Enzymes and Bioenergetics

Question 1

Define:

catalyst

Answer 1

A molecule that increases the rate of a reaction without being consumed itself.

Catalysts change the kinetics of a reaction, not the thermodynamics. In other words, they lower the activation energy without altering the equilibrium or free energy change of a reaction.

Question 2

Define:

enzyme

Answer 2

A biological catalyst that structurally facilitates a chemical reaction.

Like all catalysts, enzymes increase the rate of a chemical reaction without being consumed.

Most enzymes are proteins, but RNA molecules called ribozymes also have enzymatic activity.

Question 3

Why are enzymes generally ineffective catalysts over a broad temperature range?

Answer 3

Enzymes must be at a certain optimal temperature to maintain their structure.

The structure of an enzyme, especially in relation to its active site, is essential to its function as a catalyst. Like other proteins, enzymes denature, or lose their original conformations, above a certain temperature. However, reactions generally progress more slowly at low temperatures, further narrowing the range of optimal activity.

Question 4

How will the free energy diagram of a catalyzed reaction differ from that of an uncatalyzed reaction?

Answer 4

The catalyzed reaction will have a lower activation energy.

Note that the overall change in free energy will be the same for the catalyzed and uncatalyzed reactions.

Question 5

What effect does a reduced activation energy have on reaction kinetics?

Answer 5

A lowered activation energy increases the reaction rate.

Each reaction must overcome an activation energy barrier to progress from reactants to products. When this barrier is lower, reactants are more likely to collide with sufficient energy, speeding up the reaction.

Question 6

What happens to the equilibrium constant (K_eq) when an enzyme is added to a reaction?

Answer 6

The equilibrium constant doesn’t change.

Catalysts only impact reaction rate and have no effect on reaction equilibrium or free energy. K_eq is only altered by changes in temperature.

Question 7

Why are enzymes generally ineffective catalysts over a broad pH range?

Answer 7

Enzymes can only maintain their functional structure at a certain optimal pH.

The structure of an enzyme, especially in relation to its active site, is essential to its function as a catalyst. The active site often contains positively or negatively charged groups, which contribute to the specificity of the site. Changes in pH can protonate or deprotonate these sites, reducing enzyme activity.

Question 8

At the molecular level, why are prolonged, high fevers dangerous to human health?

Answer 8

At higher-than-optimal temperatures, most human enzymes lose function and cannot support necessary biological processes.

Most human enzymes function most efficiently at an optimal temperature of 37 ºC. While these enzymes can remain active at slightly higher temperatures, their function is impaired.

Question 9

Why is pepsin, a key digestive enzyme of the stomach, inactive in the small intestine?

Answer 9

The optimal pH for pepsin is far below that of the small intestine.

Pepsin functions most efficiently at a pH near 2, like that of the stomach. The small intestine is much more basic. This deviation from the optimal pH alters the structure of pepsin, inactivating it.

Question 10

Define:

substrate

Answer 10

A specific molecule that an enzyme acts upon, usually via interactions with the enzyme’s active site.

For example, starch is the substrate of salivary amylase. This means that amylase, an enzyme, catalyzes a reaction involving starch as a reactant.

Question 11

What is the functional significance of the active site?

Answer 11

It is the structural region where enzyme-substrate interactions take place.

In other words, the active site is the catalytic region of an enzyme. It is structured to facilitate the binding of its substrate, often through the presence of certain amino acid residues.

Question 12

Enzymes are usually limited to acting on a single substrate or class of substrate molecules. What term describes this quality?

Answer 12

specificity

The substrate(s) that can be accommodated by an enzyme are determined by the shape of its active site. For example, proteases are specific to protein substrates, while lipases act on lipid-based substrates.

Question 13

How does the lock-and-key model explain enzyme-substrate specificity?

Answer 13

It posits that a substrate will fit perfectly into the active site of its corresponding enzyme, without any conformational changes taking place.

In this model, the active site is the “lock” and its complementary substrate is the “key.”

Question 14

How does the induced fit model explain enzyme-substrate specificity?

Answer 14

It posits that active sites are flexible. When a substrate approaches the enzyme, the conformation of the active site will change to better fit the substrate.

