Microbial Metabolism: Key Concepts and Pathways

Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all chemical processes carried out by living organisms. These processes are essential for growth, reproduction, and maintenance of cellular structures.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy in the process.

• Anabolism (Biosynthesis): The synthesis of complex molecules from simpler ones, requiring an input of energy.

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

ATP: The Energy Currency of the Cell

Structure and Function of ATP

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It stores energy in its high-energy phosphate bonds, which can be released to fuel cellular processes.

• ATP Hydrolysis: The reaction $\text{ATP} \rightarrow \text{ADP} + P_i + \text{ENERGY}$ releases energy for cellular work.

• ATP as "energy currency": Used for metabolic processes that require energy.

• Example: Escherichia coli cells contain about 5 million ATP molecules, but use up to 2.5 million ATP molecules per second.

Mechanisms of ATP Generation

• Substrate-level phosphorylation: Direct transfer of a phosphate group from an organic molecule to ADP to form ATP.

• Oxidative phosphorylation: ATP is generated using energy released by electron transfer through the electron transport chain.

• Photophosphorylation: Light energy is used to generate ATP, primarily in photosynthetic organisms.

General ATP synthesis reaction: $\text{ADP} + P_i + \text{ENERGY} \rightarrow \text{ATP}$

Redox Reactions in Metabolism

Oxidation and Reduction

Redox (reduction-oxidation) reactions are central to energy transfer in cells.

• Oxidation: Removal of electrons (e-) from a molecule, releasing energy.

• Reduction: Addition of electrons (e-) to a molecule, storing energy.

• These reactions are coupled: when one molecule is oxidized, another is reduced.

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

Enzymes and Catalysis

Properties and Function of Enzymes

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions without being consumed.

• Catalyst: A substance that increases the rate of a reaction without undergoing permanent change.

• Specificity: Enzymes react with specific substrates due to the unique shape of their active site.

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

• Activation Energy: The minimum energy required to initiate a chemical reaction. Enzymes lower this barrier, increasing reaction rates.

Example: The enzyme hexokinase catalyzes the phosphorylation of glucose in glycolysis.

Enzyme-Substrate Complex

Enzyme action involves the formation of an enzyme-substrate complex, facilitating the conversion of substrates into products.

• The substrate binds to the enzyme's active site, forming the enzyme-substrate complex.

• The enzyme catalyzes the reaction, converting the substrate into product(s).

• The enzyme is released unchanged and can catalyze additional reactions.

Example: The enzyme fructose-1,6-bisphosphate aldolase splits fructose-1,6-bisphosphate into two three-carbon sugars during glycolysis.

Visual Representation of Enzyme Action

• Enzyme and substrate fit together like a lock and key, or may undergo induced fit for tighter binding.

• Enzyme-substrate complex formation is essential for catalysis.

Additional info: The figures provided illustrate the binding of substrates to enzyme active sites and the subsequent formation of products, as seen in glycolytic enzymes.