Chapter 10: Introduction to Microbial Metabolism & Energy Use

Introduction to Microbial Metabolism & Energy Use introductory chapter on metabolism explores the vast chemical repertoire used by microbes for survival and reproduction, illustrating their importance through real-world applications such as methane generation during wastewater treatment and their essential contribution to human homeostasis via the microbiota. All organisms adhere to the laws of thermodynamics, using energy from their environment to perform the three major types of cellular work: chemical synthesis (anabolism), transport, and mechanical motion. Energy transfers are primarily conserved in the high-energy nucleotide ATP, which acts as the cell’s energy currency, coupling exergonic (energy-releasing) reactions to endergonic (energy-requiring) processes. Central to energy conservation are oxidation-reduction (redox) reactions, which involve the spontaneous transfer of electrons from a donor (more negative standard reduction potential, E 0′) to an acceptor (more positive E0′), releasing free energy ($\Delta G^\circ'$). Often, these redox reactions occur sequentially within membrane-associated structures called electron transport chains (ETCs), utilizing carriers such as NAD, FAD, Coenzyme Q, and cytochromes. Cellular chemistry is organized into interconnected biochemical pathways—which can be linear, branched, or cyclic—whose intermediate and end products are termed metabolites, with metabolite flux describing their rate of turnover. Every step in these pathways is accelerated by biological catalysts, either protein enzymes or catalytic RNA ribozymes, which dramatically increase reaction speed by lowering the required activation energy (E a) without altering the equilibrium constant (K eq). Enzyme function is characterized by maximum velocity (V max) and substrate affinity (Michaelis constant, K m), and can be inhibited competitively or noncompetitively. To ensure efficiency and maintain balance (homeostasis), metabolism is regulated through three main mechanisms: metabolic channeling (e.g., compartmentation), controlling the synthesis of enzymes (gene expression), and rapid posttranslational regulation. The latter includes allosteric control (where effectors alter the enzyme shape) and covalent modification (adding or removing chemical groups). Crucially, many pathways are governed by feedback inhibition, where the pathway's final product inhibits the initial pacemaker enzyme, effectively matching supply with cellular demand.