The induced fit model is a more recent adaptation of the lock-and-key model.

Question 15

What major limitation prevents the lock-and-key model from being fully accurate?

Answer 15

It wrongly portrays all enzymes as being rigid and inflexible.

In reality, the active site of an enzyme can change its conformation to facilitate binding to the substrate.

Question 16

What is the difference between cofactors and coenzymes?

Answer 16

• Cofactors - broad group of compounds that are required for the proper functioning of enzymes
• Coenzymes - class of small, organic cofactors

Cofactors can also be inorganic substances, such as ions.

Question 17

Trace amounts of copper, zinc, and magnesium are essential for human protein function. What term describes the relationship of these metals to enzymes?

Answer 17

These metals, in their ion forms, act as cofactors.

Ions are common examples of inorganic cofactors, meaning that they are required for the proper functioning of certain enzymes.

Question 18

Name two ways in which cofactors assist enzymes in catalyzing reactions.

Answer 18

• binding to/transferring specific groups
• stabilizing the substrate via charge-based interactions

The second of these is essentially the role of Mg²⁺ in the DNA-related reactions in which it acts as a cofactor.

Question 19

Many coenzymes are derived from molecules like niacin and riboflavin, which must be consumed as part of the diet. What category describes these molecules?

Answer 19

vitamins

These are often used to synthesize coenzymes, which are non-protein organic compounds that are essential for proper enzyme function.

Question 20

Name the water-soluble vitamins.

Answer 20

1. vitamin B
2. vitamin C

The B vitamins in particular are a broad class, ranging from vitamin B₁ (thiamine) to vitamin B₁₂ (cobalamin). All of these vitamins are water-soluble (hydrophilic).

Question 21

Name the fat-soluble vitamins.

Answer 21

1. vitamin A
2. vitamin D
3. vitamin E
4. vitamin K

Since these vitamins are soluble in adipose (fat) tissue, they can build up in the body over time. This contrasts with water-soluble vitamins, which are quickly excreted.

Question 22

Define:

holoenzyme

Answer 22

It refers to an enzyme in combination with its required cofactor or coenzyme.

Question 23

Define:

apoenzyme

Answer 23

A term for an enzyme without its required cofactor or coenzyme.

Since enzymes often require their cofactors to function effectively (or at all), apoenzymes are essentially inactive.

Question 24

What happens to the rate of an enzyme-catalyzed reaction as substrate concentration increases?

Answer 24

Reaction rate initially increases but levels off as the enzyme becomes saturated with substrate.

The maximum rate of such a reaction is called V_max. Once this rate is reached, added substrate has no effect.

Question 25

An experimenter notes that the rate of an enzyme-catalyzed reaction remains **constant**, even after doubling the substrate concentration. Why might this occur?

Answer 25

The reaction is already at **V_max**. ## Footnote Above a certain substrate concentration, an enzyme will function at a maximum rate. Increasing the substrate concentration beyond this point will not affect the reaction kinetics.

Question 26

What **value** is defined as the substrate concentration present when the reaction velocity is one-half of V_max?

Answer 26

K_M ## Footnote In other words, K_M is the amount of substrate required to bind to half of an enzyme's active sites. An alternative term for this value is the **Michaelis constant**.

Question 27

The K_M of a certain reaction is calculated when the substrate concentration is 1.5 M. How will K_M change when this concentration is **doubled**?

Answer 27

K_M will **not change**. ## Footnote K_M, or the Michaelis constant, refers to the exact concentration of substrate that allows a reaction to reach one-half of V_max. Changing substrate concentration may increase or decrease the reaction rate, but it cannot alter this value.

Question 28

# True or false. The K_M of a reaction is equivalent to one-half of the V_max.

Answer 28

False ## Footnote This is a common mistaken assumption! In reality, K_M is equal to the *substrate concentration when the rate of reaction is one-half of V_max*, which is very different from simply being half of the value of V_max.

Question 29

How does the K_M of a reaction relate to the enzyme-substrate binding affinity in that reaction?

Answer 29

K_M is **inversely related** to enzyme-substrate binding affinity. ## Footnote A higher K_M means that more substrate is required to reach a rate of one-half V_max. Therefore, a high K_M denotes an inefficient reaction: one where the enzyme and substrate have *low* binding affinity.

