BCH 451 EXAM 3 REVIEW QUESTIONS AND ANSWERS WITH
COMPLETE SOLUTIONS VERIFIED
What does the Citric Acid Cycle do?
The citric acid cycle fully oxidizes acetyl-CoA to CO2, producing NADH, FADH2, and
GTP, which contribute to ATP synthesis during oxidative phosphorylation.
Location of Citric Acid Cycle
The citric acid cycle occurs in the mitochondrial matrix, except for succinate
dehydrogenase, which is located in the inner mitochondrial membrane.
Which enzyme catalyzes the condensation of acetyl-CoA and oxaloacetate to
form citrate, and why is this step significant?
Citrate synthase catalyzes the condensation of acetyl-CoA and oxaloacetate to form
citrate. It is the rate-limiting step of the citric acid cycle, highly thermodynamically
favorable, and regulated by substrate availability and product inhibition.
Coenzymes of Pyruvate Dehydrogenase Complex
The PDC requires thiamine pyrophosphate (TPP), lipoyllysine, FAD (prosthetic groups),
NAD+, and CoA-SH (co-substrates).
Regulation of Pyruvate Dehydrogenase Complex
The PDC is regulated by reversible phosphorylation of its E1 subunit.
Phosphorylation inactivates the enzyme, while dephosphorylation activates it.
This regulation is mediated by PDH kinase (activated by ATP) and PDH phosphorylase
(activated by low ATP).
,Which steps of the citric acid cycle involve oxidative decarboxylation, and what
are their products?
Step 3: Isocitrate is oxidatively decarboxylated by isocitrate dehydrogenase to form α-
ketoglutarate, producing NADH and CO2.
Step 4: α-Ketoglutarate is oxidatively decarboxylated by α-ketoglutarate dehydrogenase
to form succinyl-CoA, producing NADH and CO2.
What is a Anaplerotic Reaction in regards to the CAC?
Anaplerotic reactions replenish intermediates of the citric acid cycle that are used in
biosynthetic pathways. For example, the carboxylation of pyruvate to oxaloacetate.
How does the enzyme aconitase ensure stereospecificity in the citric acid cycle?
Aconitase produces only the R-isomer of isocitrate by three-point attachment to its
active site, ensuring the correct stereospecificity.
Why is the regulation of isocitrate dehydrogenase crucial for glycolysis and the
citric acid cycle?
Regulation of isocitrate dehydrogenase affects citrate levels. Accumulated citrate can
reverse aconitase, leave the mitochondria, and inhibit glycolysis via
phosphofructokinase 1.
What transports Pyruvate to the mitochondria before being converted to Acetyl-
CoA?
Pyruvate Translocase
The first oxidative decarboxylation of the CAC is...?
Pyruvate to Acetyl-CoA, loses CO2 and generates NADH, catalyzed by pyruvate
dehydrogenase complex.
, What is the function of Coenzyme A?
Function of CoA is to accept and carry acetyl groups
Structure of lipoyllysine
Lipoic acid covalently linked tothe enzyme via a lysine residue
What two things can lipoate carry?
Electron carrier and Acyl carrier
Thiamine pyrophosphate (TPP) is a common _______ carrier
Acetaldehyde carrier
Pyruvate dehydrogenase complex is made up of what three enzymes?
E1- Pyruvate dehydrogenase
E2- Dihyrolipoyl transacetylase
E3- Dihydrolipoyl dehydrogenase
What are the advantages of a multienzyme complex?
-Short distance between catalytic sites allows channeling of substrates from one
catalytic site to another
-Channeling minimizes side reactions
-Regulation of activity of one subunit affects the entire complex
How does citrate synthase's induced fit mechanism work?
Citrate synthase exhibits an induced fit mechanism, where binding of oxaloacetate
(OAA) triggers a conformational change in the enzyme
What are the characteristics of Citrate Synthase's open conformation?
The enzyme does not have a binding site for acetyl-CoA.
