Lecture 18 pt2

Acetyl CoA Transport to Cytosol

• Location of Acetyl CoA Production

  • Acetyl CoA is synthesized within the mitochondrial matrix.

• Need for Cytosolic Acetyl CoA

  • Acetyl CoA must be transported to the cytosol for fatty acid synthesis.

  • The transport process is energetically costly, requiring ATP.

  • Generally occurs under energy-rich conditions with surplus ATP.

• Transport Mechanism

  • Since there is no direct transporter for acetyl CoA in the inner mitochondrial membrane, an indirect method is required.

  • The overall mechanism is divided into two key parts: shuttling of acetyl CoA (above the line) and regenerating oxaloacetate (below the line).

Shuttling Acetyl CoA

• Formation of Citrate

  • Acetyl CoA combines with oxaloacetate to form citrate (first step of the TCA cycle).

  • This reaction utilizes the enzyme citrate synthase.

• Transport of Citrate

  • Citrate is transported from the mitochondria to the cytosol using a citrate transporter.

• Regeneration of Acetyl CoA and Oxaloacetate

  • In the cytosol, citrate is converted back into oxaloacetate and acetyl CoA using the enzyme citrate lyase.

  • Conversion costs 1 ATP:

  • The hydrolysis of ATP provides the energy to form the acetyl-CoA bond, making it a high-energy process.

Regeneration of Oxaloacetate

• Necessity of Regeneration

  • Continuous transport and conversion require a system to regenerate oxaloacetate.

  • The lack of a direct oxaloacetate transporter in the mitochondrial membrane necessitates an alternative route to regenerate it.

• Pathway to Regenerate Oxaloacetate

  • Conversion to Malate:

  • In the cytosol, oxaloacetate is converted to malate by the enzyme malate dehydrogenase. This consumes an NADH.

  • Transport of Malate:

  • Malate is then transported back into the mitochondrial matrix via the malate-alpha-ketoglutarate transporter.

  • Regeneration of Oxaloacetate:

  • In the matrix, malate is converted back to oxaloacetate by the same enzyme (malate dehydrogenase), which regenerates NADH.

• Alternative Pathway

  • An alternative pathway is used when generating NADPH is prioritized.

  • Malate to Pyruvate:

  • It can be converted to pyruvate via the enzyme malic enzyme, which generates NADPH through an oxidative decarboxylation reaction.

  • This pathway is utilized when there is a significant requirement for NADPH for fatty acid synthesis.

  • Pyruvate then enters the matrix where it is converted back to oxaloacetate using pyruvate carboxylase.

• Summary of Costs and Benefits

  • The overall process of shuttling acetyl CoA costs approximately 2 ATP and regenerates NADPH, which is crucial for fatty acid synthesis.

Sources of NADPH

• NADPH Production Mechanisms

  • Various pathways produce NADPH:

  • Pentose phosphate pathway.

  • Malic enzyme pathway.

  • The sources are estimated to provide NADPH roughly equally.

Fatty Acid Synthesis Specifics

• Role of Acetyl CoA Carboxylase

  • Converting acetyl CoA to malonyl CoA employs the enzyme acetyl CoA carboxylase (ACC).

  • Alternative substrates like propionyl CoA can also be used in this reaction, generating ethylphenyl CoA.

• Palmitate and Further Fatty Acid Synthesis

  • Fatty acid synthase primarily produces palmitate (16 carbons).

  • Extension of fatty acids occurs in the endoplasmic reticulum and mitochondria, typically adding two carbon units at a time.

• Common Fatty Acids

  • Stearate (18 carbons) is the most common long-chain fatty acid produced via elongation systems inside the body.

  • Other enzymes are involved in producing shorter fatty acids when needed, although fatty acid synthase consistently produces palmitate.

Cholesterol Synthesis Overview

• Cholesterol Importance

  • While often viewed negatively due to association with heart disease, cholesterol is essential for maintaining membrane fluidity and synthesizing steroid hormones.

  • It can be synthesized entirely from acetyl CoA.

• Key Intermediates

  • The early stages of cholesterol synthesis involve isoprenes and terpenes, derived from HMG CoA.

Pathway of Cholesterol Synthesis

• HMG CoA to Mevalonate

  • Conversion of HMG CoA to mevalonate is catalyzed by HMG CoA reductase, consuming two NADPH molecules.

  • Hydrolysis of HMG CoA releases CoA from mevalonate.

