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Flashcards in Enzymes and Bioenergetics Deck (88)
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1
Q

Define:

a catalyst

A

A catalyst is 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.

2
Q

Define:

an enzyme

A

An enzyme is 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.

3
Q

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

A

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.

4
Q

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

A

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.

5
Q

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

A

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.

6
Q

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

A

The equilibrium constant doesn’t change.

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

7
Q

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

A

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.

8
Q

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

A

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.

9
Q

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

A

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.

10
Q

Define:

a substrate

A

A substrate is 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.

11
Q

What is the functional significance of the active site?

A

The active site of an enzyme 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.

12
Q

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

A

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.

13
Q

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

A

The lock-and-key model 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.”

14
Q

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

A

The induced fit model 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.

15
Q

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

A

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.

16
Q

What is the difference between cofactors and coenzymes?

A

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

Cofactors can also be inorganic substances, such as ions.

17
Q

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

A

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.

18
Q

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

A

Cofactors can assist enzymes in a number of ways, but two common ones are:

  • 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.

19
Q

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

A

Vitamins

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

20
Q

Name the water-soluble vitamins.

A

The water-soluble vitamins are the B and C vitamins.

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).

21
Q

Name the fat-soluble vitamins.

A

The fat-soluble vitamins are vitamins A, D, E, and 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.

22
Q

Define:

a holoenzyme

A

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

23
Q

Define:

an apoenzyme

A

An apoenzyme is 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.

24
Q

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

A

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

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

25
Q

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

A

The reaction is already at Vmax.

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.

26
Q

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

A

KM

In other words, KM 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.

27
Q

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

A

KM will not change.

KM, or the Michaelis constant, refers to the exact concentration of substrate that allows a reaction to reach one-half of Vmax. Changing substrate concentration may increase or decrease the reaction rate, but it cannot alter this value.

28
Q

True or false:

The KM of a reaction is equivalent to one-half of the Vmax.

A

False

This is a common mistaken assumption! In reality, KM is equal to the substrate concentration when the rate of reaction is one-half of Vmax, which is very different from simply being half of the value of Vmax.

29
Q

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

A

KM is inversely related to enzyme-substrate binding affinity.

A higher KM means that more substrate is required to reach a rate of one-half Vmax. Therefore, a high KM denotes an inefficient reaction: one where the enzyme and substrate have low binding affinity.

30
Q

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

A

This is a Michaelis-Menten graph.

It shows the effect of substrate concentration ([S]) on reaction velocity (V).

31
Q

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

A

This is a Lineweaver-Burk plot, which is a linear method of depicting enzyme activity.

The x-axis is 1/[S], while the y-axis is 1/v.

32
Q

Define:

allosteric site

A

An allosteric site is a region of an enzyme, separate from the active site, where molecules can bind and affect enzyme function.

Allosteric binding can either facilitate or inhibit the binding of substrate to the active site.

33
Q

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

A

A competitive inhibitor binds to an enzyme on its active site, inhibiting the reaction.

Specifically, these inhibitors compete with the substrate and block it from binding the active site.

34
Q

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

A

A noncompetitive inhibitor binds to an enzyme on a region outside of its active site, inhibiting the reaction.

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.

35
Q

How do competitive inhibitors alter Vmax and KM?

A

Competitive inhibitors increase KM but do not change Vmax.

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, KM (the amount of substrate needed to reach one-half of Vmax) is raised in these cases.

36
Q

How do noncompetitive inhibitors alter Vmax and KM?

A

Noncompetitive inhibitors decrease Vmax but do not change KM.

Since noncompetitive inhibition acts regardless of substrate concentration, it permanently lowers Vmax. However, KM is not changed; the same substrate concentration is still required to reach one-half of the new maximum velocity.

37
Q

How can competitive inhibition be overcome?

A

Competitive inhibition can be overcome by increasing substrate concentration.

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.

38
Q

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

A

Adding substrate will have no effect.

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.

39
Q

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

A

Since the line representing the inhibitor shows an altered Vmax, this must be a noncompetitive inhibitor.

On a Lineweaver-Burk plot, the y-intercept denotes 1/Vmax. The value of the x-intercept is equal to −1/KM.

40
Q

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

A

Since the line representing the inhibitor shows an altered KM with the same Vmax, this must be a competitive inhibitor.

On a Lineweaver-Burk plot, the y-intercept denotes 1/Vmax. The value of the x-intercept is equal to −1/KM.

41
Q

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

A

An uncompetitive inhibitor binds to the enzyme-substrate (ES) complex, preventing its reaction and later dissociation into enzyme and product.

This means that an ES complex must form before an uncompetitive inhibitor can take action.

42
Q

How do uncompetitive inhibitors alter Vmax and KM?

A

Uncompetitive inhibitors decrease both KM and Vmax.

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.

43
Q

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

A

Le Chatelier’s principle

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 KM.

44
Q

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

A

No

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).

45
Q

A broad term for an inhibitor that can bind to either the enzyme alone or the enzyme-substrate complex is:

A

a mixed inhibitor.

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.

46
Q

Explain the difference between mixed and noncompetitive inhibition.

A

Noncompetitive inhibition is actually a special type of mixed inhibition.

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.

47
Q

How do mixed inhibitors alter Vmax and KM?

A

Mixed inhibitors decrease Vmax. Regarding KM, mixed inhibition may increase it, decrease it, or leave it unchanged.

The specific impact on KM depends on the enzyme’s relative affinities for the substrate and the enzyme-substrate complex.

48
Q

Name all types of inhibition that decrease Vmax.

