TCA cycle and Fatty Acid Synthesis

What is the overall purpose of the TCA cycle?

The TCA cycle oxidizes acetyl-CoA to CO₂ and captures the released energy as NADH, FADH₂, and GTP.

What are the products of one turn of the TCA cycle per acetyl-CoA?

3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂.

How does the TCA cycle ultimately use the NADH and FADH₂ it produces?

NADH and FADH₂ donate electrons to the electron transport chain, where their reducing equivalents are used to generate ATP through oxidative phosphorylation.

Why does the TCA cycle depend on oxygen even though it does not directly use O₂?

O₂ is required to reoxidize NADH and FADH₂ through the electron transport chain. Without O₂, NADH and FADH₂ accumulate and TCA-cycle activity decreases.

Which two TCA-cycle reactions produce CO₂?

Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

Which TCA-cycle reaction produces GTP?

Succinyl-CoA synthetase.

Which TCA-cycle reactions produce NADH?

Isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase.

Which TCA-cycle reaction produces FADH₂?

Succinate dehydrogenase.

What are the three major regulatory enzymes of the TCA cycle?

Citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase.

What general signals activate the TCA cycle?

Low-energy signals such as ADP and Ca²⁺ activate TCA-cycle activity.

What general signals inhibit the TCA cycle?

High-energy/reduced-state signals such as ATP and NADH inhibit TCA-cycle activity.

Why does Ca²⁺ increase TCA-cycle activity during exercise?

Ca²⁺ signals increased muscle activity and ATP demand, so it activates oxidative metabolism to increase energy production.

How does the TCA cycle respond to a high-energy state?

High ATP and NADH inhibit key regulatory enzymes, decreasing TCA-cycle activity because the cell has less need for additional energy production.

How does the TCA cycle differ between the fed and fasted states in the liver?

During fasting, TCA activity decreases because oxaloacetate is diverted toward gluconeogenesis and acetyl-CoA is diverted toward ketone-body synthesis.

Why is the TCA cycle considered amphibolic?

It has both catabolic and anabolic roles: it oxidizes acetyl-CoA for energy while also supplying intermediates for biosynthetic pathways.

How does citrate connect the TCA cycle to fatty acid synthesis?

Citrate can leave the mitochondria during the fed state and provide acetyl-CoA in the cytosol for fatty acid synthesis.

How can TCA intermediates become depleted?

Intermediates can be removed from the cycle and used as precursors for biosynthesis, such as amino acids, glucose, fatty acids, and heme.

What are anaplerotic reactions?

Reactions that replenish TCA-cycle intermediates that have been removed for other metabolic pathways.

What enzyme replenishes oxaloacetate from pyruvate?

Pyruvate carboxylase converts pyruvate to oxaloacetate.

How can amino acid metabolism replenish TCA-cycle intermediates?

For example, glutamate can form α-ketoglutarate, aspartate can form oxaloacetate, and valine/isoleucine can contribute to succinyl-CoA.

What happens to TCA-cycle activity when oxaloacetate is diverted toward gluconeogenesis during prolonged fasting?

Less oxaloacetate is available to combine with acetyl-CoA, decreasing TCA-cycle activity and favoring ketone-body production.

What enzyme links glycolysis to the TCA cycle?

The pyruvate dehydrogenase (PDH) complex.

What reaction does the PDH complex catalyze?

Pyruvate → acetyl-CoA + CO₂ while producing NADH.

Why is the PDH reaction considered irreversible?

PDH catalyzes a strongly exergonic reaction, so acetyl-CoA cannot simply be converted back into pyruvate through PDH.

What are the five cofactors/coenzymes required by PDH?

TPP, lipoic acid, CoA, FAD, and NAD⁺.

Which vitamin provides TPP for PDH?

Thiamine (vitamin B1).

Which vitamin provides FAD for PDH?

Riboflavin (vitamin B2).

Which vitamin provides NAD⁺ for PDH?

Niacin (vitamin B3).

Which vitamin provides CoA for PDH?

Pantothenic acid (vitamin B5).

What happens to PDH when it is phosphorylated?

PDH becomes inactive.

What happens to PDH when it is dephosphorylated?

PDH becomes active.

How can vitamin B1 deficiency affect TCA-cycle function?

B1 deficiency decreases TPP availability, impairing PDH activity and slowing the conversion of pyruvate to acetyl-CoA, which decreases acetyl-CoA entry into the TCA cycle.

How can vitamin B2 deficiency affect TCA-cycle function?

B2 deficiency decreases FAD availability, impairing PDH and other FAD-dependent oxidative reactions and reducing oxidative metabolism.

What happens to pyruvate when PDH activity is impaired?