• Formation of Isoprenes

  • Mevalonate is phosphorylated to forms isopentenyl pyrophosphate (IPP). The pathway includes:

  1. Phosphate addition: Converts mevalonate to its active phosphate derivatives.

  2. Decarboxylation: Produces isoprenes upon loss of carbon dioxide.

• Condensation Reactions

  • Two IPP molecules condense to form dimethylallyl pyrophosphate (DMAPP).

  • Subsequent condensation reactions lead to longer carbon chains (geranyl and farnesyl pyrophosphates).

• Squalene Synthesis

  • Squalene is synthesized from farnesyl diphosphate and undergoes further conversions to eventually yield cholesterol via several intermediates.

Cholesterol Derivatives and Medical Implications

• Steroid Hormone Synthesis

  • Steroid hormones derive from cholesterol, sharing a common four-ring structure, with additional modifications.

• Medical Concerns with Cholesterol

  • High levels of LDL cholesterol can lead to arteries being clogged by atherosclerotic plaques, resulting in heart attacks and strokes.

  • Statins like lovastatin inhibit HMG CoA reductase, lowering cholesterol production, improving health outcomes, but can cause side effects such as muscle pain.

• Regulatory Measures on Cholesterol Synthesis

  • Statins serve to reduce cholesterol levels effectively, yet must be managed carefully due to potential adverse effects.

Conclusion

• Complex interplay of metabolic processes

  • The need for cellular energy balances the cost of synthesizing vital biochemical components like fatty acids and cholesterol.

  • Understanding the interconnected pathways governing metabolism is crucial for both biological comprehension and medical applications.

Acetyl CoA Transport to Cytosol

• Location of Acetyl CoA Production

  • Acetyl CoA is primarily synthesized within the mitochondrial matrix through several pathways, including the oxidative decarboxylation of pyruvate by pyruvate dehydrogenase and the beta-oxidation of fatty acids.

• Need for Cytosolic Acetyl CoA

  • Acetyl CoA must be transported from the mitochondria to the cytosol because fatty acid synthesis, a major anabolic pathway, exclusively occurs in the cytosol.

  • This transport process is energetically costly, requiring direct or indirect expenditure of ATP, reflecting its role in energy-rich conditions.

  • It generally occurs under conditions of energy surplus, where there is an abundance of ATP and glucose, signaling the cell to store energy as fat.

• Transport Mechanism

  • There is no direct transporter in the inner mitochondrial membrane for acetyl CoA itself, necessitating an indirect shuttling mechanism.

  • The overall mechanism is precisely orchestrated and divided into two key, interconnected parts for clarity: the shuttling of acetyl CoA out of the mitochondria and the subsequent regeneration of oxaloacetate back into the mitochondria.

Shuttling Acetyl CoA

• Formation of Citrate

  • In the mitochondrial matrix, acetyl CoA (a 2-carbon unit) combines with oxaloacetate (a 4-carbon intermediate) to form citrate (a 6-carbon molecule).

  • This irreversible reaction is the first step of the tricarboxylic acid (TCA) cycle and is catalyzed by the mitochondrial enzyme citrate synthase.

  • The formation of citrate effectively links carbohydrate (via pyruvate) and fat metabolism, as citrate serves as a crucial intermediate for both energy generation and biosynthesis.

• Transport of Citrate

  • Citrate is then transported from the mitochondrial matrix to the cytosol via a dedicated citrate transporter (also known as the tricarboxylate transporter or citrate-malate shuttler) located in the inner mitochondrial membrane.

• Regeneration of Acetyl CoA and Oxaloacetate

  • Once in the cytosol, citrate is cleaved back into oxaloacetate and acetyl CoA by the cytosolic enzyme ATP-citrate lyase (often referred to simply as citrate lyase).

  • This conversion directly costs 1 ATP molecule: the hydrolysis of ATP to ADP and inorganic phosphate ($ATP \rightarrow ADP + P_i$) provides the energy required to cleave citrate and form the high-energy thioester bond of cytosolic acetyl CoA.

Regeneration of Oxaloacetate

• Necessity of Regeneration

  • For the continuous shuttling of acetyl CoA, it is critical to regenerate oxaloacetate and return it to the mitochondrial matrix, as oxaloacetate is recycled by the shuttle and not consumed.

  • Similar to acetyl CoA, there is no direct transporter for oxaloacetate across the inner mitochondrial membrane, thus requiring an indirect pathway for its regeneration in the matrix.

• Pathway to Regenerate Oxaloacetate

  • Conversion to Malate:

    • In the cytosol, oxaloacetate is converted to malate by the cytosolic enzyme malate dehydrogenase. This reaction consumes one molecule of NADH ($NADH \rightarrow NAD^+$).

