Chapter 19: The Citric Acid Cycle

Three processes play central roles in aerobic metabolism

* The citric acid cycle (TCA cycle)
* Electron transport (Chapter 20)
* Oxidative phosphorylation (Chapter 20)

Metabolism consists of 

* Catabolism: the oxidative breakdown of nutrients
* Anabolism: the reductive synthesis of biomolecules

* The citric acid cycle is______; that is, it plays a role in both______ and _________. *It is the central metabolic pathway*

* amphibolic
* catabolism; anabolism

Where does the Citric Acid Cycle Take Place?

In eukaryotes, cycle takes place in the mitochondrial matrix

The _____________ is responsible for the conversion of pyruvate to CO2 and the acetyl portion of acetyl-CoA.

Pyruvate dehydrogenase

* Five enzymes in complex (mammals): 

conversion of pyruvate to acetyl-CoA

__________________________________

* *pyruvate dehydrogenase (PDH)*
* *dihydrolipoyl transacetylase*
* *dihydrolipoyl dehydrogenase*
* *pyruvate dehydrogenase kinase*

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Control PDH

_________________________________

* *pyruvate dehydrogenase phosphatase*

Acetyl-CoA is a ________.

Thioester

What is formed when pyruvate loses CO2 in the first step?

hydroxyethylTPP (HETPP) 

1. What enzyme is bound to the active form of lipoic acid in

by an amide bond to the ε-amino group of a lysinethe second step?

dihydrolipoyl transacetylase

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Step 3: The hydroxyethyl group (HE) is oxidized and transferred to _____ of the reduced form of lipoamide.

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the sulfur atom is of the reduced form of lipoamide

What is reduced to dihydrolipoamide?

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Lipoamide

To which group is the acetyl group transferred in step 3?

sulfhydryl group of coenzyme A

What is oxidized to lipoamide in the final step?

dihydrolipoamide

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What is the two-carbon unit needed at the start of the citric acid cycle, and how is it obtained?

The two-carbon unit needed at the start of the citric acid cycle is obtained by converting pyruvate to acetyl-CoA.

What are the primary enzymes required for the conversion of pyruvate to acetyl-CoA, and what cofactors do they require?

the three primary enzymes of the pyruvate dehydogenase complex, and they require the cofactors TPP, FAD, NAD+, and lipoic acid.

What is the overall reaction catalyzed by the pyruvate dehydrogenase complex, and what are the products of this reaction?

the conversion of pyruvate, NAD+, and CoA-SH to acetyl-CoA, NADH + H+, and CO2.

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What is formed when acetyl-CoA condenses with oxaloacetate in step 1 of the process?

citrate

What is the value of ΔG°’ in the reaction of acetyl-CoA and oxaloacetate?

\-7.8 kcal/mol

What is the name of the enzyme that catalyzes the condensation of acetyl-CoA with oxaloacetate in step 1.

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 citrate synthase

Step 1 : What are the three inhibitors of this enzyme?

NADH, ATP, and succinyl-CoA

What is catalyzing the isomerization of citrate to isocitrate in step 2?

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aconitase

Does citrate have a stereocenter?

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No

Citrate is _______.

achiral

Isocitrate is _______. How many stereocenters does isocitrate have?

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Chiral.

2 but 4 are possible.

How many stereoisomers of isocitrate are formed in step 2 of the cycle?

One

What is formed along with α-ketoglutarate in step 3 of the process?

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CO2

What is the catalyzing enzyme responsible for the reaction in which α-ketoglutarate and CO2 are formed?

isocitrate dehydrogenase

What are the inhibitors and activators of the allosteric enzyme responsible for the first of two oxidative decarboxylations?

ATP and NADH

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ADP and NAD+

What is catalyzing the oxidative decarboxylation of α-ketoglutarate to succinyl-CoA in step 4?

The α-ketoglutarate dehydrogenase complex

What is required for the α-ketoglutarate dehydrogenase complex to catalyze the second of two oxidative decarboxylations?