What are the characteristics of Citrate Synthase's closed conformation?
COMPLETE SOLUTIONS VERIFIED
What does the Citric Acid Cycle do?
The citric acid cycle fully oxidizes acetyl-CoA to CO2, producing NADH, FADH2, and
GTP, which contribute to ATP synthesis during oxidative phosphorylation.
Location of Citric Acid Cycle
The citric acid cycle occurs in the mitochondrial matrix, except for succinate
dehydrogenase, which is located in the inner mitochondrial membrane.
Which enzyme catalyzes the condensation of acetyl-CoA and oxaloacetate to
form citrate, and why is this step significant?
Citrate synthase catalyzes the condensation of acetyl-CoA and oxaloacetate to form
citrate. It is the rate-limiting step of the citric acid cycle, highly thermodynamically
favorable, and regulated by substrate availability and product inhibition.
Coenzymes of Pyruvate Dehydrogenase Complex
The PDC requires thiamine pyrophosphate (TPP), lipoyllysine, FAD (prosthetic groups),
NAD+, and CoA-SH (co-substrates).
Regulation of Pyruvate Dehydrogenase Complex
The PDC is regulated by reversible phosphorylation of its E1 subunit.
Phosphorylation inactivates the enzyme, while dephosphorylation activates it.
This regulation is mediated by PDH kinase (activated by ATP) and PDH phosphorylase
(activated by low ATP).
,Which steps of the citric acid cycle involve oxidative decarboxylation, and what
are their products?
Step 3: Isocitrate is oxidatively decarboxylated by isocitrate dehydrogenase to form α-
ketoglutarate, producing NADH and CO2.
Step 4: α-Ketoglutarate is oxidatively decarboxylated by α-ketoglutarate dehydrogenase
to form succinyl-CoA, producing NADH and CO2.
What is a Anaplerotic Reaction in regards to the CAC?
Anaplerotic reactions replenish intermediates of the citric acid cycle that are used in
biosynthetic pathways. For example, the carboxylation of pyruvate to oxaloacetate.
How does the enzyme aconitase ensure stereospecificity in the citric acid cycle?
Aconitase produces only the R-isomer of isocitrate by three-point attachment to its
active site, ensuring the correct stereospecificity.
Why is the regulation of isocitrate dehydrogenase crucial for glycolysis and the
citric acid cycle?
Regulation of isocitrate dehydrogenase affects citrate levels. Accumulated citrate can
reverse aconitase, leave the mitochondria, and inhibit glycolysis via
phosphofructokinase 1.
What transports Pyruvate to the mitochondria before being converted to Acetyl-
CoA?
Pyruvate Translocase
The first oxidative decarboxylation of the CAC is...?
Pyruvate to Acetyl-CoA, loses CO2 and generates NADH, catalyzed by pyruvate
dehydrogenase complex.
, What is the function of Coenzyme A?
Function of CoA is to accept and carry acetyl groups
Structure of lipoyllysine
Lipoic acid covalently linked tothe enzyme via a lysine residue
What two things can lipoate carry?
Electron carrier and Acyl carrier
Thiamine pyrophosphate (TPP) is a common _______ carrier
Acetaldehyde carrier
Pyruvate dehydrogenase complex is made up of what three enzymes?
E1- Pyruvate dehydrogenase
E2- Dihyrolipoyl transacetylase
E3- Dihydrolipoyl dehydrogenase
What are the advantages of a multienzyme complex?
-Short distance between catalytic sites allows channeling of substrates from one
catalytic site to another
-Channeling minimizes side reactions
-Regulation of activity of one subunit affects the entire complex
How does citrate synthase's induced fit mechanism work?
Citrate synthase exhibits an induced fit mechanism, where binding of oxaloacetate
(OAA) triggers a conformational change in the enzyme
What are the characteristics of Citrate Synthase's open conformation?
The enzyme does not have a binding site for acetyl-CoA.
What are the characteristics of Citrate Synthase's closed conformation?