Question 30

With regard to enzyme kinetics, what **name** is given to this type of plot (pictured below)?

Answer 30

Michaelis-Menten graph ## Footnote It shows the effect of substrate concentration ([S]) on reaction velocity (V).

Question 31

With regard to enzyme kinetics, what **name** is given to this type of plot (pictured below)?

Answer 31

Lineweaver-Burk plot ## Footnote It is a linear method of depicting enzyme activity. The x-axis is 1/[S], while the y-axis is 1/v.

Question 32

# Define: allosteric site

Answer 32

It is the region of an enzyme, separate from the active site, where molecules can bind and affect enzyme function. ## Footnote Allosteric binding can either facilitate or inhibit the binding of substrate to the active site.

Question 33

How does a competitive inhibitor **alter** an enzyme-catalyzed reaction?

Answer 33

It binds to an enzyme **on its active site**, inhibiting the reaction. ## Footnote Specifically, these inhibitors compete with the substrate and block it from binding the active site.

Question 34

How does a noncompetitive inhibitor **alter** an enzyme-catalyzed reaction?

Answer 34

It binds to an enzyme on a region **outside of its active site**, inhibiting the reaction. ## Footnote Specifically, these inhibitors bind an allosteric site, inducing a structural change in the enzyme that decreases its efficiency. Substrates can still enter the enzyme's active site.

Question 35

How do **competitive inhibitors** alter V_max and K_M?

Answer 35

They **increase** K_M but **do not change** V_max. ## Footnote Since competitive inhibition can be overcome by adding substrate, the maximum reaction velocity is not lowered; reaching it simply requires more substrate. As a result, K_M (the amount of substrate needed to reach one-half of V_max) is raised in these cases.

Question 36

How do **noncompetitive inhibitors** alter V_max and K_M?

Answer 36

They **decrease** V_max but **do not change** K_M. ## Footnote Since noncompetitive inhibition acts regardless of substrate concentration, it permanently lowers V_max. However, K_M is not changed; the same substrate concentration is still required to reach one-half of the new maximum velocity.

Question 37

How can **competitive inhibition** be overcome?

Answer 37

**increasing** substrate concentration ## Footnote Substrates and competitive inhibitors both bind the same region of the enzyme: the active site. Increasing substrate concentration ensures that more substrate, as opposed to inhibitor, binds at that position.

Question 38

How will the action of a noncompetitive inhibitor be affected by increasing substrate concentration?

Answer 38

no effect ## Footnote A noncompetitive inhibitor binds at an allosteric site, changing the overall conformation of the enzyme. The substrate can still bind, but it cannot be converted to product, regardless of its concentration.

Question 39

Which **type of inhibitor** is represented by the orange line below?

Answer 39

Since the line representing the inhibitor shows an altered V_max, this must be a **noncompetitive inhibitor**. ## Footnote On a Lineweaver-Burk plot, the y-intercept denotes 1/V_max. The value of the x-intercept is equal to −1/K_M.

Question 40

What **type of inhibitor** is represented by the orange line below?

Answer 40

Since the line representing the inhibitor shows an altered K_M with the same V_max, this must be a **competitive inhibitor**. ## Footnote On a Lineweaver-Burk plot, the y-intercept denotes 1/V_max. The value of the x-intercept is equal to −1/K_M.

Question 41

How does an uncompetitive inhibitor alter an enzyme-catalyzed reaction?

Answer 41

It binds to the enzyme-substrate (ES) complex, **preventing** its reaction and later **dissociation** into enzyme and product. ## Footnote This means that an ES complex must form before an uncompetitive inhibitor can take action.

Question 42

How do uncompetitive inhibitors alter V_max and K_M?

Answer 42

They **decrease** both K_M and V_max. ## Footnote While uncompetitive inhibition is tested much less than competitive and noncompetitive inhibition, it still might appear on the MCAT. The fact that it impacts *both* kinetic parameters makes it easier to spot.

Question 43

What **chemical principle** explains why K_M is decreased in cases of uncompetitive inhibition?

