Biosynthesis of Lipids

Bacteria and eukaryotes both convert acetyl-CoA and malonyl-CoA into fatty acids through condensation, reduction, and dehydration. However, bacteria use a type II fatty acid synthase system where individual enzymes catalyze each step, while eukaryotes rely on a type I fatty acid synthase, a large multifunctional protein complex performing all reactions within a single assembly.

Archaea synthesize isoprenoid lipids from isopentenyl pyrophosphate via the mevalonate pathway, unlike bacteria and eukaryotes that use fatty acids. Crucially, archaea feature ether linkages between isoprenoid chains and glycerol, whereas bacteria and eukaryotes use ester linkages. This ether bonding enhances hydrolysis resistance, enabling archaea to survive extreme environments like high temperatures, acidity, and salinity.

Fatty acids are ester-linked to glycerol, forming phospholipids such as phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin. These phospholipids maintain membrane integrity and support selective permeability, cellular signaling, and protein anchoring. The ester linkage makes these lipids relatively fluid and adaptable to environmental changes, particularly in mesophilic conditions.

Certain bacteria like Bradyrhizobium and Streptomyces synthesize hopanoids from isoprenoid precursors, enhancing membrane rigidity and reducing permeability. Eukaryotes synthesize sterols such as cholesterol from the same precursors, regulating membrane fluidity by preventing excessive rigidity in cold conditions while preserving structural integrity at higher temperatures.

Certain extremophiles like Thermoplasma synthesize glycerol dibiphytanyl glycerol tetraether lipids that span the entire membrane, forming monolayers instead of bilayers. These monolayers significantly enhance membrane stability, reducing permeability and preventing structural collapse under extreme conditions such as high temperatures and acidity.

Cold-adapted bacteria fine-tune lipid composition by increasing unsaturated fatty acids, which prevent membrane rigidity and ensure functionality at lower temperatures. This adaptation maintains membrane fluidity and flexibility in cold environments, allowing the organism to survive and function properly despite reduced thermal energy.

Diverse microbial lipid compositions underscore biochemical adaptations enabling organisms to survive across varied ecological niches. The presence of ester- or ether-linked lipids and integration of sterols and hopanoids reflect evolutionary pressures shaping membrane composition in response to environmental challenges, ensuring survival in diverse and often extreme conditions.