Lecture 5 - Microbial Metabolism

Microbial Metabolism

Overview of Metabolism

• Definition: Metabolism refers to the sum of all chemical reactions in a cell, which comprises two main categories: catabolism and anabolism.

• Catabolism: This involves the breakdown of large molecules into smaller units, releasing energy in the process.

• Anabolism: In contrast, this refers to the synthesis of complex organic molecules from simpler levels, requiring an input of energy.

Biochemistry of Microbes

• Microbes utilize various nutrient sources to generate pyruvate, which is crucial in different metabolic pathways, including the Krebs cycle and fermentation processes. Common nutrient sources include glucose, lactose, and mannitol, leading to products such as lactic acid or alcohol, depending on the metabolic route taken. The pathways involved illustrate the diverse energetic options available to microbes, influencing how they produce ATP.

Energy and Work

• Energy Requirements: Energy is essential for cells to perform work necessary for survival and reproduction, involving three main types of work:

  • Chemical Work: Related to the synthesis of biological molecules.

  • Transport Work: Involving the movement of compounds across cell membranes.

  • Mechanical Work: Relates to cell movement and structure support.

The Role of ATP

• Definition: Adenosine Triphosphate (ATP) serves as the primary energy currency of the cell.

• Functionality: Energy is released from ATP when a phosphate group is cleaved off, a process involving breaking covalent bonds. The energy from ATP drives various cellular mechanisms, including powering motor proteins for movement and catalyzing chemical reactions through phosphorylation, exemplifying its critical role in metabolism.

Enzymatic Regulation in Metabolism

• Enzymes: They catalyze biochemical reactions, facilitating the conversion of substrates to products. Enzymes often consist of multiple subunits creating an active site for substrate binding.

Factors Influencing Enzyme Activity

• Enzyme activity is impacted by substrate concentration, temperature, and pH.

  • Substrate Concentration: Increased concentration raises the likelihood of enzyme-substrate collisions, but the activity will plateau once saturation (Vmax) is reached.

  • Temperature and pH: Each enzyme has an optimal temperature and pH range, deviating from which can reduce activity through denaturation or altered binding sites.

Regulation of Enzyme Activity

• Feedback Inhibition: A regulatory mechanism where the end products of metabolic pathways inhibit upstream enzymes to control the flow of metabolites.

• Competitive and Non-discriminatory Inhibition: Chemicals can bind either to the active site (competitive) or to another site, causing changes in the active site (non-competitive), impacting enzyme function.

Microbial Catabolism

• Types of Respiration: Microbial catabolism includes multiple pathways for generating energy:

  • Chemoorganotrophy: Involves the oxidation of organic compounds.

  • Chemolithotrophy: Concerns oxidation of inorganic compounds.

  • Phototrophy: Entails energy capture from sunlight.

• Fermentation: A process differentiating from oxidative phosphorylation as it does not use the electron transport chain and occurs in the absence of oxygen, producing ATP by substrate-level phosphorylation.

Aerobic Respiration

• Aerobic respiration occurs in three stages, leading to the complete oxidation of glucose via glycolysis followed by the Krebs cycle and electron transport chain. This process produces ATP and electron carriers such as NADH and FADH2, highlighting its greater efficiency over fermentation alone.

The Krebs Cycle

• Known as the tricarboxylic acid cycle, it fully oxidizes pyruvate, generating carbon dioxide and reducing agents (NADH and FADH2). Each cycle consumes one molecule of acetyl-CoA and ultimately produces substantial energy harnessed during oxidative phosphorylation.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

• The ETC comprises protein complexes that transfer electrons derived from NADH and FADH2, creating a proton motive force (PMF) essential for ATP generation. This process completes aerobic respiration by producing ATP through chemiosmosis.

ATP Synthase

• Structure: Composed of F0 (membrane-embedded proton channel) and F1 (catalytic unit) allowing proton flow to drive ATP synthesis. The rotary mechanisms facilitate the binding of ADP and inorganic phosphate, generating ATP as protons are translocated, demonstrating the efficacy of biochemical energy conversion.

Breakdown of Biological Macromolecules

• Microbial metabolism includes the breakdown of polysaccharides, lipids, and proteins to harvest energy:

  • Polysaccharides: Cleaved by exoenzymes produced by microbes to liberate glucose and other necessary components.

  • Lipids: Energy derived from triglycerides requires lipase for breakdown.

  • Proteins: Hydrolyzed by proteases into amino acids, extensively utilized by certain fungi and bacteria, showcasing the versatility in substrates used by microbes for energy.

Anabolism and Biomolecule Synthesis

• Energy in Anabolism: The synthesis of biomolecules through anabolic pathways requires energy in the form of ATP and reducing power from catabolism.

• Biosynthesis Principles: Key principles guide biosynthesis, including the efficiency of pathways, the use of metabolic intermediates as precursors, and compartmentalization of catabolic and anabolic reactions.

Precursor Metabolites and Gluconeogenesis

• Precursor metabolites are vital building blocks derived from glycolysis and TCA cycles, facilitating the synthesis of various cellular components. Gluconeogenesis aids in synthesizing glucose from non-carbohydrate precursors, exemplifying the interconnected nature of metabolic pathways in microorganisms.