Notes on Lipid and Amino Acid Metabolism

Lipid Metabolism Overview

  • Triacylglycerols: Fatty acids attached to glycerol.
  • Glycerol converts to glycerol 3-phosphate, yielding dihydroxyacetone phosphate (DHAP).
  • Fatty acids degraded into 2- and 3-carbon intermediates for the citric acid cycle.

Dietary Triacylglycerols Processing

  • Absorption of monoacylglycerol and fatty acids into lipoproteins.
  • Lipoprotein lipase hydrolyzes triacylglycerol to release fatty acids.

Fatty Acid Activation

  • Fatty acids activate to Acyl-CoA by Acyl-CoA synthetase (ATP required).
  • Acyl-CoA transported into mitochondria via carnitine.

β-Oxidation Process

  • Four reactions per cycle yield acetyl-CoA, ubiquinol (QH2), and NADH.
  • Odd-chain fatty acids produce propionyl-CoA, converting to acetyl-CoA.
  • Each two-carbon unit generates approximately 14 ATP from β-oxidation.
  • Unsaturated fatty acids require additional processing (e.g., Enoyl-CoA isomerase).

Ketone Bodies Formation

  • Acetyl-CoA forms ketone bodies (e.g., acetoacetate) when glucose is low.
  • Ketone bodies converted back to acetyl-CoA for TCA cycle use.

Fatty Acid Synthesis

  • Begins with acetyl-CoA carboxylase (requires biotin).
  • Subsequent rounds involve cyclical four-reaction steps.
  • Malonyl-CoA binds for growth of fatty acid chain.

Nitrogen Fixation and Amino Acid Metabolism

  • Nitrogen fixation by nitrogenase converts N2 to NH4+ (biochemical utility).
  • Proteins broken down into amino acids for biological synthesis.
  • Excess ammonia is detoxified and excreted as urea.

Transamination

  • Amino groups transferred between molecules via transaminases.
  • Involves glutamate dehydrogenase for free ammonia regulation.

Urea Cycle

  • Ammonia forms carbamoyl phosphate, enters urea cycle to produce urea for excretion.

Amino Acid Classification

  • Glucogenic amino acids yield gluconeogenic intermediates; ketogenic amino acids yield acetyl-CoA.

Metabolic Disorders

  • Enzyme deficiencies, e.g., phenylketonuria, limit amino acid breakdown.

Amino Acid Synthesis

  • Nonessential amino acids derived from glycolysis/citric acid cycle.
  • Essential amino acids obtained through diet; synthesized by plants/bacteria.
  • Amino acids can be metabolically interconverted into neurotransmitters/hormones.