Biochemistry of Lipids

Triglycerides

• Major source of long-term energy storage in animals.
• Composed of glycerol bonded to three fatty acids.

Phospholipids

• Integral components of cell and organelle membranes in all organisms, except archaea.
• Structurally similar to triglycerides but contain a phosphorylated head group instead of one fatty acid.

Breakdown Process

• Triglycerides and phospholipids are degraded by releasing fatty acids and/or phosphorylated head groups from the glycerol backbone.
• Lipases are enzymes that specifically catalyze the breakdown of triglycerides.
• Phospholipases are enzymes that target and break down phospholipids.

Microbial Enzyme Function

• Certain microbes, such as Staphylococcus aureus and Cryptococcus neoformans, utilize lipases and phospholipases to damage host cells and derive energy.

Products of Lipid Catabolism

• Degradation yields glycerol and fatty acids.
• Glycerol can convert to glycerol-3-phosphate, then to glyceraldehyde-3-phosphate, integrating into glycolysis.
• Fatty acids undergo β-oxidation, producing two-carbon acetyl groups along with NADH and FADH2 for ATP generation through oxidative phosphorylation.
• Acetyl groups are introduced into the Krebs cycle, resulting in the production of CO2, NADH, and FADH2.

Other Lipids

• Pathogens like Mycobacterium tuberculosis can degrade cholesterol to enhance virulence.
• The degradation of cholesterol involves the removal of side chains and a complex multi-step process for the fused rings, generating pyruvate for the Krebs cycle.

Proteins and Proteolysis

• Proteins are broken down by microbial enzymes known as proteases.

Extracellular Proteases

• Target specific amino acid sequences and cleave proteins into smaller peptides for cellular uptake.

Pathogen Identification via Proteases

• Certain pathogens produce unique extracellular proteases used for differentiation:
  • Gelatinase, produced by Proteus and Serratia, prevents gelatin from solidifying when cultured in gelatin broth.
  • Caseinase, found in Pseudomonas aeruginosa, creates a clear zone around colony growth on skim milk agar.

Further Breakdown of Peptides

• Uptaken peptides are further degraded into amino acids by intracellular proteases.
• Amino acids can undergo deamination, a process that involves the removal of the amino group.