Microbial Metabolism: Foundations, Pathways, and Regulation

Microbial Metabolism

Introduction to Metabolism

Metabolism encompasses all controlled biochemical reactions occurring within a microbe, with the ultimate function of enabling reproduction. These reactions are organized into metabolic pathways that are tightly regulated to ensure cellular efficiency and survival.

• Metabolism: The sum of all chemical reactions in a cell.

• Eight guiding principles include nutrient acquisition, energy extraction and storage (mainly as ATP), catabolism to form precursors, anabolism to build macromolecules, and regulated growth and reproduction.

Catabolism and Anabolism

Metabolic reactions are divided into two major classes: catabolic and anabolic pathways.

• Catabolic pathways: Break down larger molecules into smaller products, releasing energy (exergonic).

• Anabolic pathways: Synthesize large molecules from smaller products, requiring energy input (endergonic).

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons from an electron donor to an electron acceptor, always occurring simultaneously. Cells use electron carriers such as NAD+, NADP+, and FAD to shuttle electrons during metabolism.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron carriers are essential for energy transfer in metabolic pathways.

ATP Production and Energy Storage

Cells release energy from nutrients and store it in high-energy phosphate bonds of ATP. ATP is generated by phosphorylation of ADP in three ways:

• Substrate-level phosphorylation

• Oxidative phosphorylation

• Photophosphorylation

Anabolic pathways utilize ATP by breaking its phosphate bonds to drive biosynthetic reactions.

The Roles of Enzymes in Metabolism

Enzymes are organic catalysts that increase the likelihood of chemical reactions by lowering activation energy. They are classified by the type of reaction they catalyze:

• Hydrolases: Catalyze hydrolysis (catabolic)

• Isomerases: Rearrangement of atoms (neither catabolic nor anabolic)

• Ligases/Polymerases: Join molecules (anabolic)

• Lyases: Split molecules without water (catabolic)

• Oxidoreductases: Transfer electrons or hydrogen atoms

• Transferases: Move functional groups between molecules

Enzymes may require cofactors (inorganic ions or organic coenzymes) for activity. The combination of an apoenzyme and its cofactor forms a holoenzyme.

Enzyme Function and Regulation

• Enzymes lower the activation energy required for reactions.

• Substrate binding induces a conformational change (induced fit model).

Factors Affecting Enzyme Activity

• Temperature

• pH

• Enzyme and substrate concentrations

• Presence of inhibitors

Enzyme Regulation

• Allosteric activation: Cofactor binding at a site other than the active site activates the enzyme.

• Inhibition: Competitive inhibitors block the active site; noncompetitive inhibitors bind allosteric sites, altering enzyme activity.

• Feedback inhibition: End-product of a pathway inhibits an earlier enzyme, regulating pathway activity.

Carbohydrate Catabolism

Overview

Carbohydrate catabolism is the primary means by which cells obtain energy. Glucose is the most common substrate, catabolized by cellular respiration or fermentation.

Glycolysis

Glycolysis occurs in the cytoplasm and splits glucose into two pyruvic acid molecules, yielding a net gain of 2 ATP and 2 NADH. It consists of three stages:

• Energy-investment stage

• Lysis stage

• Energy-conserving stage

Cellular Respiration

Cellular respiration completely oxidizes pyruvic acid to produce ATP via three stages:

1. Synthesis of acetyl-CoA

2. Krebs cycle (citric acid cycle)

3. Electron transport chain (ETC)

Electron Transport Chain and Chemiosmosis

The ETC is a series of carrier molecules that transfer electrons to a final electron acceptor, generating a proton gradient used by ATP synthase to produce ATP (oxidative phosphorylation).

• Aerobic respiration: Oxygen is the final electron acceptor.

• Anaerobic respiration: Other molecules (e.g., nitrate, sulfate) serve as final acceptors.

Summary Table: Prokaryotic Aerobic Respiration

Pathway	ATP Produced	ATP Used	NADH Produced	FADH2 Produced
Glycolysis	4	2	2	0
Synthesis of acetyl-CoA & Citric Acid Cycle	2	0	8	2
Electron Transport Chain	34	0	0	0
Total	40	2
Net Total	38

Metabolic Diversity: Alternative Pathways

• Entner-Doudoroff pathway: Found in some prokaryotes, yields 1 ATP, 1 NADH, and 1 NADPH per glucose.