Answer 43

Le Chatelier's principle ## Footnote Consider the reaction E + S ↔ ES ↔ E + P. Uncompetitive inhibition essentially takes ES (the enzyme-substrate complex) out of commission, driving the first part of the reaction toward the right in an attempt to generate more ES. This increases the apparent binding affinity between E and S, and higher binding affinity corresponds to a lower K_M.

Question 44

Does an uncompetitive inhibitor bind the active site of the enzyme that it inhibits?

Answer 44

no ## Footnote Uncompetitive inhibitors are unique in that they only bind the enzyme-substrate complex. This means that the substrate is already in the active site, and the inhibitor must bind elsewhere on the enzyme (at an allosteric site).

Question 45

# Fill in the blank. A broad term for an inhibitor that can bind to either the enzyme alone or the enzyme-substrate complex is a \_\_\_\_\_\_\_\_\_\_ \_\_\_\_\_\_\_\_\_\_.

Answer 45

mixed inhibitor ## Footnote Mixed inhibition is tested the least frequently of the inhibition types on the MCAT, but it is still listed on the AAMC outline, so you should be familiar with it.

Question 46

Explain the **difference** between mixed and noncompetitive inhibition.

Answer 46

**Noncompetitive inhibition** is actually a special type of mixed inhibition. ## Footnote In mixed inhibition, the enzyme's affinity for the substrate may be greater, less, or the same as the enzyme's affinity for the enzyme-substrate (ES) complex. Noncompetitive inhibition refers specifically to mixed inhibition in which the enzyme's affinity for the substrate and its affinity for the ES are equal.

Question 47

How do mixed inhibitors alter V_max and K_M?

Answer 47

They **decrease** V_max. Regarding K_M, mixed inhibition may **increase it, decrease it, or leave it unchanged**. ## Footnote The specific impact on K_M depends on the enzyme's relative affinities for the substrate and the enzyme-substrate complex.

Question 48

Name all types of inhibition that **decrease V_max**. ## Footnote Choose from competitive, noncompetitive, uncompetitive, and mixed inhibition.

Answer 48

1. noncompetitive inhibition 2. uncompetitive inhibition 3. mixed inhibition ## Footnote Only competitive inhibition has no impact on V_max, as competitive inhibition can be overcome with the addition of a sufficient quantity of substrate.

Question 49

Name all types of inhibition that can **decrease K_M**. ## Footnote Choose from competitive, noncompetitive, uncompetitive, and mixed inhibition.

Answer 49

1. uncompetitive inhibition 2. mixed inhibition ## Footnote Uncompetitive inhibition always decreases K_M, while mixed inhibition can have a variety of effects on K_M (including decreasing it) depending on the specifics of the reaction.

Question 50

Name all types of inhibition that **change K_M** (in any way). ## Footnote Choose from competitive, noncompetitive, uncompetitive, and mixed inhibition.

Answer 50

1. competitive inhibition 2. uncompetitive inhibition 3. mixed inhibition ## Footnote Only noncompetitive inhibition has no impact on K_M. Competitive inhibition increases it, uncompetitive inhibition decreases it, and mixed inhibition can have a variety of effects.

Question 51

Phosphofructokinase, a glycolytic enzyme, is allosterically inhibited by ATP. What common homeostatic process does this example demonstrate?

Answer 51

negative feedback ## Footnote In such processes, increased concentration of a product decreases the rate of the reaction that forms that product. Glycolysis involves the breakdown of glucose to form ATP, among other products. If increased amounts of ATP are already present, glycolysis will slow.

Question 52

# Define: zymogen

Answer 52

An inactive precursor to a functional enzyme. ## Footnote Generally, zymogens must be cleaved into their active form. Zymogens are commonly found in the digestive system. Examples include pepsinogen, trypsinogen, and chymotrypsinogen.

Question 53

What is the **difference** between a zymogen and a proenzyme?

Answer 53

**Nothing**. Zymogen and proenzyme are **interchangeable terms**. ## Footnote Both terms refer to nonfunctional enzyme precursors that must be altered to produce their active enzyme forms.

Question 54

**Enterokinase** is an enzyme that converts trypsinogen to trypsin, an active digestive enzyme. **Trypsinogen** itself does not catalyze any reactions. What is trypsinogen an example of?

