Metabolism

Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions within a cell, divided into two main categories: catabolism (energy-releasing breakdown of molecules) and anabolism (energy-consuming synthesis of molecules). Microbial metabolism is central to energy production, biosynthesis, and cellular function.

• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy (exergonic).

• Anabolism: Synthesis of complex molecules from simpler ones, consuming energy (endergonic).

• ATP: The energy currency of the cell, coupling catabolic and anabolic reactions.

• Metabolism: The sum total of all chemical reactions in a cell.

Energy and ATP

ATP: Structure and Function

ATP (adenosine triphosphate) stores energy in its phosphate bonds. Hydrolysis of ATP releases energy for cellular work, while formation of ATP from ADP and phosphate requires energy input.

• Exergonic reactions: Release energy (e.g., ATP hydrolysis).

• Endergonic reactions: Require energy input (e.g., ATP synthesis).

• ATP hydrolysis: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

• ATP synthesis: $\mathrm{ADP} + \mathrm{P}_i + \text{energy} \rightarrow \mathrm{ATP} + \mathrm{H}_2\mathrm{O}$

• ATP couples catabolism and anabolism: Energy from catabolic reactions is stored in ATP, which is then used to drive anabolic reactions.

Enzymes

Enzyme Structure and Function

Enzymes are biological catalysts that speed up chemical reactions without being consumed. They are highly specific for their substrates and lower the activation energy required for reactions.

• Characteristics:

  • Speed up reactions

  • Not consumed in reactions

  • Highly specific for substrates

• How enzymes work: Bind substrates and position them to facilitate the reaction.

• Components:

  • Cofactor: Non-protein helper molecule

  • Coenzyme: Organic cofactor (often vitamin-derived)

  • Holoenzyme: Active enzyme (apoenzyme + cofactor)

Factors Affecting Enzyme Activity

• Temperature: Low temperature slows reactions; high temperature can denature enzymes.

• pH: Extreme pH values can alter enzyme shape and disrupt the active site.

• Substrate concentration: Increasing substrate increases reaction rate until enzymes are saturated.

Enzyme Inhibition

• Competitive inhibitors: Bind to the active site; inhibition can be overcome by adding more substrate.

• Noncompetitive inhibitors: Bind to an allosteric site; inhibition cannot be overcome by adding substrate.

• Feedback inhibition: End product of a pathway inhibits an early enzyme, regulating pathway activity.

Redox Reactions in Metabolism

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons between molecules. Oxidation is the loss of electrons, while reduction is the gain of electrons ("OIL RIG").

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• NAD+ / NADH: NAD+ accepts electrons (is reduced) to become NADH, which carries high-energy electrons to the electron transport chain or for biosynthesis.

Pathways of Microbial Metabolism

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It breaks down glucose into pyruvate, generating ATP and NADH.

• Inputs: Glucose, NAD+, ADP

• Outputs: 2 pyruvate, 2 ATP (net), 2 NADH

• ATP production: By substrate-level phosphorylation

• Oxygen requirement: None (anaerobic process)

• What is oxidized? Glucose

• What is reduced? NAD+ to NADH

Transition Step (Pyruvate Oxidation)

Pyruvate from glycolysis is oxidized to acetyl-CoA, producing NADH and releasing CO2.

• Inputs: 2 pyruvate, NAD+, CoA

• Outputs: 2 acetyl-CoA, 2 CO2, 2 NADH

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle completes the oxidation of acetyl-CoA, generating NADH, FADH2, ATP, and CO2.

• Inputs: 2 acetyl-CoA, NAD+, FAD, ADP

• Outputs: 4 CO2, 6 NADH, 2 FADH2, 2 ATP

• ATP production: By substrate-level phosphorylation

• What is oxidized? Acetyl-CoA

• What is reduced? NAD+ to NADH, FAD to FADH2

Electron Transport Chain (ETC) and Chemiosmosis

The ETC uses electrons from NADH and FADH2 to generate a proton gradient across a membrane, driving ATP synthesis by chemiosmosis (oxidative phosphorylation).

• Location: Cytoplasmic membrane (bacteria)

• Inputs: 10 NADH, 2 FADH2, O2

• Outputs: ~34 ATP, NAD+, FAD, H2O

• Final electron acceptor: O2 (aerobic respiration)

• ATP synthase: Enzyme that synthesizes ATP as protons flow back across the membrane

• Overall process: Chemiosmosis / oxidative phosphorylation

Summary Table: Major Pathways of Aerobic Respiration

Pathway	Inputs	Outputs	ATP Produced
Glycolysis	Glucose, NAD+, ADP	2 pyruvate, 2 NADH, 2 ATP (net)	2 (net)
Transition Step	2 pyruvate, NAD+, CoA	2 acetyl-CoA, 2 CO2, 2 NADH	0
Krebs Cycle	2 acetyl-CoA, NAD+, FAD, ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP	2
Oxidative Phosphorylation	10 NADH, 2 FADH2, O2	~34 ATP, H2O	~34

Key Concepts and Applications

• Substrate-level phosphorylation: Direct enzymatic transfer of phosphate to ADP (glycolysis, Krebs cycle).

• Oxidative phosphorylation: ATP synthesis using energy from the electron transport chain and chemiosmosis.

• ATP yield: Most ATP is produced by oxidative phosphorylation, not substrate-level phosphorylation.

• Glycolysis: Occurs in both aerobic and anaerobic organisms; does not require oxygen.

• Fermentation: Occurs when no external electron acceptor (like O2) is available; organic molecules serve as electron acceptors.

• Beta-oxidation: Fatty acids are broken down and enter the Krebs cycle as acetyl-CoA (not part of glycolysis).

• Feedback inhibition: Regulates metabolic pathways (e.g., tryptophan synthesis).

Fermentation

Definition and Process

Fermentation is the oxidation of glucose with organic molecules serving as electron acceptors. It allows ATP production in the absence of oxygen, but yields less ATP than respiration.

• ATP production: Only by glycolysis (substrate-level phosphorylation)

• Electron acceptors: Organic molecules (e.g., pyruvate, acetaldehyde)

• Examples: Lactic acid fermentation, alcoholic fermentation

Summary Table: Key Features of Metabolic Pathways

Process	O2 Required?	ATP Produced	Electron Acceptor
Glycolysis	No	2 (net)	NAD+
Krebs Cycle	Indirectly (requires O2 for ETC)	2	NAD+, FAD
Electron Transport Chain	Yes (aerobic)	~34	O2
Fermentation	No	2 (from glycolysis only)	Organic molecules

Additional Concepts

• Chemoheterotrophs: Organisms that use organic compounds for both energy and carbon.

• Common pathways: Glycolysis is found in nearly all organisms.

• ATP yield in bacteria: Complete oxidation of glucose yields about 38 ATP.

• Proton gradient: Formed during electron transport, drives ATP synthesis via ATP synthase.

• Carbon fixation: Occurs in the Calvin cycle (photosynthesis), not in respiration.

• Light reactions: In photosynthesis, use light energy to produce ATP and NADPH.

Example Applications

• Weight loss and ETC inhibitors: Inhibiting the electron transport chain reduces ATP production, causing energy to be lost as heat.

• Enzyme inhibition in medicine: Many drugs act as enzyme inhibitors (e.g., antibiotics targeting bacterial enzymes).

Additional info: Some context and explanations have been expanded for clarity and completeness, including the summary tables and examples.