8.5 Catabolism of Lipids and Proteins

Lipid Catabolism in Microorganisms

Lipid catabolism in microorganisms

Lipid catabolism is how microbes break down fats to extract energy and generate building blocks for growth. The process starts with lipolysis, where lipases hydrolyze triglycerides into glycerol and free fatty acids. From there, each component takes a different metabolic route.

Glycerol is converted into dihydroxyacetone phosphate (DHAP), which feeds into glycolysis (for energy) or gluconeogenesis (to make glucose).

Fatty acids go through β-oxidation, a cyclic process that shaves off two-carbon units as acetyl-CoA with each round. Before β-oxidation can begin, though, fatty acids must be activated by the enzyme acyl-CoA synthetase, which converts them into fatty acyl-CoA. The acetyl-CoA molecules then enter the citric acid cycle, where they're further oxidized to produce ATP.

Here's the overall sequence:

1. Lipolysis — Lipases split triglycerides into glycerol + fatty acids
2. Activation — Acyl-CoA synthetase converts fatty acids to fatty acyl-CoA
3. β-oxidation — Fatty acyl-CoA is repeatedly cleaved into two-carbon acetyl-CoA units
4. Energy harvest — Acetyl-CoA enters the citric acid cycle for further oxidation

One location detail worth knowing: in most bacteria, β-oxidation takes place in the cytoplasm. In eukaryotic cells, it occurs in the mitochondria.

Microbial identification through lipid breakdown

Not all microbes can break down the same lipids, and these differences are useful for identification.

• Genera like Pseudomonas and Acinetobacter can catabolize a broad spectrum of lipids, including complex hydrocarbons. Detecting lipase activity and the ability to degrade specific lipids helps differentiate between Pseudomonas species (e.g., P. aeruginosa vs. P. fluorescens).
• Mycobacterium tuberculosis has a lipid-rich cell wall (mycolic acids) that requires specialized lipases for degradation, making its lipid profile a distinctive diagnostic feature.

These lipid degradation patterns serve as practical diagnostic markers in clinical and environmental microbiology.

Protein Catabolism in Bacteria

Protein catabolism in bacteria

Bacteria break down proteins to harvest amino acids, which can then be used for energy or funneled into biosynthetic pathways. The process moves from outside the cell to inside:

1. Extracellular proteases hydrolyze proteins by cleaving peptide bonds, producing smaller peptides and free amino acids.
2. Transport — Peptides are brought into the cell through specific peptide transport systems (such as the Opp and Dpp systems).
3. Intracellular peptidases further degrade peptides into individual amino acids.
4. Amino acid fate — Once free, amino acids can be directly incorporated into new bacterial proteins, or they can undergo deamination. Deamination removes the amino group (released as ammonia) and generates the corresponding α-keto acid.

Those α-keto acids are versatile. They can enter the citric acid cycle for energy production, or they can serve as precursors for synthesizing other molecules like fatty acids and sugars.

Transamination reactions are also central here. These reactions transfer amino groups between amino acids and α-keto acids, allowing bacteria to interconvert amino acids and balance their metabolic needs.

Bacterial differentiation via protein degradation

Different species have very different protease toolkits, and you can exploit this for identification.

• Bacillus subtilis secretes a wide array of proteases, giving it the ability to efficiently break down many different proteins.
• Streptococcus pyogenes has a more limited protease repertoire and relies more heavily on specific peptide transport systems to acquire nutrients.

The presence or absence of particular proteases works as a diagnostic tool. For instance, Clostridium histolyticum produces collagenase, an extracellular protease that degrades collagen. This ability distinguishes it from other Clostridium species like C. difficile and C. botulinum, which lack this enzyme.

Alternative Metabolic Pathways

Ketogenesis in microorganisms

When glucose is scarce, some microorganisms turn to ketogenesis, a pathway that converts excess acetyl-CoA (from fatty acid oxidation) into ketone bodies. The three main ketone bodies are:

• Acetoacetate
• β-hydroxybutyrate
• Acetone

In eukaryotes, ketogenesis occurs in the mitochondria. Among microbes, this pathway is particularly relevant for certain anaerobic bacteria and archaea, enabling them to thrive in carbohydrate-poor environments by using lipid-derived carbon for energy.