Answer 54

zymogen ## Footnote Trypsinogen is the storage form and precursor of trypsin. Trypsinogen will not function as an enzyme until it is converted to trypsin.

Question 55

What **class of substrate** is common to maltase, sucrase, and lactase?

Answer 55

disaccharides ## Footnote Maltase breaks down maltose into two glucose molecules. Sucrase cleaves sucrose into glucose and fructose, and lactase breaks down lactose into glucose and galactose.

Question 56

What **classes of substrate** are acted upon by lipases and proteases, respectively?

Answer 56

Lipases catalyze the breakdown of **lipids** (fats), while proteases cleave **proteins**. ## Footnote In biology, the "-ase" ending generally denotes an enzyme. With digestive enzymes, the part of the word that precedes "-ase" often refers to the substance that is broken down.

Question 57

Name the **six** main classes of **enzymes**.

Answer 57

1. oxidoreductases 2. transferases 3. hydrolases 4. lyases 5. ligases 6. isomerases

Question 58

Which of the six main classes of enzymes catalyzes the **transfer of electrons** between species?

Answer 58

Oxidoreductases ## Footnote As their name implies, these enzymes catalyze redox reactions. Their reaction mechanisms often include species that can serve as electron carriers, such as FAD or NAD⁺.

Question 59

Which of the six main classes of enzymes includes **kinases**?

Answer 59

Transferases ## Footnote Predictably, transferases catalyze the transfer of a functional group from one species to another. In the case of kinases, that functional group is a phosphate group. Many other transferases are easier to spot from their names alone, such as aminotransferases.

Question 60

Which of the six main classes of enzymes catalyze the **non-hydrolytic cleavage** of a large molecule into **two smaller products**?

Answer 60

Lyases ## Footnote Examples of lyases include decarboxylases, which catalyze the splitting of a molecule into a smaller product and CO₂.

Question 61

Which of the six main classes of enzymes always catalyzes reactions that include **water as a reactant**?

Answer 61

Hydrolases ## Footnote Hydrolases catalyze the breakage of a compound using water as a reactant. (In other words, they "hydrolyze" their substrates.) Many common enzymes are hydrolases, such as lipases and phosphatases.

Question 62

Which of the six main classes of enzymes **catalyze the synthesis** of large molecules from smaller reactants? ## Footnote Hint: One member of this class is critical to DNA replication.

Answer 62

Ligases ## Footnote Ligases "ligate," or "glue together," their substrates. The most classic (and MCAT-testable) example of a ligase is DNA ligase, which connects the Okazaki fragments on the lagging strand during DNA replication.

Question 63

Which of the six main classes of enzymes catalyze the **interconversion of molecules** that have the same chemical formulae?

Answer 63

Isomerases ## Footnote The question on the front of this card is another way of asking "which class of enzymes catalyzes the interconversion of isomers?" The answer is isomerases. Common examples of isomerases include mutase enzymes.

Question 64

Step 6 of glycolysis, which involves the production of **NADH from NAD⁺**, is catalyzed by glyceraldehyde 3-phosphate dehydrogenase. This **enzyme** is a member of which larger class?

Answer 64

Oxidoreductases ## Footnote Dehydrogenases are members of the larger class of oxidoreductases. You could also answer this question by recognizing that the production of NADH from NAD⁺ must mean that the overall reaction occurring here is a redox reaction.

Question 65

Step 8 of glycolysis is catalyzed by **phosphoglycerate mutase**, which catalyzes the conversion of 3-phosphoglycerate into 2-phosphoglycerate. This **enzyme** is a member of which larger class?

Answer 65

Isomerases ## Footnote The name of this enzyme should be very helpful here, as mutase enzymes are isomerases. 3-phosphoglycerate and 2-phosphoglycerate are structural isomers.

Question 66

How does a reaction catalyzed by a kinase **differ** from one catalyzed by a phosphatase?

Answer 66

* A **kinase** catalyzes the addition of a phosphate group to a protein. * A **phosphatase** catalyzes the removal of a phosphate group from a protein. ## Footnote One classic example of a kinase is protein kinase A, which is involved in G-protein coupled receptor cascades. It phosphorylates a variety of molecules.

