Metabolism, Enzyme Function, and Regulation in Microbiology

Metabolism, Anabolism, and Catabolism

Definitions and Distinctions

Metabolism encompasses all chemical reactions occurring within an organism. These reactions are divided into two main categories: anabolism and catabolism.

• Anabolism: The set of metabolic pathways that construct molecules from smaller units. These reactions require energy and are endergonic (energy-consuming).

• Catabolism: The set of metabolic pathways that break down molecules into smaller units to release energy. These reactions are exergonic (energy-releasing).

Example: The breakdown of glucose during cellular respiration is catabolic, while the synthesis of proteins from amino acids is anabolic.

Enzyme Classification and Function

Six Basic Classes of Enzymes

Enzymes are biological catalysts that speed up chemical reactions. They are classified based on the type of reaction they catalyze.

Class	Function (Activity)	Example
Oxidoreductases	Transfer of electrons or hydrogen atoms from one molecule to another	Lactic acid dehydrogenase
Transferases	Moving a functional group from one molecule to another	Hexokinase
Hydrolases	Hydrolysis (catabolic); breaking chemical bonds with water	Lipase
Isomerases	Rearrangement of atoms within a molecule (neither anabolic nor catabolic)	Phosphoglucoisomerase
Ligases or polymerases	Joining two or more chemicals together (anabolic)	Acetyl-CoA synthetase
Lyases	Splitting a chemical into smaller parts without using water (catabolic)	Fructose

Enzyme Activity: Factors Affecting Function

Key Factors Influencing Enzyme Activity

• Temperature: Enzyme activity increases with temperature up to an optimal point. Beyond this, the enzyme denatures and loses function.

• pH: Extreme pH values can denature enzymes by disrupting hydrogen bonds and ionic interactions, altering the enzyme's structure. Each enzyme has an optimal pH.

• Substrate Concentration: As substrate concentration increases, enzyme activity rises until all active sites are occupied (saturation point). Beyond this, activity plateaus.

• Inhibitors:

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

  • Non-competitive Inhibitors: Bind to a different site, causing a conformational change that reduces or blocks enzyme activity.

Example: High fever can denature enzymes, while antacids can alter stomach pH and affect digestive enzymes.

Oxidation and Reduction Reactions

Contrast Between Oxidation and Reduction

• Oxidation: Loss of electrons, loss of hydrogen atom, or gain of oxygen atom.

• Reduction: Gain of electrons, gain of hydrogen atom, or loss of oxygen atom.

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

Negative Feedback in Enzyme Activity

Definition and Mechanism

Negative feedback occurs when the end product of a metabolic pathway inhibits an earlier step in the pathway, thus regulating the pathway's activity and preventing overproduction of the end product.

• Often involves allosteric inhibition, where the end product binds to an allosteric site on an enzyme, changing its shape and reducing its activity.

Example: The end product of the biosynthesis of isoleucine inhibits the first enzyme in its pathway.

Types of Phosphorylation

Three Main Types

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from another phosphorylated compound. $\text{ADP} + \text{Phosphorylated Substrate} \rightarrow \text{ATP} + \text{Substrate}$

• Oxidative phosphorylation: Energy from redox reactions (usually in the electron transport chain) is used to attach inorganic phosphate to ADP. $\text{ADP} + \text{P}_i \xrightarrow{\text{ETC}} \text{ATP}$

• Photophosphorylation: Light energy is used to phosphorylate ADP with inorganic phosphate, as in photosynthesis.

Regulation of Metabolism

Mechanisms of Metabolic Regulation

• Gene Expression: Cells regulate the amount and timing of enzyme production by controlling gene expression.

• Control of Enzyme Activity: Cells can activate or inhibit enzymes after they are produced, often through feedback inhibition or covalent modification.

Example: Bacteria can turn on or off genes for lactose metabolism depending on the presence of lactose.

Amphibolic Reactions

Definition

Amphibolic reactions are metabolic pathways that can function in both anabolic (building up) and catabolic (breaking down) processes. They allow cells to adapt to changing energy and biosynthetic needs.

Example: The citric acid cycle is amphibolic, serving both energy production and biosynthesis.

Biosynthesis of Major Biomolecules

Carbohydrates

• Complex carbohydrates are synthesized from simple precursors via photosynthesis (in plants) or gluconeogenesis (in animals and some bacteria).

• Common precursors include pyruvate, lactate, amino acids, and glycerol.

• Newly synthesized sugars can be stored as starch or glycogen, or converted into other essential carbohydrates.

Lipids

• Lipids are synthesized from acetyl-CoA and other precursors.

• Fatty acids are typically synthesized via the fatty acid synthase complex.

• Other lipids, such as steroids, are synthesized via complex pathways involving isoprenoid intermediates.

Amino Acids

• Amino acids are synthesized from precursor metabolites derived from glycolysis, the citric acid cycle, and the pentose phosphate pathway.

• Cells can also synthesize many amino acids from other amino acids via transamination reactions.

Cellular Regulation of Sugar Utilization

Glucose vs. Lactose Utilization in Bacteria

When multiple energy sources are available, bacteria preferentially metabolize the most energy-efficient one (often glucose). This is regulated by mechanisms such as catabolite repression:

• In the presence of glucose, the genes for lactose utilization (e.g., the lac operon) are repressed.

• Once glucose is depleted, repression is lifted, and enzymes for lactose metabolism are produced.

Example: Escherichia coli uses the lac operon to regulate lactose metabolism based on glucose availability.