Choose from competitive, noncompetitive, uncompetitive, and mixed inhibition.

A

Noncompetitive, uncompetitive, and mixed inhibition

Only competitive inhibition has no impact on Vmax, as competitive inhibition can be overcome with the addition of a sufficient quantity of substrate.

49
Q

Name all types of inhibition that can decrease KM.

Choose from competitive, noncompetitive, uncompetitive, and mixed inhibition.

A

Uncompetitive and mixed inhibition

Uncompetitive inhibition always decreases KM, while mixed inhibition can have a variety of effects on KM (including decreasing it) depending on the specifics of the reaction.

50
Q

Name all types of inhibition that change KM (in any way).

Choose from competitive, noncompetitive, uncompetitive, and mixed inhibition.

A

Competitive, uncompetitive, and mixed inhibition

Only noncompetitive inhibition has no impact on KM. Competitive inhibition increases it, uncompetitive inhibition decreases it, and mixed inhibition can have a variety of effects.

51
Q

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

A

This is an example of negative feedback. 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.

52
Q

Define:

a zymogen

A

A zymogen is an inactive precursor to a functional enzyme. Generally, zymogens must be cleaved into their active form.

Zymogens are commonly found in the digestive system. Examples include pepsinogen, trypsinogen, and chymotrypsinogen.

53
Q

What is the difference between a zymogen and a proenzyme?

A

Nothing. Zymogen and proenzyme are interchangeable terms.

Both terms refer to nonfunctional enzyme precursors that must be altered to produce their active enzyme forms.

54
Q

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?

A

Trypsinogen is a zymogen.

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

55
Q

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

A

The substrates of these enzymes are all disaccharides.

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

56
Q

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

A

Lipases catalyze the breakdown of lipids (fats), while proteases cleave proteins.

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.

57
Q

Name the six main classes of enzymes.

A

The six main classes of enzymes are:

  • oxidoreductases
  • transferases
  • hydrolases
  • lyases
  • ligases
  • isomerases
58
Q

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

A

Oxidoreductases

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+.

59
Q

Which of the six main classes of enzymes includes kinases?

A

Transferases

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.

60
Q

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

A

Lyases

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

61
Q

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

A

Hydrolases

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.

62
Q

Which of the six main classes of enzymes catalyze the synthesis of large molecules from smaller reactants?

Hint: One member of this class is critical to DNA replication.

A

Ligases

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.

63
Q

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

A

Isomerases

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.

64
Q

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?

A

Oxidoreductases

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.

65
Q

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?

A

Isomerases

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

66
Q

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

A
  • A kinase catalyzes the addition of a phosphate group to a protein.
  • A phosphatase catalyzes the removal of a phosphate group from a protein.

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.

67
Q

Define:

bioenergetics

A

Bioenergetics is the study of energy transformations within organisms. 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.

68
Q

What is the difference between anabolic and catabolic processes?

A

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.

69
Q

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

A

The change in Gibbs free energy, or ΔG, determines reaction spontaneity.

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.

70
Q

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

A

Thermodynamically favorable reactions are termed exergonic. 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.

71
Q

True or false:

Exergonic reactions are always exothermic.

A

False

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

72
Q

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?

A

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.

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.

73
Q

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

A

This pairing is known as coupling. 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.

74
Q

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?

A

This situation is the result of coupling the reaction with another, more favorable, process. Specifically, this reaction is coupled with the hydrolysis of ATP.

glucose + Pi → glucose-6-phosphate + H2O ΔGº = +13.8 kJ/mol

ATP + H2O → ADP + Pi 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.

75
Q

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

A

This molecule is adenosine triphosphate (ATP), the major form of cellular energy.

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.

76
Q

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

A

These are phosphoanhydride bonds, which exist between phosphate groups on molecules like ATP (shown) and ADP.

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).

77
Q

True or false:

ATP is a nucleotide.

A

True

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.

78
Q

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?

A
  • 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 Pi, which are highly stabilized by resonance. ATP has less resonance stabilization overall.
  • Similarly, ADP and Pi are more stabilized than ATP by surrounding water molecules, due to their additional negative charges.
79
Q

The molecule ATP contains ________ high-energy bond(s), while the molecule ADP contains ________ high-energy bond(s).

Complete the sentence above with a number in each blank.

A

The molecule ATP contains two high-energy bonds, while the molecule ADP contains one high-energy bond.

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.

80
Q

Name two methods of phosphoryl group transfer that form ATP.

A

Oxidative phosphorylation and substrate-level phosphorylation

81
Q

Define:

oxidative phosphorylation

A

Oxidative phosphorylation is the production of ATP from ADP by ATP synthase.

This occurs immediately after the electron transport chain.

82
Q

Define:

substrate-level phosphorylation

A

Substrate-level phosphorylation is the transfer of a phosphoryl group from a substrate directly to ADP.

Such substrates must include phosphate groups, of course. One example of a substrate that participates in this form of phosphorylation is 1,3-bisphosphoglycerate.

83
Q

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

A
  • 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).
84
Q

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

A

These molecules are involved in oxidation-reduction (redox) reactions.

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.

85
Q

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

A

Its ΔG value should be negative, while its E value, or reaction potential, should be positive.

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.

86
Q

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

A

FADH2 is the reduced form, 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.

87
Q

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

A

NAD+ is the oxidized form, 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.

88
Q

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

A

Eight molecules of NAD+, if reduced to NADH, would hold a total of sixteen electrons.

Remember that both common electron carriers (NAD+ and FAD) can accept two electrons per molecule.