Question 67

# Define: bioenergetics

Answer 67

It is the study of energy transformations within organisms. ## Footnote Specifically, it concerns the energy released and used during the formation and breaking of chemical bonds. Bioenergetics is closely related to thermodynamics, especially Gibbs free energy.

Question 68

What is the **difference** between anabolic and catabolic processes?

Answer 68

* **Catabolic processes** involve the biological breakdown of molecules into smaller units. Catabolism is accompanied by the generation of energy. * **Anabolic processes** involve the creation of larger biomolecules from smaller units. Anabolism requires energy input.

Question 69

What single comparison between reactants and products can be made to determine whether a reaction will proceed?

Answer 69

The change in Gibbs free energy, or **ΔG**, determines reaction spontaneity. ## Footnote A negative ΔG means the reaction will be spontaneous, while a positive ΔG denotes a nonspontaneous reaction. To calculate ΔG, use the equation ΔG = ΔH − TΔS. Note that while reactions are favored when ΔH (the enthalpy change) is negative and when ΔS (the entropy change) is positive, neither of these quantities alone can guarantee spontaneity.

Question 70

What term can be used to describe a thermodynamically favorable reaction?

Answer 70

exergonic ## Footnote In such reactions, ΔG is always negative, meaning that free energy is released. The opposite type of reactions are **endergonic**. In these processes, ΔG is positive and the reaction will not proceed spontaneously.

Question 71

# True or false. Exergonic reactions are always exothermic.

Answer 71

False ## Footnote Exergonic (meaning spontaneous) and exothermic (meaning energy-releasing) are different terms! A reaction can be exergonic and *endo*thermic as long as it involves a sufficient increase in entropy.

Question 72

A student is analyzing a particular physiological reaction and finds that ΔGº = +1.6 kJ/mol. From this, he assumes that the reaction will not proceed spontaneously in the body. What is the flaw in this student's reasoning?

Answer 72

The student is using the standard ΔG value (ΔGº). This value only determines if a reaction is favorable under standard-state conditions, which may not be applicable. ## Footnote Standard state dictates that the temperature must be 25 ºC, gases must be under a pressure of 1 atm, and all reactants must be present in 1 M concentrations. In the human body, temperature is roughly 37 ºC, so we already know that standard-state conditions do not apply.

Question 73

What **name** is given to the promotion of a thermodynamically unfavorable reaction by pairing it with a second, favorable process?

Answer 73

coupling ## Footnote If the sum of the reactions' ΔG values is negative, they will proceed. Often, an unfavorable reaction is paired with the hydrolysis of adenosine triphosphate (ATP), which is extremely exergonic.

Question 74

The phosphorylation of glucose into glucose-6-phosphate has a ΔGº of +13.8 kJ/mol. However, this reaction proceeds as the initial step of glycolysis. What **process** is responsible?

Answer 74

This situation is the result of **coupling** the reaction with another, more favorable, process. Specifically, this reaction is coupled with the hydrolysis of ATP. ## Footnote glucose + P_i → glucose-6-phosphate + H₂O ΔGº = +13.8 kJ/mol ATP + H₂O → ADP + P_i ΔGº = −30.5 kJ/mol Together, these coupled reactions have a ΔGº of (+13.8 kJ/mol) + (−30.5 kJ/mol) = −16.7 kJ/mol. With such a negative value, this process will be spontaneous.

Question 75

What **molecule** is shown below, and what is its **biological role**?

Answer 75

**Adenosine Triphosphate** (ATP), the major form of cellular energy. ## Footnote ATP consists of three phosphate groups bound to the ribonucleoside adenosine. The cleavage of the third (and second) phosphate bonds facilitates the release of energy, which the cell harnesses to drive biological processes.

Question 76

What **name** is given to the bonds circled in the structure below?

Answer 76

**Phosphoanhydride bonds**, which exist between phosphate groups on molecules like ATP (shown) and ADP. ## Footnote The name of these bonds comes from the reaction that forms them: dehydration (removal of H₂O). Predictably, they can be broken by the reverse reaction, hydrolysis (addition of H₂O).

