Fatty Acid Catabolism II Summary

Lecture Overview

Lecture 32

• Focus on fatty acid catabolism, specifically β-oxidation.

Learning Objectives

• Understand lower ATP yield in β-oxidation of unsaturated fatty acids vs. saturated.

• Recognize that human fatty acids can't convert to glucose.

• Identify metabolic fate of odd-chain fatty acids.

• Know three ketone bodies and their production organ; understand their role during fasting.

• Explain ketosis susceptibility in children and type I diabetics; why the liver can't utilize ketone bodies.

β-oxidation Mechanisms

• Monounsaturated Fatty Acids:

  • cis-Δ3 bond can't proceed in β-oxidation; converted to trans-Δ2 by enoyl-CoA isomerase.

• Polyunsaturated Fatty Acids:

  • β-oxidation continues until encountering cis-Δ3; further modification required since dienoyl-CoA isn't a substrate for hydratase.

  • A reductase reduces unwanted double bonds before proceeding.

Odd-chain Fatty Acid Breakdown

• Odd-chain FA produce propionyl-CoA at final stage of β-oxidation.

• Propionyl-CoA converts to succinyl-CoA via three enzyme-catalyzed steps:

  1. Propionyl-CoA Carboxylase: Requires biotin.

  2. Methylmalonyl-CoA Epimerase: Converts D to L form.

  3. Methylmalonyl-CoA Mutase: Requires vitamin B12, redistributes carboxylate group.

Coenzyme B12 Importance

• B12 plays critical roles in methylmalonyl-CoA mutase activity; deficiency leads to pernicious anemia, affecting absorption and causing various health issues.

Peroxisomal vs Mitochondrial Fatty Acid Oxidation

• Peroxisomes: Handle very-long-chain FA (VLCA); produce H2O2 instead of ATP initially.

• Mitochondria: β-oxidation produces FADH2 for ATP generation.

Diseases Related to Fatty Acid Oxidation

• Zellweger syndrome (peroxisome deficiency).

• X-linked adrenoleukodystrophy (defect in VLCFA import).

Fatty Acids and Glucose Metabolism

• Fatty acids are not glucogenic; cannot convert acetate (from acetyl-CoA) to glucose in animals.

Ketone Bodies in Energy Production

• Produced during fasting; involve conversion of excess acetyl-CoA in the liver into acetoacetate and β-hydroxybutyrate.

• Important energy sources for tissues under energy deficit.

Children's Susceptibility to Ketosis

• Children achieve fasting metabolic state quickly, leading to higher ketone body levels during starvation.

Summary Points

• Animals convert fatty acids to ketone bodies rather than glucose.

• Ketone bodies serve as an alternative energy source during fasting or energetic deficits, playing a critical role in metabolic balance.

Lecture Overview

Lecture 32

• Focus on fatty acid catabolism, specifically the intricate mechanisms of β-oxidation, and the subsequent metabolic fates of fatty acid products.

Learning Objectives

• Understand the reasons for the lower ATP yield encountered during the β-oxidation of unsaturated fatty acids compared to saturated fatty acids.

• Recognize the metabolic constraint that human fatty acids cannot be directly converted to glucose.

• Identify the specific metabolic fate and pathway for odd-chain fatty acids.

• Know the three primary ketone bodies, the organ responsible for their production, and their crucial role as an alternative energy source during periods of fasting or metabolic stress.

• Explain the physiological basis for increased ketosis susceptibility in children and individuals with type I diabetes, and understand why the liver, despite producing ketone bodies, cannot utilize them for its own energy needs.

β-oxidation Mechanisms

• Monounsaturated Fatty Acids:

  • The presence of a cis-Δ3 double bond, common in naturally occurring unsaturated fatty acids, poses a challenge as it cannot directly proceed through the standard enoyl-CoA hydratase step of β-oxidation. Enoyl-CoA hydratase acts only on trans double bonds.

  • To overcome this, the enzyme enoyl-CoA isomerase catalyzes the conversion of the cis-Δ3 double bond to a trans-Δ2 bond, which is a suitable substrate for enoyl-CoA hydratase. This modification ensures the fatty acid can continue through the β-oxidation spiral, but bypasses one step of FADH2 production, leading to a slightly lower ATP yield.

