Microbial Metabolism: Study Guide and Key Concepts

Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all controlled biochemical reactions that occur within cells, enabling life by managing energy and molecular transformations. It is divided into two main processes: catabolism (breaking down molecules to release energy) and anabolism (building complex molecules, requiring energy input).

• Catabolism: Degradative, exergonic reactions that break down molecules and release energy.

• Anabolism: Biosynthetic, endergonic reactions that use energy to build cellular components.

• Energy Storage: Energy from catabolic reactions is stored in adenosine triphosphate (ATP).

• Precursor Metabolites: Small molecules generated by catabolism, used in anabolic pathways.

Example: Glucose catabolism provides both energy (as ATP) and precursor metabolites for biosynthesis.

Redox Reactions and Electron Carriers

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons from an electron donor to an electron acceptor. These reactions are central to energy production in cells.

• Oxidation: Loss of electrons (often as hydrogen atoms).

• Reduction: Gain of electrons.

• Electron Carriers: Molecules that shuttle electrons during metabolic reactions.

Carrier	Oxidized Form	Reduced Form	Role
Nicotinamide adenine dinucleotide	NAD+	NADH	Catabolic reactions (glycolysis, Krebs cycle, ETC)
Nicotinamide adenine dinucleotide phosphate	NADP+	NADPH	Anabolic reactions (biosynthesis)
Flavine adenine dinucleotide	FAD	FADH2	Krebs cycle, ETC

Equation Example:

$\mathrm{NAD}^+ + 2\mathrm{H} \rightarrow \mathrm{NADH} + \mathrm{H}^+$

ATP Production and Phosphorylation

ATP: The Energy Currency

ATP stores energy in its high-energy phosphate bonds. Hydrolysis of ATP releases energy for cellular work.

$\mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

Types of Phosphorylation

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP from a phosphorylated substrate (occurs in glycolysis and Krebs cycle).

• Oxidative phosphorylation: Energy from redox reactions in the electron transport chain (ETC) is used to generate ATP via ATP synthase.

• Photophosphorylation: Light energy is used to phosphorylate ADP (in photosynthetic organisms).

Example: Substrate-level phosphorylation produces ATP during glycolysis.

Enzymes and Their Regulation

Enzyme Structure and Function

Enzymes are biological catalysts, mostly proteins, that speed up chemical reactions by lowering activation energy. They are highly specific for their substrates.

• Active Site: Region where substrate binds and reaction occurs.

• Induced-fit Model: Enzyme changes shape slightly to fit substrate.

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins) required by some enzymes.

• Ribozymes: RNA molecules with catalytic activity.

Factors Affecting Enzyme Activity

• Temperature: High or low temperatures can denature enzymes.

• pH: Extreme pH values disrupt enzyme structure and function.

• Ionic Concentration: Changes can affect enzyme shape and activity.

• Substrate Concentration: Affects reaction rate up to a saturation point.

• Enzyme Concentration: More enzyme increases reaction rate (if substrate is available).

Denaturation: Permanent loss of enzyme structure and function due to extreme conditions.

Enzyme Inhibition

• Competitive Inhibitors: Resemble substrate and bind to active site, blocking substrate access.

• Noncompetitive Inhibitors: Bind to allosteric site, changing enzyme shape and reducing activity.

Example: Sulfanilamide is a competitive inhibitor of an enzyme in folic acid synthesis.

Applications: Many drugs (e.g., penicillin, aspirin) act as enzyme inhibitors.

Macromolecules: Proteins and Amino Acids

Proteins

• Functions: Structure, catalysis (enzymes), regulation, transport, defense.

• Sensitivity: Proteins are sensitive to pH, temperature, and ionic concentration.

Amino Acids

• Monomers of proteins (21 types in cells).

• Peptide Bonds: Covalent bonds linking amino acids.

• Structure: Side chains (R groups) determine protein properties and interactions.

Carbohydrate Catabolism

Overview

Most organisms use carbohydrates, especially glucose, as their primary energy source. Glucose catabolism occurs via two main pathways: cellular respiration and fermentation.

• Cellular Respiration: Complete oxidation of glucose to CO2 and H2O (glycolysis, Krebs cycle, ETC).

• Fermentation: Partial oxidation of glucose; organic molecules serve as electron acceptors.

Glycolysis

• Occurs in cytoplasm of all cells.

• Glucose (6C) split into two pyruvate (3C each).

• Net gain: 2 ATP (substrate-level phosphorylation), 2 NADH, 2 pyruvate.