Question 77

# True or false. ATP is a **nucleotide**.

Answer 77

True ## Footnote While we typically think of nucleotides in the context of nucleic acids (DNA and RNA), a nucleotide simply must contain a sugar, a nitrogenous base, and at least one phosphate group. ATP contains all of these things (in particular, it contains three phosphate groups) and is therefore a nucleotide.

Question 78

While the breaking of bonds usually requires energy, hydrolysis of a phosphoanhydride bond in ATP is extremely exergonic. What **structural features **of ATP explain this phenomenon?

Answer 78

* At a pH of 7.4, the phosphate groups on ATP have four negative charges, creating large amounts of **repulsion**. This is *minimized* when the molecule is **hydrolyzed**. * The products of hydrolysis are ADP and P_i, which are highly stabilized by **resonance**. ATP has *less* resonance stabilization overall. * Similarly, **ADP and P_i are more stabilized** than ATP by surrounding water molecules, due to their additional negative charges.

Question 79

# Fill in the blank. The molecule ATP contains \_\_\_\_\_\_\_\_ high-energy bond(s), while the molecule ADP contains \_\_\_\_\_\_\_\_ high-energy bond(s). ## Footnote Complete the sentence above with a number in each blank.

Answer 79

two, one ## Footnote An easy way to remember this is to think about how many phosphate groups can be removed from each molecule. ATP can lose two phosphates to become AMP (which does not lose its final phosphate group), while ADP can lose only one phosphate.

Question 80

Name **two methods** of phosphoryl group transfer that form ATP.

Answer 80

1. Oxidative phosphorylation 2. Substrate-level phosphorylation

Question 81

# Define: oxidative phosphorylation

Answer 81

It is the production of ATP from ADP by **ATP synthase**. ## Footnote This occurs immediately after the electron transport chain.

Question 82

# Define: substrate-level phosphorylation

Answer 82

It is the **transfer** of a phosphoryl group from a substrate **directly to ADP**. ## Footnote Such substrates must include phosphate groups, of course. One example of a substrate that participates in this form of phosphorylation is 1,3-bisphosphoglycerate.

Question 83

In what part(s) of a **eukaryotic cell** do oxidative and substrate-level phosphorylation occur, respectively?

Answer 83

* Oxidative phosphorylation occurs only in the **mitochondria**. Specifically, ATP synthase is located along the inner mitochondrial membrane. * Substrate-level phosphorylation can occur in the **cytosol** (as in glycolysis) or the **mitochondrial matrix** (as in the Krebs cycle).

Question 84

What type of **chemical reaction** most closely relates to the biological molecules FAD and NAD⁺?

Answer 84

oxidation-reduction | (redox) ## Footnote Note that the full name of FAD is flavin adenine dinucleotide, while that of NAD⁺ is nicotinamide adenine dinucleotide. Both molecules are common electron carriers.

Question 85

For a biological oxidation-reduction reaction to proceed spontaneously, what should be the signs of its ΔG and E values?

Answer 85

Its ΔG value should be **negative**, while its E value, or reaction potential, should be **positive**. ## Footnote Note that, as in most biological reactions, nonspontaneous reactions can occur if coupled with spontaneous processes. In that case, the sum of their ΔG values must still be negative.

Question 86

Is FADH₂ the **reduced** or **oxidized** form of flavin adenine dinucleotide?

Answer 86

reduced ## Footnote Meaning that it is currently carrying electrons. FAD is the oxidized form. When FADH₂ donates its electrons to another molecule, as in the electron transport chain, it is acting as a reducing agent.

Question 87

Is NAD⁺ the **reduced** or **oxidized** form of nicotinamide adenine dinucleotide?

Answer 87

oxidized ## Footnote Meaning that it is not currently carrying electrons. NADH is the reduced form. When NAD⁺ accepts electrons to become NADH, it is acting as an oxidizing agent.

Question 88

How many **total electrons** can be accepted by eight molecules of NAD⁺?

Answer 88

If reduced to NADH, it would hold a total of **sixteen electrons**. ## Footnote Remember that both common electron carriers (NAD⁺ and FAD) can accept two electrons per molecule.