Coenzyme A, thiamine pyrophosphate, lipoic acid, FAD, and NAD+

What is the significance of the reaction catalyzed by the α-ketoglutarate dehydrogenase complex?

it is the second of two oxidative decarboxylations, which results in the formation of succinyl-CoA.

What is hydrolyzed in the formation of succinate during step 5 of the cycle?

The thioester bond of succinyl-CoA

Are the two CH2-COO- groups of succinate equivalent?

Yes

Which step of the cycle is the first energy-yielding step?

Step 5

Is the overall reaction of the cycle slightly exergonic?

Yes, slightly.

What is oxidized to fumarate in step 6 of the process, and which enzyme catalyzes this reaction?

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Succinate,

and succinate dehydrogenase catalyzes it.

What is the name of the enzyme that catalyzes the hydration of fumarate to L-malate in step 7 of the process?

Fumarase

Which step involves the oxidation of succinate to fumarate, and which enzyme is responsible for catalyzing this reaction?

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Step 6, and the enzyme succinate dehydrogenase catalyzes this reaction.

What reaction occurs in step 7 of the process, and which enzyme catalyzes this reaction?

The hydration of fumarate to L-malate , and the enzyme fumarase catalyzes this reaction.

What is oxidized to Oxaloacetate in step 8?

malate

Which enzyme catalyzes the oxidation of malate to Oxaloacetate?

malate dehydrogenase

How many molecules of CO2 are produced from one molecule of pyruvate in the citric acid cycle and the pyruvate dehydrogenase reaction as a result of oxidative decarboxylation?

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Three m______

What are the reduced electron carriers involved in the oxidations during the citric acid cycle and the pyruvate dehydrogenase reaction?

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NAD+ to NADH ; FAD to FADH2

What molecule is phosphorylated to GTP during the citric acid cycle?

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GDP

What is one purpose of the citric acid cycle?

energy production

What are the two products of the citric acid cycle that are used to produce energy?

\[NADH and FADH2\]

How many control points are there within the cycle?

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3 within the cycle.

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Which molecule inhibits pyruvate dehydrogenase in the conversion of pyruvate to acetyl-CoA?

ATP and NADH, and acetyl-CoA product inhibits it too.

Which molecule activates isocitrate dehydrogenase in step 3 of the cycle?

ADP and NAD+, while ATP and NADH inhibit it.

Which molecule inhibits α-ketoglutarate dehydrogenase complex in step 4 of the cycle?

ATP, NADH, and succinyl CoA inhibit α-ketoglutarate dehydrogenase complex, while ADP and NAD+ activate it.

Is the control point for pyruvate dehydrogenase inside or outside the cycle?

outside

Is step 8 of the TCA cycle exergonic or endergonic?

strongly endergonic.

Which step is paired with step 1 of the TCA cycle?

Step 8

Are the overall reactions of the TCA cycle exergonic or endergonic?

exergonic

What effect do high ATP/ADP and NADH/NAD+ ratios have on enzyme activity?

shut down enzyme activity.

When do cells have low energy requirements?

they are not actively carrying out metabolic processes.

What is a high "energy charge"?

refers to a high concentration of ATP relative to ADP and AMP.

What happens to enzymes when they are "shut down"?

they are no longer active and cannot catalyze metabolic reactions.

What signals the enzymes to "turn on"?

Low ATP/ADP and NADH/NAD+ ratios

Why do cells need enzymes to turn on?

because they need energy.

When do enzymes turn on?

when the ATP/ADP and NADH/NAD+ ratios are low.

What is the name of the cycle that bypasses the two oxidative decarboxylations of the citric acid cycle?

The glyoxylate cycle

In which organisms is there a modification of the citric acid cycle to produce four-carbon dicarboxylic acids and eventually glucose?

plants and some bacteria

Which two steps of the citric acid cycle are bypassed by the glyoxylate cycle?

steps 3 and 4

What are the key enzymes in the glyoxylate cycle?

isocitrate lyase and malate synthase.

In which organelle does the glyoxylate cycle take place in plants?

glyoxysomes, which are specialized organelles for this cycle.

Where does the glyoxylate cycle occur in yeast and algae?

the cytoplasm in yeast and algae.

