosphate. This is the first enzyme used in glycolysis, which catalyzes the phosphorylation of glucose. The negative charge of the phosphate prevents glucose-6-phosphate from leaving the cell. When this enzyme is inhibited, glucose diffuses out of the cell, and does not become a substrate for the respiration pathways in that tissue.

Question 86

What is phosphofructokinase?

Answer 86

A kinase enzyme that phosphorylates fructose-6-phosphate in glycolysis. If this enzyme is inhibited, glucose-6-phosphate will accumulate. It is the main enzyme controlled in glycolysis. High levels of ATP, citrate, or a lower, more acidic pH decrease the enzyme’s activity. An increase in citrate concentration can occur because of a blockage in the citric acid cycle.

Question 87

What is pyruvate kinase?

Answer 87

The enzyme involved in the last step of glycolysis, which catalyzes the transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, yielding one molecule of pyruvate and one molecule of ATP. After glycolysis, pyruvate can proceed to be catabolized or converted into the amino acid alanine. If no more energy is needed and alanine is in adequate supply, this enzyme is inhibited. Its activity increases when fructose-1,6-bisphosphate levels increase. The regulation of pyruvate kinase involves phosphorylation by a kinase (pyruvate kinase kinase), resulting in a less-active enzyme. Dephosphorylation by a phosphatase reactivates it. It is also regulated by ATP (a negative allosteric effect).

Question 88

How is conversion of pyruvate to acetyl CoA regulated?

Answer 88

If more energy is needed in a cell, more pyruvate will be converted into acetyl CoA through the action of pyruvate dehydrogenase. If acetyl groups or NADH accumulate, the rate decreases. Pyruvate dehydrogenase is also regulated by phosphorylation. A kinase phosphorylates it to form an inactive enzyme, and a phosphatase reactivates it. The kinase and the phosphatase are also regulated.

Question 89

Which enzymes catalyze the reactions that make the first two molecules of NADH and so control the citric acid cycle?

Answer 89

Isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase.

Question 90

How is alpha-ketoglutarate dehydrogenase regulated?

Answer 90

It is negatively regulated by ATP, NADH, and succinyl CoA. A decrease in activity of this enzyme may result in the alpha-ketoglutarate being used by the cell for amino acid (glutamate) synthesis.

Question 91

How is the electron transport chain inhibited?

Answer 91

The enzymes of the electron transport chain are unaffected by feedback inhibition, but the chain is slowed by the buildup of ATP (indicating lower levels of energy usage).