Microbial Metabolism: Enzymes, Catabolism, and Anabolism (Chapter 7 Study Guide)

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Metabolism and the Role of Enzymes

Biochemical Pathways and Enzyme Function

Metabolism in cells consists of all chemical reactions and is organized into biochemical pathways. Each reaction is catalyzed by a specific enzyme, and these pathways are divided into catabolic and anabolic processes.

Biochemical pathway: A sequence of chemical reactions in which the product of one reaction becomes the reactant for the next.
Catabolism: The breakdown of larger molecules into smaller ones, releasing energy.
Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input.

Enzymes are biological catalysts that speed up reactions by lowering the activation energy required. They are not consumed during the reaction and are highly specific for their substrates.

Enzyme inhibition: Occurs when a molecule (the inhibitor) binds to an enzyme and decreases its activity. Inhibition can be competitive (inhibitor binds to the active site) or noncompetitive (inhibitor binds elsewhere, changing the enzyme's shape).
Cofactors and coenzymes: Many enzymes require non-protein helpers. Cofactors are inorganic ions, while coenzymes are organic molecules (often derived from vitamins).

Types of Cellular Work

Cells perform three main types of work:

Chemical work: Synthesis of complex molecules.
Transport work: Movement of substances across membranes.
Mechanical work: Movement of cell structures or the cell itself.

The Pursuit and Utilization of Energy

Energy in Biological Systems

Energy is the capacity of a system to perform work. In cells, energy is stored in chemical bonds and released in redox reactions (oxidation-reduction reactions).

Endergonic reactions: Require energy input (e.g., anabolism).
Exergonic reactions: Release energy (e.g., catabolism).
Redox reactions: Involve the transfer of electrons. Oxidation is the loss of electrons, while reduction is the gain of electrons.
Biological oxidations: Often occur via dehydrogenation (removal of hydrogen atoms).

ATP: The Energy Currency

Adenosine triphosphate (ATP) is the main energy currency in cells. Energy released from catabolic reactions is used to synthesize ATP, which then powers cellular work.

ATP is produced by substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation.
Energy released from ATP hydrolysis is used for cellular processes.

Catabolism

Respiration and Fermentation

Catabolism of carbohydrates, such as glucose, yields energy and high-energy electrons. There are two main pathways:

Respiration: Complete oxidation of glucose to CO2 and H2O, yielding maximum ATP. Involves glycolysis, the Krebs cycle, and the electron transport chain (ETC).
Fermentation: Partial oxidation of glucose, yielding less ATP. End products include organic acids, alcohols, and gases.

Glycolysis

Glycolysis is the pathway that degrades glucose to pyruvate, producing ATP and NADH.

In aerobic respiration, pyruvate enters the Krebs cycle and is fully oxidized.
In fermentation, pyruvate is converted to organic end products.

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle further oxidizes pyruvate, generating NADH and FADH2 for the electron transport chain.

Electron Transport Chain (ETC) and Chemiosmosis

The ETC transfers electrons from NADH and FADH2 to oxygen (in aerobic respiration), creating a proton gradient across the membrane. This gradient powers ATP synthesis via chemiosmosis.

Proton motive force (pmf): The energy stored in the proton gradient.
ATP synthase: Enzyme that uses pmf to synthesize ATP from ADP and phosphate.

Summary Table: Catabolic Pathways

Pathway	Final Electron Acceptor	ATP Yield	End Products
Aerobic Respiration	O2	Up to 38 ATP	CO2, H2O
Anaerobic Respiration	Inorganic molecules (e.g., NO3-, SO42-)	Varies (less than aerobic)	CO2, reduced inorganic compounds
Fermentation	Organic molecules	2 ATP	Organic acids, alcohols, gases

Anabolism and the Crossing Pathways of Metabolism

Anabolic Pathways

Anabolism is the process of synthesizing complex molecules from simpler ones, requiring energy input. It is highly integrated with catabolic pathways.

Glycolysis and Krebs cycle: Provide intermediates for biosynthesis.
Carbohydrate biosynthesis: Glucose is synthesized from non-carbohydrate precursors via gluconeogenesis.
Amino acid, protein, and nucleic acid synthesis: Proteins and nucleic acids are built from amino acids and nucleotides, respectively.

Assembly of the Cell

Cell components are synthesized and assembled continuously. Nutritional requirements for biosynthesis vary among species due to genetic differences.

Macromolecules: Carbohydrates, proteins, lipids, and nucleic acids are essential for cell structure and function.
Cell division: Requires synthesis of new macromolecules to form daughter cells.

Key Terms and Definitions

Metabolism: The sum of all chemical reactions in a cell.
Enzyme: A protein that catalyzes chemical reactions.
Catabolism: Breakdown of molecules to release energy.
Anabolism: Synthesis of complex molecules from simpler ones.
ATP: Adenosine triphosphate, the main energy carrier in cells.
Redox reaction: Chemical reaction involving electron transfer.
Glycolysis: Pathway that breaks down glucose to pyruvate.
Krebs cycle: Series of reactions that oxidize acetyl-CoA to CO2 and generate NADH and FADH2.
Electron transport chain: Series of proteins that transfer electrons and generate a proton gradient for ATP synthesis.
Fermentation: Anaerobic process that produces energy and organic end products.

Important Equations

ATP Hydrolysis:

General Redox Reaction:

Glycolysis Net Reaction:

Aerobic Respiration (overall):

Additional info:

Some details on the integration of catabolism and anabolism, and the role of macromolecules in metabolism, were expanded for clarity.
Definitions and equations were added to ensure the notes are self-contained and suitable for exam preparation.