Questions and Answers
Which enzyme is responsible for the cleavage of β-HMG-CoA to form acetoacetate?
In which part of the cell does ketogenesis primarily occur?
Which statement regarding the utilization of ketone bodies is correct?
What is the concentration of total ketone bodies in the blood of a well-fed individual?
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Which molecule serves as the precursor for ketone body synthesis?
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Which ketone body is formed by the reduction of acetoacetate?
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During which metabolic circumstance does the production of ketone bodies become particularly significant?
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How do ketone bodies benefit peripheral tissues?
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What is the primary source of 2-carbon fragments used in the microsomal elongation of palmitate?
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Which of the following fatty acids can be directly synthesized from palmitate through desaturation?
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Which enzyme is responsible for introducing double bonds into fatty acids during desaturation?
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What distinguishes β-hydroxybutyrate from acetoacetate in terms of structure?
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Which of the following fatty acids cannot be synthesized by mammals due to the lack of specific enzymes?
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From which acid can arachidonic acid be synthesized in mammals?
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Which of the following statements about ketone bodies is true?
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Which mechanism primarily facilitates the metabolism of short and medium-chain fatty acids in mitochondria?
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Study Notes
Synthesis of Long Chain Fatty Acids from Palmitate
- Fatty acid elongase in the microsomal enzyme system elongates palmitate using 2-carbon fragments from malonyl-CoA and reducing equivalents from NADPH.
- Mitochondrial elongation of fatty acids is less active, primarily elongating fatty acids with less than 16 carbons, mainly short and medium chain fatty acids, using acetyl-CoA.
- Fatty acyl CoA desaturase system in microsomes forms unsaturated fatty acids, requiring flavin-dependent cytochrome b5 reductase, NADH, and molecular oxygen.
- Oleic acid (from stearate) and palmitoleic acid (from palmitate) are synthesized as monounsaturated fatty acids.
- Mammals cannot introduce double bonds between the 10th carbon and the methyl terminal (ω) end, making linoleic and linolenic acids essential dietary components.
- Arachidonic acid can be synthesized from linoleic acid via desaturation and chain elongation, serving as a precursor for eicosanoids like prostaglandins and thromboxanes.
Metabolism of Ketone Bodies
- Ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone) are water-soluble energy sources, with acetone exhaled and not metabolized.
- Ketogenesis occurs exclusively in the liver, within the mitochondrial matrix of hepatic cells, utilizing acetyl-CoA derived from fatty acid oxidation, pyruvate, or specific amino acids.
- Ketogenesis involves the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA, followed by condensation with another acetyl-CoA to create β-HMG-CoA, regulated by HMG-CoA synthase.
- HMG-CoA is then cleaved into acetoacetate and acetyl-CoA by HMG-CoA lyase, while β-hydroxybutyrate is produced by reducing acetoacetate using β-hydroxybutyrate dehydrogenase.
- Acetoacetate can spontaneously decarboxylate into acetone, with blood ketone body concentrations not exceeding 0.2 mmol/L in well-fed conditions.
Utilization of Ketone Bodies
- Ketone bodies (acetoacetate and β-hydroxybutyrate) serve as significant energy sources for peripheral tissues, including skeletal muscle and cardiac muscle.
- Tissues such as erythrocytes, lacking mitochondria, cannot utilize ketone bodies.
- The production and utilization of ketone bodies increase significantly under conditions of low glucose availability, observed in starvation or diabetes mellitus.
- During prolonged starvation, ketone bodies become a primary fuel source for the brain and other central nervous system regions.
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Description
This quiz covers the synthesis of long chain fatty acids from palmitate, detailing the processes involving microsomal and mitochondrial elongation. Explore the roles of enzymes like fatty acid elongase and the metabolic pathways involved in fatty acid elongation. Test your understanding of these biochemical mechanisms.
