Microbial Metabolism: Enzymes, Catabolism, and Energy Production

Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all controlled biochemical reactions that occur within cells, enabling life by managing energy and molecular transformations. It is divided into two main processes: catabolism (breaking down molecules to release energy) and anabolism (building complex molecules, requiring energy input).

• Catabolism: Exergonic reactions that break down molecules, releasing energy.

• Anabolism: Endergonic reactions that synthesize complex molecules from simpler ones, consuming energy.

• Energy Storage: Energy from catabolic reactions is stored in adenosine triphosphate (ATP).

• Enzymes: Biological catalysts that facilitate metabolic reactions by lowering activation energy.

Figure: Metabolism is composed of catabolic and anabolic reactions, with energy transfer mediated by ATP and enzymes.

Enzymes and Their Roles in Metabolism

Structure and Function of Enzymes

Enzymes are primarily proteins that catalyze chemical reactions in cells without being consumed or permanently altered. They function by reducing the activation energy required for reactions, thus increasing reaction rates.

• Active Site: The region on the enzyme where substrates bind and reactions occur.

• Cofactors and Coenzymes: Some enzymes require non-protein helpers for activity. Cofactors are inorganic ions, while coenzymes are organic molecules.

• Holoenzyme: The complete, active enzyme with its cofactor(s) or coenzyme(s).

Enzyme Activity and Regulation

Enzyme activity is influenced by several factors, including temperature, pH, ionic concentration, substrate concentration, and the presence of inhibitors. Extreme changes can lead to denaturation, rendering the enzyme nonfunctional.

• Competitive Inhibitors: Molecules that bind to the active site, blocking substrate access.

• Noncompetitive Inhibitors: Molecules that bind elsewhere (allosteric site), altering the enzyme's shape and function.

• Denaturation: Loss of protein structure and function due to environmental changes.

Mechanism of Enzyme Action

Enzymes bind substrates to form an enzyme-substrate complex, facilitating the conversion to products and releasing the enzyme for reuse.

Oxidation-Reduction (Redox) Reactions

Redox Reactions in Metabolism

Redox reactions involve the transfer of electrons between molecules, essential for energy extraction in cells. Oxidation is the loss of electrons (or hydrogen), while reduction is the gain of electrons (or hydrogen).

• Electron Carriers: Molecules such as NAD+, NADP+, and FAD shuttle electrons during metabolic reactions.

• Key Electron Carriers:

  • Nicotinamide adenine dinucleotide: NAD+ → NADH

  • Nicotinamide adenine dinucleotide phosphate: NADP+ → NADPH

  • Flavin adenine dinucleotide: FAD → FADH2

ATP Production and Phosphorylation

Mechanisms of ATP Synthesis

ATP is synthesized by adding a phosphate group to ADP, a process known as phosphorylation. There are three main types:

• Substrate-level phosphorylation: Direct transfer of phosphate from a substrate to ADP.

• Oxidative phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis.

• Photophosphorylation: ATP synthesis using light energy (in photosynthetic organisms).

Carbohydrate Catabolism

Overview of Glucose Catabolism

Carbohydrates, especially glucose, are primary energy sources for most organisms. Glucose catabolism occurs via two main pathways: cellular respiration and fermentation.

• Cellular Respiration: Complete oxidation of glucose to CO2 and H2O, involving glycolysis, the Krebs cycle, and the electron transport chain (ETC).

• Fermentation: Partial oxidation of glucose, producing organic end products (e.g., lactate, ethanol) and regenerating NAD+ for glycolysis.

Glycolysis

Glycolysis is the metabolic pathway that converts glucose (6 carbons) into two molecules of pyruvate (3 carbons each), generating a net gain of ATP and NADH.

• Occurs in the cytoplasm of nearly all cells.

• Net products: 2 ATP (substrate-level phosphorylation), 2 NADH, 2 pyruvate.

Cellular Respiration: Krebs Cycle and Electron Transport Chain

After glycolysis, pyruvate is converted to acetyl-CoA, which enters the Krebs cycle. The cycle generates NADH and FADH2, which donate electrons to the ETC, driving ATP synthesis via oxidative phosphorylation.

• Krebs Cycle: Occurs in the cytosol (prokaryotes) or mitochondrial matrix (eukaryotes). Produces ATP, NADH, FADH2, and CO2.

• Electron Transport Chain (ETC): Located in the inner mitochondrial membrane (eukaryotes) or cytoplasmic membrane (prokaryotes). Electrons from NADH and FADH2 are transferred through a series of carriers, creating a proton gradient used by ATP synthase to generate ATP.

ATP Yield from Aerobic Respiration

The total ATP yield from one molecule of glucose in prokaryotes is approximately 38 ATP, including contributions from glycolysis, the Krebs cycle, and the ETC.

Pathway	ATP Produced	ATP Used	NADH Produced	FADH2 Produced
Glycolysis	4	2	2	0
Synthesis of acetyl-CoA and the citric acid cycle	2	0	8	2
Electron transport chain	34	0	0	0
Total	40	2
Net Total	38

Fermentation

Fermentation allows cells to regenerate NAD+ in the absence of an electron transport chain, enabling glycolysis to continue. It is less efficient than respiration and produces various organic end products.

• Common products: lactic acid, ethanol, propionic acid, acetone.

• Fermentation products are often useful in food and industrial processes.

Comparison of Aerobic Respiration, Anaerobic Respiration, and Fermentation

	Aerobic Respiration	Anaerobic Respiration	Fermentation
Oxygen Required	Yes	No	No
Type of Phosphorylation	Substrate-level and oxidative	Substrate-level and oxidative	Substrate-level
Final Electron (Hydrogen) Acceptor	Oxygen	NO3-, SO42-, CO32-, or externally acquired organic molecules	Cellular organic molecules
Potential Molecules of ATP Produced per Molecule of Glucose	~38 in prokaryotes, ~36 in eukaryotes	2-36	2

Integration and Regulation of Metabolism

Metabolic Pathways and Regulation

Cells regulate metabolism by controlling enzyme synthesis and activity. Catabolic enzymes are produced only when substrates are available, and anabolic pathways are suppressed if end products are present in the environment. Many metabolic pathways are amphibolic, serving both catabolic and anabolic roles.

Key Terms and Concepts

• Activation Energy: The minimum energy required to initiate a chemical reaction.

• Peptide Bond: Covalent bond linking amino acids in proteins.

• Oligosaccharide: Short-chain carbohydrate composed of 3-10 monosaccharide units.

• Amphibolic Pathway: A metabolic pathway that functions in both anabolism and catabolism.