• Pentose phosphate pathway: Produces precursor metabolites and NADPH for biosynthesis.

Fermentation

Fermentation provides an alternative to respiration when cells cannot fully oxidize glucose. It regenerates NAD+ by transferring electrons to organic molecules, producing various end-products (e.g., lactic acid, ethanol).

Comparison Table: Aerobic Respiration, Anaerobic Respiration, and Fermentation

	Aerobic Respiration	Anaerobic Respiration	Fermentation
Oxygen Required	Yes	No	No
Type of Phosphorylation	Substrate-level & oxidative	Substrate-level & oxidative	Substrate-level
Final Electron Acceptor	Oxygen	NO3-, SO42-, CO32-, etc.	Cellular organic molecules
ATP Yield (per glucose)	38 (prokaryotes)	4–36	2

Other Catabolic Pathways

Lipid and Protein Catabolism

Lipids and proteins can be catabolized to provide energy and precursor metabolites for biosynthesis. Lipids are broken down by lipases and beta-oxidation; proteins are degraded by proteases and deaminated before entering central metabolic pathways.

Photosynthesis

Overview and Structures

Photosynthesis is the process by which organisms synthesize organic molecules from CO2 and H2O using light energy. Chlorophylls and photosystems are key components, with light-dependent and light-independent (Calvin-Benson cycle) reactions.

Light-Dependent and Light-Independent Reactions

• Light-dependent reactions: Use light energy to generate ATP and NADPH via photophosphorylation (cyclic and noncyclic).

• Light-independent reactions: Use ATP and NADPH to fix carbon dioxide into glucose (Calvin-Benson cycle).

Comparison Table: Types of Phosphorylation

	Source of Phosphate	Source of Energy	Location in Eukaryotes	Location in Prokaryotes
Substrate-Level	Organic molecule	High-energy phosphate bond	Cytosol, mitochondrial matrix	Cytosol
Oxidative	Inorganic phosphate	Proton motive force	Inner mitochondrial membrane	Cytoplasmic membrane
Photophosphorylation	Inorganic phosphate	Proton motive force	Thylakoid of chloroplast	Thylakoid of cytoplasmic membrane

Other Anabolic Pathways

Anabolism and Precursor Metabolites

Anabolic reactions synthesize macromolecules using energy (ATP) and precursor metabolites derived from catabolic pathways. Many pathways are amphibolic (reversible).

Table: 12 Precursor Metabolites and Their Uses

Metabolite	Pathway	Macromolecule Synthesized	Functional Use
Glucose 6-Phosphate	Glycolysis	Lipopolysaccharide	Outer membrane
Fructose 6-Phosphate	Glycolysis	Peptidoglycan	Cell wall
Glyceraldehyde 3-Phosphate	Glycolysis	Glycerol (lipids)	Energy storage
Phosphoglyceric Acid	Glycolysis	Amino acids	Enzymes
Phosphoenolpyruvic Acid	Glycolysis	Amino acids	Enzymes
Pyruvic Acid	Glycolysis	Amino acids	Enzymes
Ribose 5-Phosphate	Pentose phosphate	DNA, RNA	Genome, enzymes
Erythrose 4-Phosphate	Pentose phosphate	Amino acids	Enzymes
Acetyl-CoA	Citric acid cycle	Fatty acids	Membranes
α-Ketoglutaric Acid	Citric acid cycle	Amino acids	Enzymes
Succinyl-CoA	Citric acid cycle	Heme	Electron carrier
Oxaloacetate	Citric acid cycle	Amino acids	Enzymes

Integration and Regulation of Metabolic Function

Cells regulate metabolism by controlling enzyme synthesis and activity, choosing energy sources, and using feedback inhibition. Eukaryotes compartmentalize pathways in organelles, and amphibolic pathways are regulated by coenzyme specificity.

• Control of gene expression: Regulates enzyme production.

• Control of metabolic expression: Regulates enzyme activity post-production.

Summary: Microbial metabolism is a complex, highly regulated network of catabolic and anabolic pathways that enable microbes to grow, reproduce, and adapt to their environments. Understanding these processes is fundamental to microbiology and biotechnology.