• Polyunsaturated Fatty Acids:

  • In the case of polyunsaturated fatty acids (PUFAs), β-oxidation proceeds normally until it encounters a double bond. If a cis-Δ3 bond is encountered, enoyl-CoA isomerase handles it as described above.

  • However, if the double bond is encountered at a different position (e.g., cis-Δ4), or if two consecutive double bonds arise from further rounds of β-oxidation (e.g., a trans-Δ2, cis-Δ4 dienoyl-CoA), further enzymatic modifications are required. The key is that dienoyl-CoA is not a substrate for enoyl-CoA hydratase.

  • A 2,4-dienoyl-CoA reductase is required to reduce one of the double bonds (using NADPH), typically converting a trans-Δ2, cis-Δ4 dienoyl-CoA into a trans-Δ3-enoyl-CoA. This intermediate is then acted upon by enoyl-CoA isomerase to shift it to a trans-Δ2 bond, allowing β-oxidation to continue. This additional step also reduces the overall ATP yield due to the consumption of NADPH and the bypass of certain steps.

Odd-chain Fatty Acid Breakdown

• Odd-chain fatty acids, upon successive rounds of β-oxidation, ultimately produce a 3-carbon molecule called propionyl-CoA at the final stage, alongside acetyl-CoA units. This is because the chain length cannot be perfectly divided into 2-carbon units.

• Propionyl-CoA is not directly usable in common metabolic pathways and must be converted to succinyl-CoA, a tricarboxylic acid (TCA) cycle intermediate, to enter central metabolism. This conversion occurs via three distinct enzyme-catalyzed steps:

  1. Propionyl-CoA Carboxylase: This mitochondrial enzyme, requiring biotin (a vitamin B7 derivative) as a cofactor and ATP, catalyzes the ATP-dependent carboxylation of propionyl-CoA to $D extit{-methylmalonyl-CoA}$.

  2. Methylmalonyl-CoA Epimerase: This enzyme interconverts the stereoisomers $D extit{-methylmalonyl-CoA}$ to $L extit{-methylmalonyl-CoA}$, which is the substrate for the next enzyme.

  3. Methylmalonyl-CoA Mutase: This enzyme, which critically requires coenzyme B12 (specifically deoxyadenosylcobalamin), catalyzes a complex intramolecular rearrangement of $L extit{-methylmalonyl-CoA}$ to succinyl-CoA. The succinyl-CoA can then enter the TCA cycle, making odd-chain fatty acids weakly glucogenic (as succinyl-CoA can be converted to oxaloacetate, a glucose precursor).

Coenzyme B12 Importance

• Vitamin B12 (cobalamin) plays pivotal roles in various metabolic pathways, particularly as a cofactor for methylmalonyl-CoA mutase and methionine synthase.

• A deficiency in B12 can lead to serious health issues, most notably pernicious anemia, which is characterized by defective red blood cell formation due to impaired DNA synthesis. This often stems from an inability to absorb dietary B12, frequently caused by a lack of intrinsic factor (a glycoprotein secreted by gastric parietal cells essential for B12 absorption in the ileum).

• Beyond anemia, B12 deficiency can also cause severe neurological symptoms, including peripheral neuropathy, cognitive impairment, and ataxia, primarily due to the accumulation of methylmalonic acid and propionic acid within the body, which interferes with myelin synthesis and neuronal function.

Peroxisomal vs Mitochondrial Fatty Acid Oxidation

• Peroxisomes: These organelles are particularly important for handling very-long-chain fatty acids (VLCFAs, typically >20 carbons) and branched-chain fatty acids. Unlike mitochondrial β-oxidation, the initial step in peroxisomes, catalyzed by acyl-CoA oxidase, generates $H<em>{2}O</em>{2}$ (hydrogen peroxide) instead of directly producing FADH2 for ATP generation. The $H<em>{2}O</em>{2}$ is then immediately degraded by catalase. Peroxisomes shorten VLCFAs until they are of a length suitable for further oxidation in the mitochondria, where the bulk of ATP is generated.

• Mitochondria: The primary site for the complete β-oxidation of most fatty acids. Here, each round of β-oxidation directly produces FADH2 and NADH, which feed into the electron transport chain to generate a substantial amount of ATP through oxidative phosphorylation.

Diseases Related to Fatty Acid Oxidation

• Zellweger syndrome: A severe, often fatal, autosomal recessive disorder characterized by the absence or reduction of functional peroxisomes, leading to the accumulation of VLCFAs and other peroxisomal substrates in multiple organs. It results in profound neurological dysfunction, developmental delays, and liver, kidney, and bone abnormalities.