$\text{Glucose} + 2\,\mathrm{NAD}^+ + 2\,\mathrm{ADP} + 2\,\mathrm{P_i} \rightarrow 2\,\text{Pyruvate} + 2\,\mathrm{NADH} + 2\,\mathrm{ATP} + 2\,\mathrm{H}_2\mathrm{O}$

Cellular Respiration

• Conversion of Pyruvate to Acetyl-CoA: Pyruvate dehydrogenase converts pyruvate to acetyl-CoA, producing CO2 and NADH.

• Krebs Cycle: Occurs in cytosol (prokaryotes) or mitochondrial matrix (eukaryotes). Produces ATP, NADH, FADH2, CO2.

• Electron Transport Chain (ETC): NADH and FADH2 donate electrons to ETC, generating a proton gradient used to make ATP.

Location of ETC: Inner mitochondrial membrane (eukaryotes), cytoplasmic membrane (prokaryotes).

Electron Acceptors in ETC

• Aerobic Respiration: O2 is the final electron acceptor.

• Anaerobic Respiration: Other molecules (e.g., SO42-, NO3-, CO32-) serve as final electron acceptors.

Oxidative Phosphorylation and Chemiosmosis

• Energy from electrons is used to pump H+ across the membrane, creating a proton gradient.

• ATP synthase uses the proton motive force to phosphorylate ADP to ATP.

$\mathrm{ADP} + \mathrm{P_i} \xrightarrow{\text{ATP synthase}} \mathrm{ATP}$

Peter Mitchell: Proposed the chemiosmotic theory explaining ATP generation via proton gradients.

ATP Yield from Aerobic Respiration

Pathway	ATP Produced (Prokaryotes)
Glycolysis	2
Pyruvate to Acetyl-CoA	0
Krebs Cycle	2
Electron Transport Chain	~34
Total	~38

*Additional info: In eukaryotes, the total ATP yield is slightly lower (~36) due to transport costs across mitochondrial membranes.

Fermentation

• Used by cells lacking ETC or when O2 is absent.

• Regenerates NAD+ for glycolysis by reducing organic molecules.

• Yields less ATP than respiration.

• Common products: lactic acid, ethanol, acetate.

Commercial Applications: Fermentation is used to produce bread, yogurt, cheese, alcoholic beverages, and more.

Integration and Regulation of Metabolism

Metabolic Regulation

• Catabolic Enzymes: Synthesized only when substrate is available.

• Anabolic Pathways: Synthesis stops if end product is available in the environment.

• Amphibolic Reactions: Pathways that function in both catabolism and anabolism.

Summary Table: Comparison of Metabolic Pathways

Pathway	Final Electron Acceptor	ATP Yield	End Products
Aerobic Respiration	O2	~38 (prokaryotes)	CO2, H2O
Anaerobic Respiration	Inorganic molecules (e.g., NO3-)	Varies (<38)	CO2, H2O, other reduced compounds
Fermentation	Organic molecules	2	Lactic acid, ethanol, etc.

Key Terms and Definitions

• Metabolism: All chemical reactions in a cell.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones.

• Redox Reaction: Coupled oxidation and reduction reactions.

• ATP: Main energy currency of the cell.

• Enzyme: Biological catalyst that speeds up reactions.

• Substrate: Molecule upon which an enzyme acts.

• Activation Energy: Energy required to start a reaction.

• Active Site: Region on enzyme where substrate binds.

• Allosteric Site: Site on enzyme where non-substrate molecules bind, affecting activity.

• Competitive/Noncompetitive Inhibitor: Molecules that reduce enzyme activity by different mechanisms.

• Cellular Respiration: Complete oxidation of glucose to CO2 and H2O, producing ATP.

• Fermentation: Partial oxidation of glucose without ETC; regenerates NAD+.

• Glycolysis: Splitting of glucose into pyruvate.

• Krebs Cycle: Series of reactions oxidizing acetyl-CoA to CO2.

• Electron Transport Chain: Series of carriers transferring electrons to generate ATP.

• ATP Synthase: Enzyme that synthesizes ATP using proton gradient.

• Chemiosmosis: Movement of ions across a membrane to generate ATP.

• Beta-oxidation: Catabolism of fatty acids.

• Deamination: Removal of amino group from amino acids during protein catabolism.

Additional info: Understanding these concepts is essential for mastering microbial metabolism and its applications in biotechnology, medicine, and environmental science.