Chapter 17: Citric Acid Cycle

Question 1

is a central pathway for recovering energy from several metabolic fuels, including
carbohydrates, fatty acids, and amino acids, that are broken down to acetyl-CoA for oxidation.

Answer 1

citric acid cycle

Question 2

what does the citric acid cycle oxidize

Answer 2

acetyl-CoA to two molecules of CO2

Question 3

what does the citric acid cycle oxidizes in a manner that conserves what

Answer 3

liberated free energy in
the reduced compounds NADH and FADH2

Question 4

The cycle is named after the
product of its first reaction

Answer 4

citrate

Question 5

One complete round of the citric acid cycle yields

Answer 5

two molecules of CO2

three NADH

one FADH2

GTP or ATP

Question 6

is consumed in the first step of the citric acid cycle is regenerated in the last step of the cycle.

Answer 6

oxaloacetate

Question 7

the citric acid cycle acts as a multistep catalyst that can what

Answer 7

oxidize an unlimited number of acetyl groups

Question 8

In eukaryotes, all the enzymes of the citric acid cycle are located in the

Answer 8

mitochondria

Question 9

all substrates, including what must be generated in the mitochondria or be
transported into mitochondria from the cytosol.

Answer 9

NAD+ and GDP

Question 10

all the products of the citric acid cycle must be consumed where and transported where

Answer 10

mitochondria

cytosol.

Question 11

however, the net effect of each round of the cycle is
the oxidation of

Answer 11

one acetyl group to 2 CO2.

Question 12

Acetyl-CoAis formed from pyruvate through oxidative decarboxylation by a
multienzyme complex named

Answer 12

pyruvate dehydrogenase

Question 13

This complex contains multiple
copies of three enzymes

Answer 13

pyruvate dehydrogenase (E1)

dihydrolipoyl transacetylase
(E2)

dihydrolipoyl dehydrogenase (E3)

Question 14

Decarboxylates pyruvate yielding a hydroxyethyl-tpp carbanion

Answer 14

Thiamine pyrophosphate (tpp)

Question 15

What are advantages of multienzymes:

Answer 15

1. The distance that substrates must diffuse between active sites is minimized, thereby
   enhancing the reaction rate.
2. The channeling of metabolic intermediates between successive enzymes in a
   metabolic pathway reduces the opportunity for these intermediates to react with other molecules, thereby minimizing side reactions.
3. The reactions catalyzed by a multienzyme complex can be coordinately controlled.

Question 16

Accepts the hydroxyethyl carbanion from TPP as an acetyl group

Answer 16

Lipoic acid

Question 17

Accepts the acetyl group from lipoamide

Answer 17

Coenzyme A (CoA)

Question 18

Reduced by lipoamide

Answer 18

Flavin adenine

Question 19

Reduced by FADH2

Answer 19

Nicotinamide adenine dinucleotide (NAD+)

Question 20

acts as a swinging arm

Answer 20

lipoamide

Question 21

what does lipoamide do

Answer 21

swings disulfide group from E1 (where it picks up a hydroxyethyl group)

to the E2 active site (where the
hydroxyethyl group is transferred to form acetyl-CoA)

to E3 (where the reduced disulfide is reoxidized)

Question 22

are toxic because they bind to sulfhydryl compounds (including lipoamide) that can form bidentate adducts.

Answer 22

arsenite and organic arsenicals

Question 23

The inactivation of lipoamide-containing enzymes by what

Answer 23

arsenite

Question 24

brings respiration to a halt

Answer 24

pyruvate dehydrogenase

Question 25

This differential toxicity is the basis for the early twentieth century use of organic arsenicals in the treatment of

Answer 25

syphilis

Question 26

Aconitase contains a

Answer 26

[4Fe–4S] iron–sulfur cluster

Question 27

that presumably coordinates the OH group of citrate to facilitate its elimination

Answer 27

[4Fe–4S] iron–sulfur cluster

Question 28

normally participate in redox
processes

Answer 28

Iron–sulfur clusters

Question 29

The α-ketoglutarate dehydrogenase reaction chemically resembles the reaction catalyzed by

Answer 29

pyruvate dehydrogenase multienzyme complex

Question 30

is the only membrane-bound enzyme of the citric acid
cycle

Answer 30

Succinate dehydrogenase

Question 30

catalyzes the stereospecific dehydrogenation of succinate to fumarate

Answer 30

Succinate dehydrogenase

Question 31

In eukaryotes, the products of the pyruvate dehydrogenase reaction, NADH and acetyl-CoA, also activate the

Answer 31

pyruvate dehydrogenase kinase

Question 32

the hormone that signals
fuel abundance

Answer 32

Insulin

Question 33

reverses the inactivation by activating pyruvate dehydrogenase phosphatase,
which removes the phosphate groups from pyruvate dehydrogenase.

Answer 33

Insulin

Question 34

The citric acid cycle’s flux is controlled primarily by three simple mechanisms:

Answer 34

(1) substrate
availability

(2) product inhibition

(3) competitive feedback inhibition by intermediates further along the cycle.

Question 35

Perhaps the most crucial regulators of the citric acid cycle are

Answer 35

its substrates acetyl-CoA

and oxaloacetate,

and NADH

Question 36

Since the citric acid cycle is a cyclical pathway, any of its
intermediates can be converted to oxaloacetate and used for gluconeogenesis.

Answer 36

Gluconeogenesis

Question 37

is a cytosolic process that requires acetyl-CoA. Acetyl-CoA is generated in the mitochondrion but transported across the mitochondrial membrane as
citrate

Answer 37

Fatty acid biosynthesis

Question 38

An increase in the concentrations of citric
acid cycle intermediates supports increased
flux of

Answer 38

acetyl groups through the cycle

Question 39

Pyruvate carboxylase― senses the need for more citric acid cycle intermediates through
its activator

Answer 39

acetyl-CoA

Question 40

subsists largely on lipids, using the citric acid cycle to produce precursors for amino acid synthesis and using the glyoxylate cycle to produce
carbohydrate precursors.

Answer 40

glyoxylate cycle

Question 41

also stimulates cell growth and differentiation by increasing the synthesis of glycogen,
proteins, and triacylglycerols.

Answer 41

Insulin

Question 42

Muscle cells and adipocytes express an insulin-sensitive glucose transporter known as

Answer 42

GLUT4.

Question 43

inhibits transcription of the
genes encoding the gluconeogenic enzymes PEP
carboxykinase, fructose-1,6-bisphosphatase, and
glucose-6-phosphatase and stimulates transcription
of the genes for the glycolytic enzymes glucokinase
and pyruvate kinase.

Answer 43

Insulin

Question 44

Most of the brain’s energy
production powers the

Answer 44

plasma membrane

Question 45

which maintains the membrane potential
required for nerve impulse transmission.

Answer 45

plasma membrane

Question 46

is the brain’s primary fuel.

Answer 46

glucose

Question 47

blood glucose concentration of less than half the normal
value of what results in brain dysfunction.

Answer 47

~5 mM