How does the glyoxylate cycle help plants grow in the dark?

Seeds are able to convert lipids into oxaloacetate and other intermediates to produce carbohydrates, which can sustain the plant's growth in the absence of light.

What do seeds contain that is rich in fatty acids?

They are rich in lipids.

What happens to the acetyl-CoA produced during fatty acid oxidation in plants?

it is produced during fatty acid oxidation to produce oxaloacetate and other intermediates for carbohydrate synthesis.

When do glyoxysomes disappear in plants?

once they begin photosynthesis and can fix CO2.

Are metabolic pathways related to each other, and do they operate simultaneously?

Yes, all metabolic pathways are related to each other, and all of them operate simultaneously.

What happens to macromolecules in catabolic pathways?

They are are broken down into smaller molecules such as sugars, fatty acids, and amino acids.

What happens to small molecules in catabolic pathways, and where do the end products frequently enter?

They are processed further, and enter the citric acid cycle, which plays a key role in metabolism.

What is the role of the citric acid cycle in biosynthesis?

it is the source of starting materials

Why must a component of the citric acid cycle be replaced if it is taken out for biosynthesis?

to maintain the cycle's functionality.

How is oxaloacetate replenished in the citric acid cycle?

by the carboxylation of pyruvate by pyruvate carboxylase in a process called anaplerotic reaction.

What is an anaplerotic reaction?

They replenish a citric acid cycle intermediate, such as the carboxylation of pyruvate by pyruvate carboxylase to replace oxaloacetate.

What is the purpose of the anabolic reaction that uses the citric acid intermediate α-ketoglutarate?

competes with the rest of the cycle to help mammals keep an adequate supply of metabolic intermediates.

Which metabolic intermediate signals allosteric activation of pyruvate carboxylase, leading to the production of more oxaloacetate?

The concentration of acetyl-CoA rises.

How do mammals ensure an adequate supply of metabolic intermediates?

They use an anabolic reaction that competes with the rest of the cycle and signal allosteric activation of pyruvate carboxylase through rising concentrations of acetyl-CoA to produce more oxaloacetate.

Where do the anabolic reactions of gluconeogenesis take place?

the cytosol.

Is oxaloacetate transported across the mitochondrial membrane?

NO! It is not.

How can PEP be formed during gluconeogenesis?

from oxaloacetate in the mitochondrial matrix and subsequently transported across the membrane.

How can malate be converted to oxaloacetate during gluconeogenesis?

via malate dehydrogenase in the cytosol

What is the purpose of the transfer of starting materials of gluconeogenesis from the mitochondrion to the cytosol?

to facilitate carbohydrate anabolism.

Where does lipid anabolism begin and take place?

In the cytosol with acetyl-CoA as its starting molecule.

What is the primary source of acetyl-CoA?

mainly produced in mitochondria from the catabolism of fatty acids and carbohydrates.

What is the indirect transfer mechanism involving citrate?

it is converted to acetyl-CoA and oxaloacetate in a reaction catalyzed by ATP-citrate lyase

What are the products of the reaction catalyzed by ATP-citrate lyase?

Acetyl-CoA, Oxaloacetate, ADP, and Pi.

What is the role of oxaloacetate in lipid anabolism?

provides a means for the production of NADPH, which is needed for biosynthesis.

What molecule is reduced to malate in the TCA cycle?

Oxaloacetate

What is the net effect of the two reactions involving malate in the TCA cycle?

The replacement of NADH by NADPH.

How can malate move into and out of mitochondria?

The active transport processes.

What is the principal source of NADPH?

the pentose phosphate pathway.

What do the anabolic reactions that produce amino acids and biomolecules begin with?

The TCA cycle molecules that are transported into the cytosol.

What is the final product of the reaction involving malate, NADP+?

pyruvate, CO2, NADPH, and H+.

What is the origin of pathways that lead to the synthesis of sugars, fatty acids, and amino acids?

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originate with components of the citric acid cycle.

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Can animals convert lipids to carbohydrates?

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No, they can’t.

What can animals convert into lipids?

carbohydrates