Microbial Metabolism Overview

Microbial Metabolism

• Metabolism encompasses all chemical reactions within a cell.
• It's divided into two parts:
  • Catabolism: Breaking down reactions.
  • Anabolism: Building up reactions.

Catabolism

• Fueling reactions: Provide energy to the cell.
• Energy-conserving reactions: Store energy in easily usable forms.
• Provides a ready source or reducing power (electrons).
• Generates precursors for biosynthesis.

Anabolism

• Synthesis of complex organic molecules from simpler ones.
• Requires energy from catabolic reactions.

Major Nutritional Types of Microorganisms

• Photolithotrophic autotrophs (Photolithoautotrophy): Use light energy to synthesize organic compounds from inorganic sources like carbon dioxide. Examples include algae and some bacteria.
• Photoorganotrophic heterotrophs (Photoorganoheterotrophy): Use light energy, but require organic molecules as a source of carbon. Examples include purple nonsulfur bacteria and green nonsulfur bacteria.
• Chemolithotrophic autotrophs (Chemolithoautotrophy): Obtain energy through inorganic chemical reactions and use inorganic carbon as a carbon source. Examples include sulfur-oxidizing, hydrogen, and nitrifying bacteria.
• Chemoorganotrophic heterotrophs (Chemoorganoheterotrophy): Obtain energy by breaking down organic molecules and use organic carbon. Examples include protozoa, fungi, and most nonphotosynthetic bacteria (including pathogens).

Types of Work Carried Out by Organisms

• Energy: The capacity to cause changes or do work.
• Chemical work: Synthesis of complex molecules.
• Transport work: Uptake of nutrients, elimination of wastes, and ion balances.
• Mechanical work: Movement of organisms or cells and movement of internal structures.

Energy Currency of Cells

• ATP: Used to transfer energy from the cell's energy-conserving systems to the systems that carry out cellular work.
• Other nucleotides (GTP, UTP, CTP) also involved in energy transfer.

The Cell's Energy Cycle

• ATP is generated through aerobic respiration, anaerobic respiration, fermentation, and photosynthesis.
• The generated ATP powers chemical, transport, and mechanical work within the cell.

Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed. Chemical energy in food converts to chemical energy in ATP and then to mechanical energy.
• Second Law: Energy transformations result in a loss of usable energy. 25% of gasoline's chemical energy is useful, rest is lost as heat, similar inefficiencies exist with ATP use in muscle.

Entropy

• Measure of randomness or disorder.
• Organized forms of energy have low entropy.
• Unorganized forms have high entropy.
• Energy conversions result in heat increasing the universe's entropy.

Free Energy

• Free energy (G): The amount of energy released or used in a chemical reaction.
• ΔG: Change in free energy.
  • ΔG < 0 (negative): Reaction favors products (spontaneous reaction).
  • ΔG > 0 (positive): Reaction favors reactants and requires energy input.

Types of Reactions

• Exergonic reactions: Release energy (ΔG < 0).
• Endergonic reactions: Absorb energy (ΔG > 0).
• Reversible reactions: Free energy difference near zero; reaction at equilibrium (A ⇌ B).

Oxidation-Reduction (Redox) Reactions

• Transfer of electrons from a donor to an acceptor.
• Reducing agent (reductant): Donates electrons.
• Oxidizing agent (oxidant): Accepts electrons.
• Release energy important to the cell.

Electron Carriers

• Act as temporary storage for the transferred electrons during reactions.
• Examples include NAD, NADP, FAD, FMN, and Coenzyme Q (CoQ/Ubiquinone).

Biochemical Pathways

• Enzymes can be linked to form linear, cyclic, or branching pathways.
• Pathways overlap and feed into each other, forming complex networks.
• Dynamic pathways used to monitor changes in metabolite concentrations.

Enzymes

• Protein catalysts that speed up reactions without being consumed.
• Highly specific for the reactions catalyzed and molecules they act on.
• Substrates are reacting molecules that bind to enzymes.
• Products are the substances formed.

Enzyme Structure and Classification

• Some enzymes are made up of solely polypeptides.
• Others consist of polypeptides and non-protein components.
• Apoenzyme: Protein component of an enzyme.
• Cofactor: Non-protein component (prosthetic group—firmly attached versus coenzyme—loosely attached).
• Holoenzyme: Entire active enzyme with both apoenzyme and cofactor.
• Enzymes classified according to the reactions they catalyze. (e.g., Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases)

The Mechanism of Enzyme Reactions

• Enzyme-substrate complex forms.
• Transition-state complex resembles both substrates and products.

Activation Energy

• Energy needed to start a reaction, lower by enzymes.

How Enzymes Lower Activation Energy

• Increase substrate concentrations at the active site.
• Orient substrates properly for complex formation.

Interaction of Enzymes and Substrates

• Lock and key model: Enzymes and substrates have complementary shapes that fit together.
• Induced fit model: Active site changes shape to accommodate the substrate, creating a tighter fit after binding.

Effect of Substrate Concentration

• Reaction rate increases with increasing substrate concentration until all enzyme molecules are saturated.
• Beyond this point, adding more substrate will not increase reaction rate.

Effect of pH and Temperature

• Each enzyme has a specific pH and temperature optimum for highest activity.
• Denaturation: Loss of enzyme structure and activity when these conditions deviate significantly from optimal.

Enzyme Inhibition

• Competitive inhibition: Inhibitor directly competes with substrate for the active site.
• Noncompetitive inhibition: Inhibitor binds to a site other than the active site, changing the enzyme's shape and decreasing activity

Ribozymes

• RNA molecules that act as catalysts.
• Examples include self-splicing RNA and enzymes involved in replication and other reactions.

Regulation of Metabolism

• Essential for maintaining energy and material balance despite environmental changes.
• Mechanisms include metabolic channeling (controlling localization of enzymes and substrates), enzyme regulation (enzyme synthesis/degradation, and post-translation enzyme modifications), and feedback inhibition (product of pathway stops the reaction)

Metabolic Channeling

• Differential localization of enzymes and metabolites in different compartments of the cell.
• Can create marked variations in metabolite concentrations.

Post-Translational Regulation

• Reversible addition or removal of chemical groups (e.g., phosphorylation) alters enzyme activity.

Allosteric Regulation

• Effector molecules bind to regulatory sites, changing enzyme shape and thus activity.

Feedback Inhibition

• Product of a pathway inhibits earlier enzymes in the same pathway.
• Helps regulate overall pathway activity.

Isoenzymes

• Different enzymes catalyzing the same reaction.

This quiz focuses on microbial metabolism, covering key concepts such as catabolism and anabolism. It explores the major nutritional types of microorganisms, including photolithotrophic autotrophs and photoorganotrophic heterotrophs. Test your understanding of how these processes fuel cellular functions.