More pyruvate is converted to lactate, contributing to increased lactate production.

What is the clinical significance of severe thiamine deficiency?

It can impair energy metabolism and is associated with conditions such as wet beriberi and Wernicke-Korsakoff syndrome.

What is the basic structure of a fatty acid?

A fatty acid consists of an alkyl hydrocarbon chain with a terminal carboxyl group.

What is the difference between a saturated and unsaturated fatty acid?

A saturated fatty acid has no carbon-carbon double bonds, whereas an unsaturated fatty acid contains one or more double bonds.

What does a notation such as 18:2 mean for a fatty acid?

It means the fatty acid contains 18 carbons and 2 double bonds.

What is characteristic about the double bonds in most naturally occurring unsaturated fatty acids?

They are usually in the cis configuration, creating a kink in the hydrocarbon chain.

Why can't mitochondrial acetyl-CoA directly enter the cytosol for fatty acid synthesis?

Acetyl-CoA cannot cross the mitochondrial membrane directly.

How is mitochondrial acetyl-CoA transported to the cytosol for fatty acid synthesis?

Acetyl-CoA combines with oxaloacetate to form citrate; citrate is transported to the cytosol and then cleaved by citrate lyase to regenerate acetyl-CoA and oxaloacetate.

What is the role of citrate lyase in fatty acid synthesis?

Citrate lyase regenerates cytosolic acetyl-CoA from citrate, providing the acetyl-CoA needed for fatty acid synthesis.

What is the role of malic enzyme in fatty acid synthesis?

Malic enzyme converts malate to pyruvate while generating NADPH, providing reducing power for fatty acid synthesis.

Where does fatty acid synthesis occur?

In the cytosol, primarily in the liver during the fed state.

What is the rate-limiting enzyme of fatty acid synthesis?

Acetyl-CoA carboxylase (ACC).

What reaction does acetyl-CoA carboxylase catalyze?

Acetyl-CoA + CO₂ → malonyl-CoA.

What does acetyl-CoA carboxylase require?

ATP, biotin (vitamin B7), and CO₂.

Why is malonyl-CoA important in fatty acid synthesis?

Malonyl-CoA provides the two-carbon units used to elongate the growing fatty acid chain.

What are the major stages of the fatty acid synthase cycle?

Loading → condensation → reduction → dehydration → reduction.

What reducing agent is required during fatty acid synthesis?

NADPH.

What is the major product of fatty acid synthase?

Palmitate, a saturated 16-carbon fatty acid (16:0).

Why is palmitate considered the starting point for many other fatty acids?

Other fatty acids can be produced by modifying palmitate through elongation and/or desaturation.

Where does elongation of fatty acids beyond 16 carbons primarily occur?

In the smooth endoplasmic reticulum.

What happens during fatty acid elongation?

Two-carbon units are sequentially added to fatty acyl-CoA molecules using malonyl-CoA and NADPH.

What happens when palmitate undergoes one round of elongation?

Palmitate (C16:0) is converted to stearate (C18:0).

Where does fatty acid desaturation occur?

In the endoplasmic reticulum.

What enzyme introduces double bonds into fatty acids?

Fatty acyl-CoA desaturases.

What does fatty acid desaturation require?

O₂, NADH, and cytochrome b5.

Why are some fatty acids considered essential?

Human desaturases cannot introduce double bonds beyond carbon 9, so certain fatty acids must be obtained from the diet.

Which essential fatty acids are specifically identified in the lecture?

Linoleic acid and linolenic acid.

What is arachidonic acid derived from?

Linoleic acid can be elongated and further desaturated to produce arachidonic acid.

What is arachidonic acid important for?

It is a precursor for eicosanoids, which act as short-lived local signaling molecules involved in processes such as inflammation, fever, pain, and reproductive function.

What are triglycerides synthesized from?

Glycerol-3-phosphate and three fatty acyl-CoA molecules.

What is phosphatidic acid's role in triglyceride synthesis?

The first two fatty acids are attached to glycerol-3-phosphate to form phosphatidic acid, which is an intermediate in triglyceride synthesis.

Where are triglycerides synthesized?

In the liver and adipose tissue, primarily associated with the endoplasmic reticulum.

What is the major purpose of triglyceride synthesis in adipose tissue?

To store energy as triglycerides that can later be mobilized when energy is needed.

What is an important purpose of triglyceride synthesis in the liver?

The liver can store triglycerides and package them into VLDL particles for export.

What is a key difference in glycerol-3-phosphate production between liver and adipose tissue?

The liver expresses glycerol kinase and can use free glycerol to make glycerol-3-phosphate, whereas adipose tissue primarily obtains glycerol-3-phosphate from glucose metabolism.