Microbial Metabolism: Enzymes, Energy, and Pathways

Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses all the chemical reactions that occur within microorganisms to maintain life. These reactions are essential for energy production, growth, and cellular maintenance. Metabolic processes are broadly categorized as catabolic (breaking down molecules to release energy) or anabolic (building complex molecules from simpler ones, requiring energy).

Exergonic and Endergonic Reactions

Energy in Metabolic Reactions

• Exergonic Reactions: Release energy during the process (e.g., cellular respiration).

• Endergonic Reactions: Require an input of energy to proceed (e.g., biosynthesis of macromolecules).

Redox Reactions

Oxidation and Reduction in Metabolism

• Redox Reactions: Involve the transfer of electrons between molecules; consist of oxidation (loss of electrons) and reduction (gain of electrons).

• Electron Acceptor: The molecule that gains electrons and is reduced.

• Electron Donor: The molecule that loses electrons and is oxidized.

• Reduction: Often involves gaining an electron on a hydrogen atom.

• Oxidation: Involves losing an electron or hydrogen atom, or gaining an oxygen atom. Loss of a hydrogen atom is called a dehydration reaction.

Electron Carriers

NAD+, NADP+, and Their Reduced Forms

• NAD+ (Nicotinamide adenine dinucleotide): Reduced to NADH during catabolic reactions.

• NADP+ (Nicotinamide adenine dinucleotide phosphate): Reduced to NADPH, primarily used in anabolic reactions.

ATP: The Energy Currency

Properties and Functions of ATP

• ATP (Adenosine triphosphate): A water-soluble molecule that can accumulate to high concentrations in cells without causing harm.

• ATP can lose one or two phosphate groups, forming ADP (adenosine diphosphate) or AMP (adenosine monophosphate).

• ATP donates phosphate groups to other molecules, transferring energy.

ATP Synthesis Mechanisms

1. Substrate-level phosphorylation: Transfer of a phosphate group from a phosphorylated compound directly to ADP.

2. Oxidative phosphorylation: Energy from redox reactions of respiration is used to attach inorganic phosphate to ADP.

3. Photophosphorylation: Light energy is used to add inorganic phosphate to ADP (in photosynthetic organisms).

Enzymes in Metabolism

Role and Structure of Enzymes

• Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy.

• Enzyme names typically end in -ase and often reflect the substrate they act upon.

• Enzymes are necessary because living organisms cannot achieve the high reactant concentrations or temperatures required for many reactions to occur spontaneously.

Enzyme Components

• Apoenzyme: The protein portion of an enzyme, inactive without its cofactor.

• Cofactor: Non-protein component required for enzyme activity; can be inorganic ions (e.g., Fe2+, Mg2+, Zn2+, Cu+) or organic molecules (coenzymes, often derived from vitamins).

• Coenzymes: Organic cofactors such as NAD+, NADP+, and FAD.

• Holoenzyme: The complete, active enzyme with its apoenzyme and cofactor(s) bound.

Types of Enzymes (by Function)

Enzyme Class	Function	Example
Hydrolases	Add water to break bonds (hydrolysis, catabolic)	Lipase
Isomerases	Rearrange atoms within a molecule	Phosphoglucoisomerase
Ligases/Polymerases	Join two molecules, often using ATP (anabolic)	DNA polymerase
Lysases	Split large molecules without water (catabolic)	Aldolase
Oxidoreductases	Remove or add electrons (catabolic and anabolic)	Lactate dehydrogenase
Transferases	Transfer functional groups between molecules (anabolic)	Transaminase

Enzyme-Substrate Interaction

• Specificity: Enzymes are highly specific for their substrates, often described by the "lock and key" model.

• Induced Fit Model: The enzyme's active site changes shape to better fit the substrate upon binding.

