Microbiology Study Guide: Enzymes, Metabolism, and Energy Pathways

Enzymes and Their Mechanisms

Definition and Mechanism of Enzyme Action

Enzymes are biological catalysts that speed up chemical reactions in living organisms by lowering the activation energy required for the reaction. They are highly specific for their substrates and function through a series of well-defined steps.

• Substrate: The molecule upon which an enzyme acts.

• Active Site: The region on the enzyme where the substrate binds.

• Substrate-Enzyme Complex: The intermediate formed when a substrate binds to the enzyme's active site.

• Activation Energy: The minimum energy required to initiate a chemical reaction.

• Transition State: A high-energy state during the reaction where old bonds are breaking and new bonds are forming.

• Cofactors and Coenzymes: Non-protein molecules that assist enzymes in catalyzing reactions. Cofactors are often metal ions, while coenzymes are organic molecules (e.g., NAD+, FAD).

Example: The enzyme sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Illustration: (Students should draw a diagram showing substrate binding to the active site, formation of the enzyme-substrate complex, transition state, and release of products.)

Thermodynamics in Biology

First Law of Thermodynamics

The first law of thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. In biological systems, this means that the energy from nutrients is converted into usable forms such as ATP.

• Equation: $\Delta E = q + w$ (where $\Delta E$ is the change in internal energy, $q$ is heat, and $w$ is work)

Metabolic Pathways

Anabolism, Catabolism, and Reaction Types

Metabolism consists of all chemical reactions in a cell, divided into two main types:

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy (endergonic reactions).

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy (exergonic reactions).

• Endergonic Reactions: Reactions that absorb energy ($\Delta G > 0$).

• Exergonic Reactions: Reactions that release energy ($\Delta G < 0$).

Application: Catabolic reactions provide the energy needed for anabolic processes in cells.

ATP and Energy Coupling

ATP Cycle and Energy Transfer

ATP (adenosine triphosphate) is the primary energy carrier in cells. The breakdown (hydrolysis) of ATP releases energy that can be used to drive endergonic reactions.

• ATP Hydrolysis: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

• Energy Coupling: The process by which the energy released from ATP hydrolysis is used to power other cellular reactions.

Redox Reactions in Metabolism

Oxidation and Reduction

Redox reactions involve the transfer of electrons between molecules. Oxidation is the loss of electrons, while reduction is the gain of electrons.

• Example: In cellular respiration, glucose is oxidized and oxygen is reduced.

Energy Sources: Chemotrophs and Phototrophs

Types of Organisms Based on Energy and Electron Sources

Organisms can be classified based on how they obtain energy and electrons:

• Phototrophs: Use light as an energy source.

• Chemotrophs: Use chemical compounds as an energy source.

• Chemoorganotrophs: Use organic compounds as electron donors.

• Chemolithotrophs: Use inorganic compounds as electron donors.

Example: Escherichia coli is a chemoorganotroph; Nitrosomonas is a chemolithotroph.

Aerobic Respiration

Stages and Substrates

Aerobic respiration is a multi-step process that converts glucose into ATP in the presence of oxygen. It occurs in both prokaryotes and eukaryotes, though the location of the steps may differ.

• Glycolysis: Glucose is broken down into pyruvate, producing ATP and NADH.

• Krebs Cycle (Citric Acid Cycle): Pyruvate is further oxidized, generating NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through protein complexes, leading to ATP synthesis.

Substrates and Products: Glucose (substrate) → CO2, H2O, and ATP (products).

Oxidative Phosphorylation and Chemiosmosis

ATP Synthesis via Chemiosmosis

Oxidative phosphorylation is the process by which ATP is formed as electrons are transferred to oxygen in the ETC. Chemiosmosis refers to the movement of protons across a membrane, generating a proton gradient that drives ATP synthesis via ATP synthase.

• Location: Prokaryotes: plasma membrane; Eukaryotes: inner mitochondrial membrane.

• Equation: $\mathrm{ADP} + \mathrm{P_i} \rightarrow \mathrm{ATP}$ (driven by proton motive force)

Fermentation vs. Aerobic Respiration

Comparison of Metabolic Pathways

Fermentation and aerobic respiration are two pathways for ATP production:

Pathway	Oxygen Required?	ATP Yield (per glucose)	End Products
Fermentation	No	2	Lactic acid, ethanol, CO2
Aerobic Respiration	Yes	~36-38	CO2, H2O

Which produces more ATP? Aerobic respiration produces the most ATP; fermentation produces the least.

Fermentation Pathways

Types and Examples

Fermentation is an anaerobic process that allows cells to regenerate NAD+ for glycolysis by converting pyruvate into various end products.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid (e.g., in muscle cells, Lactobacillus).

• Alcoholic Fermentation: Pyruvate is converted to ethanol and CO2 (e.g., in yeast).

• Mixed Acid Fermentation: Produces a variety of acids and gases (e.g., in Escherichia coli).

Example: Yogurt production uses lactic acid fermentation by bacteria.

Exoenzymes

Definition and Example

Exoenzymes are enzymes secreted by cells to function outside the cell, breaking down large molecules into smaller ones that can be absorbed.

• Example: Amylase, secreted by bacteria and fungi, breaks down starch into sugars.