  • Transport of Malate:

    • Malate is then transported back into the mitochondrial matrix via the malate-alpha-ketoglutarate transporter (which is also the tricarboxylate transporter).

  • Regeneration of Oxaloacetate:

    • In the mitochondrial matrix, malate is converted back to oxaloacetate by the mitochondrial malate dehydrogenase, which regenerates NADH ($NAD^+ \rightarrow NADH$).

    • This pathway effectively recycles oxaloacetate, maintaining the continuous operation of the citrate shuttle while also interconverting cytosolic and mitochondrial NADH.

• Alternative Pathway

  • An alternative pathway for oxaloacetate regeneration is utilized, especially when generating NADPH for reductive biosynthesis (e.g., fatty acid synthesis) is prioritized over NADH production.

  • Malate to Pyruvate:

    • In this alternative, cytosolic malate (instead of being transported back directly) is converted to pyruvate via the cytosolic enzyme malic enzyme.

    • This reaction is an oxidative decarboxylation, generating one molecule of NADPH from $NADP^+$ ($NADP^+ \rightarrow NADPH$) and releasing carbon dioxide ($CO_2$) in the process.

  • Pyruvate Entry and Conversion:

    • Pyruvate then enters the mitochondrial matrix via a specific pyruvate transporter in the inner mitochondrial membrane.

    • Inside the matrix, pyruvate is converted back to oxaloacetate by the enzyme pyruvate carboxylase.

    • This reaction requires the input of one ATP molecule ($ATP \rightarrow ADP + P_i$) and is activated by high levels of acetyl CoA, signaling a need to replenish oxaloacetate for both the TCA cycle and further citrate shuttling.

• Summary of Costs and Benefits

  • The overall process of shuttling acetyl CoA to the cytosol and regenerating oxaloacetate via the malic enzyme pathway (which also produces NADPH) costs approximately 2 ATP molecules per acetyl CoA transported (1 ATP for citrate lyase, 1 ATP for pyruvate carboxylase) and efficiently regenerates NADPH, which is crucial for reducing power in fatty acid synthesis.

Sources of NADPH

• NADPH Production Mechanisms

  • NADPH is a vital reducing agent for various biosynthetic pathways, including fatty acid and cholesterol synthesis, and for maintaining cellular redox balance against oxidative stress.

  • Several pathways produce NADPH within the cell:

    • The pentose phosphate pathway (also known as the hexose monophosphate shunt) is a primary cytosolic source, particularly in adipose tissue and the liver.

    • The malic enzyme pathway (as described above) is another significant cytosolic source, directly coupled to the acetyl CoA shuttle.

  • The contributions from these sources are estimated to provide NADPH in roughly equal proportions depending on the cell type and metabolic state.

Fatty Acid Synthesis Specifics

• Role of Acetyl CoA Carboxylase

  • The committed and rate-limiting step in fatty acid synthesis is the conversion of acetyl CoA to malonyl CoA.

  • This carboxylation reaction is catalyzed by the cytosolic enzyme acetyl CoA carboxylase (ACC), which requires biotin as a prosthetic group and consumes one molecule of ATP ($ATP \rightarrow ADP + P_i$).

  • ACC is a major point of regulation for fatty acid synthesis, being activated by citrate and insulin and inhibited by acyl-CoAs.

  • While acetyl CoA is the primary substrate, alternative substrates like propionyl CoA (a 3-carbon unit from odd-chain fatty acid breakdown or certain amino acids) can also be used, leading to the synthesis of odd-chain fatty acids.

• Palmitate and Further Fatty Acid Synthesis

  • The primary product of fatty acid synthase, a multi-enzyme complex found in the cytosol, is palmitate (a 16-carbon saturated fatty acid).

  • Further extension of fatty acids (elongation) beyond 16 carbons occurs primarily in the endoplasmic reticulum and, to a lesser extent, in the mitochondria, typically by adding two-carbon units at a time from malonyl CoA.

• Common Fatty Acids

  • Stearate (an 18-carbon saturated fatty acid) is the most common long-chain fatty acid produced in the body via these elongation systems.

  • While fatty acid synthase consistently produces palmitate, other elongase and desaturase enzymes are involved in creating a diverse range of fatty acids, including unsaturated ones, to meet specific cellular needs.

Cholesterol Synthesis Overview

• Cholesterol Importance

  • Cholesterol, while often associated with negative health outcomes, is an absolutely essential lipid for various biological functions.