• Mechanism:

  1. Enzyme and substrate associate.

  2. Form an enzyme-substrate complex (induced fit).

  3. Substrate is converted to product.

  4. Enzyme releases product and resumes original shape.

Enzyme Regulation

• Denaturation: High temperature or extreme pH can disrupt noncovalent bonds, causing loss of enzyme structure and function. This may be reversible or irreversible.

• Competitive Inhibition: Inhibitors resemble the substrate and bind to the active site, blocking substrate access. This can be reversible or permanent.

• Example: Sulfanilamide inhibits the enzyme that converts PABA to folic acid, essential for nucleic acid synthesis.

• Allosteric Regulation: Molecules bind to an allosteric site (not the active site), causing conformational changes that activate or inhibit the enzyme.

• Feedback Inhibition (Negative Feedback): The end product of a metabolic pathway inhibits an earlier enzyme, conserving energy and resources.

Metabolic Pathways

Glycolysis and Alternatives

• Glycolysis: The oxidation of glucose to pyruvic acid, producing ATP and NADH.

• During glycolysis, two molecules of G3P (glyceraldehyde-3-phosphate) are oxidized to pyruvic acid.

• Pentose Phosphate Pathway: An alternative to glycolysis, important for generating NADPH and pentoses for biosynthesis.

• Entner-Doudoroff (ED) Pathway: Another alternative, found only in prokaryotes.

Fermentation

• Fermentation: Partial oxidation of sugar or other metabolites to release energy, using an organic molecule from within the cell as the final electron acceptor.

• Produces organic waste products (e.g., lactic acid, ethanol).

Acetyl-CoA Formation and the Krebs Cycle

• Acetyl-CoA: Formed from pyruvic acid, with the loss of two CO2 molecules per glucose.

• The bond between acetyl and coenzyme A is high-energy.

• Krebs Cycle (Citric Acid Cycle): Completes the oxidation of acetyl-CoA, generating NADH, FADH2, ATP, and CO2. Also used for catabolism of lipids and proteins.

• By the end of the Krebs cycle, all carbons from glucose have been released as CO2.

Electron Transport Chain (ETC) and Chemiosmosis

• ETC: A series of membrane-bound carriers that transfer electrons, creating a proton gradient across the membrane.

• ATP Generation: The proton gradient drives ATP synthesis via chemiosmosis.

Major Electron Carriers in the ETC

Carrier	Description	Source
Flavoproteins	Integral membrane proteins with flavin coenzyme (from vitamin B2, e.g., FAD)	Vitamin B2
Ubiquinones	Lipid-soluble, non-protein carriers, universally present	Vitamin K
Metal-containing proteins	Integral proteins with iron, sulfur, or copper atoms (copper only in photosynthetic ETC)	Various
Cytochromes	Proteins associated with heme groups	Heme

Summary Table: Key Metabolic Pathways

Pathway	Main Function	Key Products	Notes
Glycolysis	Glucose breakdown	ATP, NADH, pyruvate	Occurs in cytoplasm
Pentose Phosphate Pathway	Alternative glucose breakdown	NADPH, pentoses	Important for biosynthesis
Entner-Doudoroff Pathway	Alternative glucose breakdown (prokaryotes)	ATP, NADPH, pyruvate	Not found in eukaryotes
Krebs Cycle	Oxidation of acetyl-CoA	ATP, NADH, FADH2, CO2	Central to metabolism
Fermentation	Energy production without ETC	Organic acids/alcohols, ATP	Final electron acceptor is organic

Key Equations

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

• General Redox Reaction: $\text{AH}_2 + \text{B} \rightarrow \text{A} + \text{BH}_2$

• Glycolysis (Net Reaction): $\mathrm{Glucose} + 2\ \mathrm{NAD}^+ + 2\ \mathrm{ADP} + 2\ \mathrm{P_i} \rightarrow 2\ \mathrm{Pyruvate} + 2\ \mathrm{NADH} + 2\ \mathrm{ATP} + 2\ \mathrm{H}_2\mathrm{O}$

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the summary tables and equations.