Microbial Metabolism: Enzymes, Pathways, and Cellular Respiration

Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses the complete set of chemical reactions that occur within a microorganism. The primary goal of metabolism is to support the reproduction and survival of the organism by managing energy and matter transformation.

• Metabolism is divided into two main types: catabolism (breaking down molecules to release energy) and anabolism (synthesizing complex molecules from simpler ones).

• Metabolic pathways are highly regulated and interconnected, ensuring efficient use of resources.

• Understanding microbial metabolism is crucial for biotechnology, medicine, and environmental science.

Example: Plastic-eating bacteria can metabolize synthetic polymers, offering solutions for waste management.

Why Study Microbial Metabolism?

Applications and Importance

Microbial metabolism has significant implications for human health, industry, and the environment.

• Microbes can be engineered to produce pharmaceuticals, such as antibiotics or antimalarial drugs.

• Research shows that yeast can function with human genes, highlighting evolutionary conservation and potential for genetic engineering.

• Microbes can degrade pollutants, such as plastics, contributing to environmental sustainability.

Metabolic Pathways

Catabolic and Anabolic Pathways

Metabolic pathways are sequences of enzymatically catalyzed chemical reactions in a cell. Each step is facilitated by a specific enzyme.

• Catabolic pathways break down large molecules into smaller ones, releasing energy (usually in the form of ATP).

• Anabolic pathways use energy to build complex molecules from simpler ones.

ATP: The Energy Currency

Structure and Function of ATP

Adenosine triphosphate (ATP) is the primary energy carrier in cells.

• ATP consists of an adenine base, a ribose (5-carbon sugar), and three phosphate groups.

• Energy is stored in the high-energy bonds between phosphate groups and released upon hydrolysis to ADP (adenosine diphosphate).

Redox Reactions in Metabolism

Oxidation and Reduction

Redox reactions are central to energy transfer in metabolism.

• Oxidation involves the loss of electrons (OIL: Oxidation Is Loss).

• Reduction involves the gain of electrons (RIG: Reduction Is Gain).

• Electron carriers such as NAD+ and FAD are reduced during catabolic reactions and later oxidized to generate ATP.

Enzymes and Catalysis

Enzyme Structure and Function

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

• Most enzymes are proteins; some are RNA molecules called ribozymes.

• Enzymes lower the activation energy required for a reaction to proceed.

• Enzyme activity depends on the shape of the active site, which is specific to the substrate.

• Some enzymes require cofactors (inorganic ions or organic molecules) to be active. The complete, active enzyme is called a holoenzyme.

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

Types of Enzyme Reactions

• Hydrolases: Catalyze hydrolysis reactions (breaking bonds with water).

• Isomerases: Rearrange atoms within a molecule.

• Polymerases: Join two molecules together.

• Lyases: Split molecules without using water.

• Oxidoreductases: Transfer electrons between molecules.

• Transferases: Move functional groups from one molecule to another.

Enzyme Inhibition and Regulation

• Inhibitors are substances that decrease or block enzyme activity.

• Competitive inhibitors bind to the active site, preventing substrate binding.

• Non-competitive inhibitors bind to an allosteric site, changing the enzyme's shape and function.

• Feedback inhibition occurs when the end product of a pathway inhibits an earlier enzyme, regulating the pathway's activity.

Glycolysis

Overview and Stages

Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP and NADH. It is the first step in both aerobic and anaerobic respiration.

• Occurs in the cytoplasm of cells.

• Does not require oxygen.

• Net products per glucose: 2 ATP, 2 NADH, and 2 pyruvate molecules.

Stages of Glycolysis

• Energy-Investment Stage: 2 ATP are used to phosphorylate glucose and its intermediates.

• Lysis Stage: The six-carbon sugar is split into two three-carbon molecules.

• Energy-Conserving Stage: 4 ATP and 2 NADH are produced (net gain: 2 ATP).

Cellular Respiration

Overview

Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into ATP, and release waste products.

• Includes glycolysis, the Krebs cycle (TCA or citric acid cycle), and the electron transport chain.

• Involves a series of redox reactions to extract energy from organic molecules.

The Krebs Cycle (TCA Cycle, Citric Acid Cycle)

The Krebs cycle is a series of enzyme-catalyzed reactions that oxidize acetyl-CoA to CO2 and transfer energy to NAD+ and FAD.

• Occurs in the cytoplasm of prokaryotes and mitochondria of eukaryotes.

• Each turn of the cycle produces:

  • 3 NADH

  • 1 FADH2

  • 1 GTP (or ATP)

  • 2 CO2

• For each glucose molecule (2 acetyl-CoA):

  • 6 NADH

  • 2 FADH2

  • 2 ATP (or GTP)

  • 4 CO2

Main Types of Reactions in the Krebs Cycle

• Anabolism of citric acid

• Isomerization

• Hydration

• Redox reactions

• Decarboxylation

Summary Table: Products of Glycolysis and Krebs Cycle

Pathway	ATP Produced	NADH Produced	FADH2 Produced	CO2 Produced
Glycolysis (per glucose)	2	2	0	0
Krebs Cycle (per glucose)	2	6	2	4

Additional info: The electron transport chain, which follows the Krebs cycle, uses NADH and FADH2 to generate a large amount of ATP via oxidative phosphorylation.