  • It is crucial for maintaining the fluidity and structural integrity of biological membranes, serves as a precursor for the synthesis of all steroid hormones (e.g., testosterone, estrogen, cortisol, aldosterone), bile acids (for fat digestion), and vitamin D.

  • It can be synthesized entirely within the body from simple two-carbon acetyl CoA units, highlighting its anabolic significance.

• Key Intermediates

  • The early stages of cholesterol synthesis involve the formation of isoprene units (5-carbon building blocks) and subsequently terpenes (molecules composed of multiple isoprene units), all derived from HMG CoA (hydroxymethylglutaryl-CoA).

Pathway of Cholesterol Synthesis

• HMG CoA to Mevalonate

  • The conversion of HMG CoA (a 6-carbon molecule) to mevalonate (also 6-carbon) is a crucial, committed, and rate-limiting step in cholesterol synthesis.

  • This reaction is catalyzed by the enzyme HMG CoA reductase, which is located in the endoplasmic reticulum membrane and consumes two NADPH molecules ($2 NADPH \rightarrow 2 NADP^+$).

  • This step is a major target for pharmacological regulation and feedback inhibition by cholesterol itself.

  • The hydrolysis of HMG CoA also releases CoA, as mevalonate does not contain CoA.

• Formation of Isoprenes

  • Mevalonate undergoes subsequent phosphorylation steps, consuming ATP ($ATP \rightarrow ADP + P_i$), to convert it into its active, high-energy derivatives.

  • This is followed by a decarboxylation reaction (loss of $CO_2$) to produce isopentenyl pyrophosphate (IPP), the activated 5-carbon isoprene unit.

• Condensation Reactions

  • Two molecules of IPP are isomerized and then condense to form dimethylallyl pyrophosphate (DMAPP), another 5-carbon isoprene unit.

  • Subsequent condensation reactions involving IPP and DMAPP, as well as their elongated products, lead to the formation of longer carbon chains:

    • IPP + DMAPP $\rightarrow$ Geranyl pyrophosphate (C10)

    • Geranyl pyrophosphate + IPP $\rightarrow$ Farnesyl pyrophosphate (C15)

• Squalene Synthesis

  • Two molecules of farnesyl pyrophosphate (C15 each) then condense head-to-head to form squalene, a 30-carbon linear hydrocarbon.

  • Squalene subsequently undergoes a complex series of cyclization and modification reactions involving several intermediates (e.g., lanosterol) to eventually yield the 27-carbon cholesterol molecule.

Cholesterol Derivatives and Medical Implications

• Steroid Hormone Synthesis

  • All steroid hormones, including glucocorticoids (e.g., cortisol), mineralocorticoids (e.g., aldosterone), and sex hormones (e.g., testosterone, estradiol), are synthesized from cholesterol.

  • They all share a common four-ring cyclopentanoperhydrophenanthrene structure, with specific modifications dictating their diverse biological activities.

• Medical Concerns with Cholesterol

  • While essential, dysregulation of cholesterol metabolism, particularly high levels of low-density lipoprotein (LDL) cholesterol (often termed “bad cholesterol” due to its role in transporting cholesterol from the liver to peripheral tissues), can lead to serious health issues.

  • High LDL levels contribute to the formation and buildup of atherosclerotic plaques in arteries, which can narrow blood vessels, impede blood flow, and result in life-threatening events such as heart attacks and strokes.

  • Conversely, high-density lipoprotein (HDL) cholesterol (”good cholesterol”) is beneficial as it mediates reverse cholesterol transport, removing excess cholesterol from peripheral tissues and returning it to the liver.

• Regulatory Measures on Cholesterol Synthesis

  • Statins, such as lovastatin, are a class of drugs widely prescribed to lower cholesterol levels.

  • They function by acting as competitive inhibitors of HMG CoA reductase, thereby significantly reducing the rate-limiting step of cholesterol production in the liver.

  • While highly effective in improving cardiovascular health outcomes, statins must be managed carefully due to potential adverse effects, including muscle pain (myopathy) and liver enzyme elevation.

  • Cholesterol synthesis is also highly regulated by dietary intake and cellular demand through complex feedback mechanisms, including the sterol regulatory element-binding protein (SREBP) pathway.

Conclusion

• Complex interplay of metabolic processes

  • The synthesis, transport, and regulation of vital biochemical components like fatty acids and cholesterol involve an intricate and tightly controlled interplay of various metabolic pathways.

  • These processes are finely tuned to balance the cellular need for energy storage and structural components against the metabolic cost of their synthesis. A comprehensive understanding of these interconnected pathways is crucial for both fundamental biological comprehension and the development of effective medical applications and interventions for metabolic disorders.