Biochemistry Lecture 19: TCA Cycle

Questions and Answers

What is the primary function of the pyruvate dehydrogenase complex?

To convert acetyl CoA to glucose

To produce ATP from pyruvate

To convert pyruvate into acetyl CoA (correct)

To oxidize isocitrate to a-ketoglutarate

How many subunits make up the E1 component of pyruvate dehydrogenase in mammalian cells?

294

30 (correct)

What is the intermediate formed during the isomerization of citrate by aconitase?

Acetyl CoA

D-isocitrate

Cis-aconitate (correct)

a-ketoglutarate

Which subunit is a tetramer composed of a2b2 in the pyruvate dehydrogenase complex?

E1 Signup and view all the answers

Where are the TCA cycle enzymes located in eukaryotic cells?

Mitochondrial matrix Signup and view all the answers

What type of transport mechanism allows pyruvate to enter the mitochondria?

Symport Signup and view all the answers

Which reaction catalyzed by isocitrate dehydrogenase connects isocitrate to nitrogen metabolism?

Oxidative decarboxylation of isocitrate to a-ketoglutarate Signup and view all the answers

What reaction occurs first in the TCA cycle involving acetyl CoA?

Condensation to form citrate Signup and view all the answers

What role does FADH2 play in the electron transport chain?

It binds tightly to enzymes and transfers electrons. Signup and view all the answers

What is the function of fumarase in the TCA cycle?

It adds water across the carbon-carbon double bond. Signup and view all the answers

Which product is formed from the final step of the citric acid cycle?

Oxaloacetate Signup and view all the answers

Which intermediate is crucial in the synthesis of the porphyrin ring of heme groups?

Succinyl CoA Signup and view all the answers

How does the TCA cycle contribute to amino acid synthesis?

By transamination of α-ketoglutarate. Signup and view all the answers

What is the role of ubiquinone in the electron transport chain?

It transports electrons through the membrane. Signup and view all the answers

Which of the following statements about malate dehydrogenase is true?

It catalyzes the conversion of malate to oxaloacetate. Signup and view all the answers

What is a significant function of the TCA cycle aside from energy production?

Synthesis of purines and pyrimidines. Signup and view all the answers

What is the primary purpose of the TCA cycle?

To oxidize acetyl CoA and produce energy-rich molecules Signup and view all the answers

Which of the following is NOT a direct product of the TCA cycle per one molecule of acetyl CoA?

2 molecules of ATP Signup and view all the answers

What must occur before pyruvate can enter the TCA cycle?

It must be converted to acetyl CoA Signup and view all the answers

Which enzyme catalyzes the conversion of pyruvate to acetyl-CoA?

Pyruvate dehydrogenase Signup and view all the answers

Which of the following cofactors is produced during the conversion of pyruvate to acetyl CoA?

NADH Signup and view all the answers

Where in the cell does the TCA cycle primarily occur?

In the matrix of mitochondria Signup and view all the answers

What happens to the NADH and FADH2 produced in the TCA cycle?

They are oxidized in the electron transport chain. Signup and view all the answers

What is the primary fuel source that generates acetyl CoA for the TCA cycle?

Sugars, fats, and proteins Signup and view all the answers

Study Notes

TCA Cycle Overview

The TCA cycle, also known as the citric acid cycle or Krebs cycle, describes a series of reactions that fully oxidize sugars, fats, and proteins.
In 1937, Sir Hans Krebs summarized existing scientific knowledge to define this cycle, with contributions from other scientists like Albert Szent-Györgyi identifying related enzymes.

Function and Products

The cycle processes one acetyl CoA to produce:
- 2 molecules of CO2
- 3 NADH
- 1 FADH2
- 1 GTP
The NADH and FADH2 produced are used in the electron transport chain for ATP production.

Entry into the TCA Cycle

Pyruvate, generated from glycolysis, must be converted to acetyl CoA via the pyruvate dehydrogenase complex.
This conversion releases CO2 and produces NADH.

Pyruvate Dehydrogenase Complex

Catalyzes oxidative decarboxylation of pyruvate to form acetyl-CoA, found in bacterial cytosol and mitochondrial matrix.
Composed of three subunits (E1, E2, E3); mammalian cells have a total of 294 protein subunits.

TCA Cycle Reactions

Reaction 1: Acetyl CoA combines with oxaloacetate to form citrate, catalyzed by citrate synthase.
Reaction 2: Aconitase converts citrate to D-isocitrate via an intermediate, cis-aconitate.
Reaction 3: Isocitrate dehydrogenase converts isocitrate to α-ketoglutarate through oxidative decarboxylation, linking to nitrogen metabolism and glutamate synthesis.

Additional Enzymatic Steps

Fumarase catalyzes the hydration of fumarate to form L-malate.
Malate dehydrogenase regenerates oxaloacetate, producing NADH in the final step of the TCA cycle.

Biosynthetic Pathway Precursors

The TCA cycle supplies intermediates for various biosynthetic pathways:
- Fatty acids synthesized from citrate.
- Amino acids derived from α-ketoglutarate via transamination.
- Purine and pyrimidine nucleotides synthesized from α-ketoglutarate and oxaloacetate.
- Gluconeogenesis conversion of oxaloacetate to glucose.
- Succinyl CoA involved in heme group synthesis.

Subcellular Localization

In mammalian cells, TCA cycle enzymes are located in the mitochondrial matrix.
In prokaryotic cells (e.g., E. coli), all enzymes function in the cytosol.
Pyruvate enters mitochondria through a symporter, consuming energy during transport.

Clinical Relevance

Understanding the TCA cycle is vital for comprehending cellular metabolism and energy production, with implications in biochemistry, physiology, and medicine.

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Description

This quiz covers Lecture 19 of the Biochemistry and Molecular Biology course, focusing on the TCA cycle, also known as the citric acid or Krebs cycle. It explores major catabolic pathways including glycolysis and the pentose phosphate pathway, as well as b-oxidation.