• X-linked adrenoleukodystrophy (X-ALD): An X-linked recessive genetic disorder caused by mutations in the ABCD1 gene, which encodes a peroxisomal membrane transporter responsible for importing VLCFAs into peroxisomes. This defect leads to the accumulation of VLCFAs in tissues, particularly in the brain and adrenal glands, causing demyelination, adrenal insufficiency, and progressive neurological impairment.

Fatty Acids and Glucose Metabolism

• A critical point in animal metabolism is that fatty acids are not glucogenic; they cannot be converted into glucose. Although fatty acids are catabolized to acetyl-CoA, and acetyl-CoA enters the TCA cycle, the two carbons from acetyl-CoA are ultimately released as $CO_{2}$ in the TCA cycle. The pyruvate dehydrogenase complex reaction, which converts pyruvate to acetyl-CoA, is irreversible in animals, meaning there is no net pathway to convert acetyl-CoA back to pyruvate or other gluconeogenic precursors.

• This contrasts with plants and some microorganisms, which possess the glyoxylate cycle, allowing them to convert acetyl-CoA into four-carbon intermediates that can then be used for glucose synthesis.

Ketone Bodies in Energy Production

• Ketone bodies are water-soluble molecules produced in the liver during prolonged fasting, starvation, very low-carbohydrate diets, or uncontrolled diabetes. They serve as an essential alternative fuel source for various extrahepatic tissues when glucose availability is limited.

• The liver converts excess acetyl-CoA (derived primarily from fatty acid β-oxidation) into three ketone bodies through a process called ketogenesis in the mitochondrial matrix:

  1. Acetoacetate: The primary ketone body formed from the condensation of two acetyl-CoA molecules to acetoacetyl-CoA, followed by further steps involving HMG-CoA synthase and HMG-CoA lyase. Acetoacetate can then either be directly used or further metabolized.

  2. D extit{-eta-hydroxybutyrate}: Acetoacetate can be reversibly reduced to D extit{-eta-hydroxybutyrate} by D extit{-eta-hydroxybutyrate} dehydrogenase, primarily using NADH.

  3. Acetone: Acetoacetate can spontaneously decarboxylate to form acetone, a volatile compound that is largely exhaled. It is not metabolically utilized.

• These ketone bodies are released into the bloodstream and can be transported to extrahepatic tissues (e.g., heart, skeletal muscle, brain, kidney cortex). In these tissues, acetoacetate and D extit{-eta-hydroxybutyrate} are converted back into acetyl-CoA (via D extit{-eta-hydroxybutyrate} dehydrogenase and $$eta$extit{-ketoacyl-CoA transferase}$, also known as succinyl-CoA:3-oxoacid CoA transferase or SCOT) and then oxidized in the TCA cycle to generate ATP. The brain, in particular, adapts to utilize ketone bodies as a major fuel source during prolonged fasting, significantly reducing its reliance on glucose.

Children's Susceptibility to Ketosis

• Children are more susceptible to developing ketosis and ketoacidosis during periods of fasting or illness compared to adults. This is primarily due to several physiological factors:

  • Smaller glycogen stores: Children have relatively smaller hepatic glycogen reserves than adults, which are depleted more rapidly during fasting.

  • Higher metabolic rate: Their higher metabolic rate relative to their body mass means they consume energy (and thus glycogen) faster.

  • Rapid transition to fat metabolism: These factors cause children to transition rapidly from glucose utilization to fatty acid oxidation and ketone body production as their primary energy source, potentially leading to higher circulating ketone body levels more quickly.

Summary Points

• In animals, fatty acids are primarily converted to ketone bodies, which serve as an alternative energy source during states of energetic deficit, rather than being converted to glucose.

• Ketone bodies, particularly acetoacetate and D extit{-eta-hydroxybutyrate}, play a critical role in metabolic balance by providing fuel to extrahepatic tissues, including the brain, when glucose is scarce.

• The liver, the primary site of ketone body synthesis, cannot utilize ketone bodies for its own energy needs because it lacks the enzyme $\beta$ extit{-ketoacyl-CoA transferase}$$ (SCOT), which is essential for converting acetoacetate back to acetoacetyl-CoA.