Chapter 7: Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses all chemical reactions and physical workings within a microbial cell. It is divided into two main processes: anabolism (biosynthesis) and catabolism (degradation). These processes are essential for cell growth, maintenance, and reproduction.

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

• Anabolism: Synthesis of cell molecules and structures; requires energy input.

• Catabolism: Breakdown of larger molecules into smaller ones; releases energy.

Enzymes: Catalysts of Life

Enzymes are biological catalysts that accelerate chemical reactions without being consumed or becoming part of the products. They are essential for metabolic processes.

• Increase the rate of chemical reactions.

• Do not create reactions; only speed up those that would occur naturally.

• Can be used repeatedly.

How Enzymes Work

• Provide a physical site for substrate binding and reaction.

• Have a unique active site specific to their substrate ("lock-and-key" fit).

• Bind substrates, participate in their transformation, and release products.

• Are larger than their substrates and are not consumed in the reaction.

Enzyme Structure

• Simple enzymes: Consist of protein only.

• Conjugated enzymes (holoenzymes): Contain protein (apoenzyme) and nonprotein cofactors.

• Apoenzyme: Protein portion of a holoenzyme.

• Cofactors: Nonprotein components; can be coenzymes (organic, often vitamin-derived) or inorganic (metal ions).

Enzyme-Substrate Interactions

• Temporary enzyme-substrate complex forms at the active site.

• Specificity is high; the fit is often described as "lock-and-key."

• After the reaction, the product is released and the enzyme is free to catalyze another reaction.

Cofactors and Coenzymes

• Microorganisms require trace elements (e.g., iron, copper, magnesium) as cofactors.

• Cofactors help bring the active site and substrate together and may participate directly in the reaction.

• Coenzymes are organic cofactors, often derived from vitamins, that transfer chemical groups, hydrogen atoms, or electrons between molecules.

Naming Enzymes

• Named based on substrate, type of reaction, or both.

• Names typically end in -ase (e.g., carbohydrase, protease, lipase).

Constitutive and Regulated Enzymes

• Constitutive enzymes: Always present, produced in constant amounts.

• Regulated enzymes: Produced only when substrates are present or in response to environmental changes.

Regulation of Enzyme Action

• Enzyme activity is influenced by temperature, pH, and osmotic pressure.

• Denaturation: Loss of enzyme structure (and function) due to environmental changes.

Direct Controls on Enzyme Activity

• Competitive inhibition: A molecule similar to the substrate occupies the active site, blocking the actual substrate.

• Noncompetitive inhibition: A molecule binds to a regulatory site, altering enzyme activity regardless of substrate presence.

Controls on Enzyme Synthesis

• Enzyme repression: Synthesis of enzymes is suppressed by the accumulation of end-products.

• Enzyme induction: Enzymes are produced only when their substrate is present.

Example: E. coli produces lactase only when lactose is present; switches to sucrase production when sucrose is the available sugar.

Metabolic Pathways

Metabolic reactions are organized into pathways, each catalyzed by a specific enzyme. Pathways can be linear, cyclic, branched (divergent or convergent), and are often interconnected.

• Linear: Product of one reaction is the substrate for the next (e.g., glycolysis).

• Cyclic: Starting molecule is regenerated (e.g., Krebs cycle).

• Branched: Pathways diverge or converge, allowing flexibility in metabolism.

Energy in Cells: ATP and Redox Reactions

Cells store energy primarily as ATP (adenosine triphosphate). Energy is managed through chemical reactions involving bond formation/breakage and electron transfer.

• Exergonic reactions: Release energy.

• Endergonic reactions: Require energy input.

• Exergonic and endergonic reactions are often coupled.

ATP Structure and Function

• Composed of adenine (nitrogen base), ribose (5-carbon sugar), and three phosphate groups.

• High-energy phosphate bonds store energy; removal of terminal phosphate releases energy for cellular work.

Oxidation and Reduction (Redox) Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox reactions are essential for energy transformations in cells.

• Common electron carriers: NAD+ (nicotinamide adenine dinucleotide), FAD (flavin adenine dinucleotide).

Equation:

Catabolic Pathways in Microbial Metabolism

• Aerobic respiration: Glucose is converted to CO2 and energy (ATP) using oxygen as the final electron acceptor. Includes glycolysis, Krebs cycle, and electron transport chain.

• Anaerobic respiration: Similar to aerobic, but uses inorganic molecules other than oxygen (e.g., nitrate, sulfate) as final electron acceptors.

• Fermentation: Incomplete oxidation of glucose without oxygen; organic compounds serve as final electron acceptors.

Glycolysis

• First step in all three pathways; converts glucose to two molecules of pyruvic acid.

• Generates a small amount of ATP and NADH.

Krebs Cycle (Citric Acid Cycle)

• Transfers energy from acetyl CoA to NAD+ and FAD, producing NADH and FADH2.

• Regenerates oxaloacetic acid and releases CO2.

Electron Transport Chain (ETC)

• Series of redox carriers embedded in membranes (cytoplasmic in bacteria, mitochondrial in eukaryotes).

• Electrons from NADH and FADH2 are passed through the chain, creating a proton gradient used to synthesize ATP (oxidative phosphorylation).

• Oxygen (aerobic) or other inorganic molecules (anaerobic) serve as final electron acceptors.

ATP Yield Table

Fermentation

• Occurs when oxygen is absent or ETC is not functional.

• Pyruvic acid serves as the electron acceptor, producing products like lactic acid, ethanol, and others.

• Regenerates NAD+ for glycolysis.

Catabolism of Noncarbohydrate Compounds

• Polysaccharides are broken into sugars for glycolysis.

• Lipids are hydrolyzed by lipases; glycerol enters glycolysis, fatty acids undergo beta-oxidation to form acetyl CoA.

• Proteins are degraded by proteases; amino acids are deaminated and converted to metabolic intermediates.

Anabolic Pathways and Amphibolism

Anabolism uses intermediates from catabolic pathways to synthesize macromolecules. Amphibolism refers to the integration of catabolic and anabolic pathways for efficient resource use.

• Intermediates from glycolysis and Krebs cycle are used for amino acid, nucleotide, and lipid biosynthesis.

• Cells can import building blocks or synthesize them de novo, depending on genetic capability.

Carbohydrate Biosynthesis

• Glucose is central to energy storage and cell wall synthesis.

• Glycogen and peptidoglycan are synthesized from glycolytic intermediates.

Protein and Nucleic Acid Synthesis

• Proteins are built from 20 amino acids; some microbes can synthesize all, others require some from the environment.

• Nucleic acids (DNA, RNA) are synthesized from nucleotides derived from metabolic intermediates.

Cell Assembly and Division

• Continuous synthesis of cell components occurs as long as nutrients are available.

• Cell division requires sufficient macromolecules, DNA replication, and membrane/cell wall expansion.

Key Terms and Concepts

• Metabolism: All chemical reactions in a cell.

• Anabolism: Building up molecules; requires energy.

• Catabolism: Breaking down molecules; releases energy.

• Enzyme: Biological catalyst.

• Cofactor: Nonprotein helper for enzymes.

• Coenzyme: Organic cofactor, often vitamin-derived.

• ATP: Main energy currency of the cell.

• Redox reaction: Transfer of electrons between molecules.

• Amphibolism: Integration of catabolic and anabolic pathways.