Lipid and Amino Acid Metabolism

Lipid Metabolism: Focus on Cholesterol

• Overview of Cholesterol

  • Synthesis of cholesterol discussed with implications for health
  • Connection to cardiovascular disease and treatments

• Cholesterol Biosynthetic Pathway

  • Brown and Goldstein's research on cholesterol elucidation at UT Southwestern
  • Developed statin drugs to reduce serum cholesterol
  • High levels of serum cholesterol linked to heart disease
  • Cholesterol circulates in vesicles through the bloodstream
  • Can become entrapped in the endothelial lining of blood vessels
  • Development of atherosclerotic plaques from trapped vesicles

• Atherosclerotic Plaque Formation

  • Immune cells attempt to clear these trapped vesicles
  • Erosion of plaques can cause blood clots; potential for heart attacks

• Blood Serum and Cholesterol Levels

  • Testing blood serum for lipid count as a diagnostic tool
  • Example image comparison of healthy vs. high lipid serum levels
  • Left panel: normal lipid count
  • Right panel: excess lipids present

• Synthesis and Regulation of Cholesterol

  • Daily cholesterol production by liver is approximately 1 gram
  • Minimal dietary intake; importance of endogenous synthesis
  • Begins with acetyl CoA, specifically using three acetyl CoA molecules
  • Primary enzyme: HMG CoA reductase converting HMG CoA to mevalonate
  • Statins inhibit HMG CoA reductase: therapeutic implications
  • Four stages of cholesterol synthesis detailed
  • 27-carbon cholesterol produced via progressive transformations

• Uses of Cholesterol

  • Stored in lipid droplets
  • Membrane incorporation
  • Affects membrane fluidity: higher cholesterol reduces lateral movement
  • Important for maintaining membrane structure
  • Converted into bile salts with taurine or glycine modifications

Therapeutics Targeting Cholesterol

• Cholesterol Treatments

  • Reducing serum cholesterol by altering bile salt pathways
  • Bile salts synthesized from cholesterol and stored in gallbladder
  • Resin treatment binds bile salts and promotes direct cholesterol usage

• Lipoproteins and Their Roles

  • Various lipoproteins facilitate lipid transport:
  • Chylomicrons:
  • Largest lipoprotein; primarily triacylglycerols (95%) and cholesterol (5%)
  • Low-Density Lipoprotein (LDL):
  • Higher cholesterol percentage; often termed “bad cholesterol”
  • High-Density Lipoprotein (HDL):
  • Lower cholesterol; collects excess cholesterol for return to liver, termed “good cholesterol”

Mechanisms of Cholesterol Scavenging

• HDL Functionality

  • HDL scavenges excess cholesterol and transfers it back to the liver
  • Interaction with peripheral cells through apolipoproteins
  • Apolipoprotein A-I activates lecithin-cholesterol acyltransferase

• Atherosclerosis Recap

  • Plaque development from LDL entrapment in endothelial linings
  • Immune response and subsequent lipid breakdown exacerbate plaque formation

Regulation of Cholesterol Homeostasis

• Cholesterol Regulation via Gene Expression

  • Low cholesterol stimulates transcription factors (SREBPs) to increase production of LDL receptors
  • Enhances uptake of cholesterol from circulating LDL
  • For therapeutic models, statins reduce cholesterol synthesis while increasing receptor synthesis

• Inhibition of HMG CoA Reductase

  • Statins structurally mimic HMG CoA, blocking its reductase activity
  • Classes of statins available; derived from fungi or designed synthetically

• Additional Treatment Approaches

  • Inhibition of intestinal cholesterol absorption through transporter inhibition
  • Limited success as most high cholesterol cases arise from endogenous production rather than diet

Transition to Amino Acid Metabolism

• Overview of Amino Acid Metabolism

  • Introduction to nitrogen assimilation and biosynthetic pathways
  • Sourcing nitrogen gas (N2) from atmosphere for amino acid synthesis
  • Processes of nitrogen fixation by specialized bacteria and plants

• Importance of Nitrogen Fixation

  • Reflects critical need for nitrogen in biological systems
  • Various mechanisms:
  • Symbiotic relationships between bacteria and plants (e.g., legumes)
  • Industrial fixation for agriculture
  • Atmospheric fixation from natural events (e.g., lightning)

Nitrogen Assimilation

• Mechanisms for Nitrogen Assimilation

  • Comprises processes to incorporate fixed nitrogen into organic molecules like amino acids
  • Specific enzymes and their pathways (glutamine synthetase, glutamate synthase) involved
  • Importance of ATP and redox reactions in synthesizing amino acids

• Key Enzymatic Reactions

  • Glutamine synthesis requiring ATP and ammonia incorporation
  • Glutamate restoration through two-step conversion involving alpha-ketoglutarate

• Regulation of Pathway Activity

  • Feedback inhibition of synthetase by downstream metabolites
  • Covalent modifications involving adenylation to modulate synthetase activity

• Aminotransferases Role in Nitrogen Cycling

  • Transfer amine groups to generate new amino acids

Conclusion

• Comprehensive understanding of cholesterol synthesis, regulation, and implications for cardiovascular health
• Awareness of amino acid metabolism, emphasizing nitrogen fixation and assimilation processes