Microbial Metabolism: Catabolism and Anabolism of Lipids, Proteins, and Photosynthesis

Microbial Metabolism: Catabolism and Anabolism

Lipid Catabolism

Lipids, particularly triglycerides, are important energy sources for many microorganisms. Their breakdown involves several key steps and enzymes.

• Beta-Oxidation: This is the process by which fatty acids are degraded. In prokaryotes, the enzymes for beta-oxidation are located in the cytosol, while in eukaryotes, they are found in the mitochondria.

• Triglyceride Catabolism:

  • Lipases hydrolyze triglycerides, separating glycerol from fatty acid chains.

  • Glycerol is converted to dihydroxyacetone phosphate (DHAP), which enters glycolysis.

  • Fatty acids undergo beta-oxidation, which removes two-carbon units at a time, attaching them to coenzyme A to form acetyl-CoA. Acetyl-CoA then enters the citric acid cycle.

  • Beta-oxidation also produces FADH2 and NADH, which are electron carriers that feed into the electron transport chain (ETC).

• Energy Yield: Triglycerides are the most energy-efficient energy storage form in cells.

• Phospholipids: The most common form of lipid synthesized in cells.

• Carotenoids: These are also classified as lipids.

Protein Catabolism

Proteins are generally too large to cross the cytoplasmic membrane and must be broken down before use as an energy or carbon source.

• Protease Secretion: Prokaryotes secrete proteases to degrade proteins into amino acids outside the cell.

• Amino Acid Uptake and Deamination:

  • Amino acids are transported into the cell.

  • Enzymes remove the amino group (deamination), producing ammonia (NH3) or ammonium ion (NH4+), or recycling the NH2 group to synthesize other amino acids.

Anabolism and Photosynthesis

Anabolism refers to the biosynthetic processes that build complex molecules from simpler ones. Photosynthetic organisms use light energy to drive the synthesis of organic compounds.

• Photosynthetic Organisms: Include cyanobacteria, purple sulfur bacteria, green sulfur bacteria, green nonsulfur bacteria, purple nonsulfur bacteria, algae, green plants, and some protozoa.

• Chlorophylls and Bacteriochlorophylls:

  • Chlorophyll a and b differ from bacteriochlorophyll a in the length and structure of their hydrocarbon tails and the atoms extending from their active sites.

  • Green plants, algae, photosynthetic protozoa, and cyanobacteria primarily use chlorophyll a.

  • Green and purple bacteria mainly use bacteriochlorophylls.

Photosynthetic Electron Transport and ATP Generation

Photosynthetic organisms use light energy to generate ATP and reducing power (NADPH) through electron transport chains.

• Proton Pumping:

  • In prokaryotes, protons are pumped out of the cell.

  • In eukaryotes, protons are pumped from the stroma into the thylakoid space (inside the thylakoid).

• Cyclic Photophosphorylation: Occurs in all photosynthetic organisms. Electrons cycle back to the photosystem, generating ATP via chemiosmosis (ADP → ATP).

• Noncyclic Photophosphorylation: Occurs in all plants, algae, photosynthetic protozoa, and some photosynthetic bacteria. Generates both ATP and NADPH. Electrons are replaced by splitting water (in oxygenic organisms) or by using inorganic compounds like H2S (in anoxygenic bacteria).

Calvin-Benson Cycle (Light-Independent Reactions)

The Calvin-Benson cycle is the primary pathway for carbon fixation in photosynthetic organisms.

1. Carbon Fixation: Three molecules of CO2 are attached to three molecules of ribulose 1,5-bisphosphate (RuBP) by the enzyme rubisco, forming six molecules of 3-phosphoglyceric acid (3-PGA).

   • Equation: $3 \text{CO}_2 + 3 \text{RuBP} \xrightarrow{\text{rubisco}} 6 \text{3-PGA}$

2. Reduction: Six molecules of 3-PGA are reduced by NADPH to form six molecules of glyceraldehyde 3-phosphate (G3P).

3. Regeneration: Five out of six G3P molecules are used to regenerate three RuBP molecules; the remaining G3P can be used to synthesize glucose and other carbohydrates.

Central Metabolic Pathways and Precursor Metabolites

Key metabolic pathways such as glycolysis, the citric acid cycle, and the pentose phosphate pathway provide essential precursor metabolites for biosynthesis.

• 12 Precursor Metabolites: These are the building blocks for all macromolecules and cellular structures. Escherichia coli can synthesize all 12.

Gluconeogenesis

Gluconeogenesis is the synthesis of sugars from non-carbohydrate precursors. It is a highly endergonic process, requiring significant energy input.

• G3P (Glyceraldehyde 3-phosphate): Can be converted into various sugars, including starch and cellulose (in algae), peptidoglycan, and glycogen (in animals and protozoa).

• Energy Requirement: Gluconeogenesis is highly endergonic.

Table: Comparison of Cyclic and Noncyclic Photophosphorylation

Feature	Cyclic Photophosphorylation	Noncyclic Photophosphorylation
Organisms	All photosynthetic organisms	All plants, algae, photosynthetic protozoa, some bacteria
Electron Flow	Cyclic (returns to photosystem)	Noncyclic (does not return; replaced by external source)
Products	ATP only	ATP and NADPH
Electron Source	Original chlorophyll	Water (oxygenic) or inorganic compounds (anoxygenic)

Table: Key Enzymes and Their Functions

Enzyme	Function	Pathway
Lipase	Hydrolyzes triglycerides into glycerol and fatty acids	Lipid catabolism
Protease	Degrades proteins into amino acids	Protein catabolism
Rubisco	Catalyzes CO2 fixation to RuBP	Calvin-Benson cycle

Additional info: The 12 precursor metabolites referenced are intermediates from glycolysis, the pentose phosphate pathway, and the citric acid cycle, which serve as starting points for the biosynthesis of amino acids, nucleotides, lipids, and carbohydrates.