Microbial Metabolism: Key Concepts and Study Guide

Microbial Metabolism

Introduction to Metabolism

Metabolism refers to the sum of all chemical reactions that occur within a living organism. These reactions are essential for growth, reproduction, and maintenance of cellular structures. Metabolism is divided into two main categories: catabolism and anabolism.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. (ATP)

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input. (Using ATP)

• ATP (Adenosine Triphosphate): The primary energy carrier in cells, linking catabolic and anabolic processes.

• Collision theory: States that chemical reactions can occur when atoms, ions, and molecules collide.

• Activation energy Ea: Amount of energy necessary to push the reactants over an energy barrier to break the bonds. {the higher the activation energy the slower the reaction.}

• Reaction rate: Is the frequency of collisions with enough energy to bring about a reaction.

Enzymes and Enzyme Activity

Enzymes are biological catalysts that speed up chemical reactions without being consumed. They are crucial for metabolic processes.

• Components of Enzymes: Most enzymes consist of a protein part (apoenzyme) and a non-protein cofactor (which may be a metal ion i.e. Na+ or an organic molecule i.e. Vitamins, NAD+ called a coenzyme). Holoenzyme = Apoenzyme and the Coenzyme (whole enzyme)

• Mechanism of Enzymatic Action: Enzymes lower the activation energy of reactions by binding substrates at their active site, forming an enzyme-substrate complex, and converting substrates into products. The enzyme remains unchanged.

• Factors Influencing Enzyme Activity: Temperature, pH, substrate concentration, and the presence of inhibitors or activators. Low Temp there is no collision so no reaction, at optimal temperature there is Collison at its peak from the temperature, at very high temperatures the enzyme is denatured.

• Saturation point: All enzymes are bound with substrates so no more enzymes for substrates to bind to.

Enzyme Inhibition

Enzyme activity can be regulated by inhibitors, which are molecules that decrease enzyme function.

• Competitive Inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive Inhibitors: Bind to an allosteric site, changing the enzyme's shape and reducing activity.

• Feedback Inhibition: The end product of a metabolic pathway inhibits an earlier step, regulating pathway activity.

• Negative feedback: Stops the reaction. (Red light) Inhibits

• Positive Feeback: Up regulates the reaction. (Green light) stimulates

• EXAMPLE: Sulfanilamide is a drug which inhibits PABA, PABA is necessary for bacteria to make folic acid. (Analogue)

Oxidation and Reduction (Redox Reactions) LEO GER (Loose Electrons Oxidation Gain Electrons Reduction).

Redox reactions involve the transfer of electrons between molecules, playing a central role in energy production.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Example: In cellular respiration, glucose is oxidized, and oxygen is reduced.

• Respiration: Process of breaking down glucose to make lots of ATP

ATP Generation and Phosphorylation

Cells generate ATP through three main types of phosphorylation:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate generating ATP. (Uses an enzyme and a substrate). Used during Glycolysis and Krep Cycle.

• Oxidative phosphorylation: ATP is generated using high energy from the electron transport chain and chemiosmosis.

• Photophosphorylation: ATP is produced using light energy during photosynthesis.

Major Metabolic Pathways

Microorganisms use several interconnected pathways to extract energy from nutrients.

• Glycolysis: The breakdown of 1 glucose to 2pyruvate, producing 2ATP and 2NADH. (Occurs in the cytoplasm) Transition stage of pyruvate into acetyl-CoA.

• Krebs Cycle (Citric Acid Cycle): Oxidizes acetyl-CoA to 4CO2, generating 6NADH, 2FADH2, and 2ATP. (Occurs in the cytoplasm form prokaryotes and in the Mitochondrial matrix for eukaryotes)

• Electron Transport Chain (ETC): Transfers electrons from NADH and FADH2 to oxygen (in aerobic respiration), producing a proton gradient used to synthesize 34ATP. (Occurs in the inner mitochondrial membrane)

• 1 NADH = 3 ATP

• 1 FADH2 = 2 ATP

Summary Table: Major Products of Metabolic Pathways

Pathway	Main Products	ATP Yield (per glucose)
Glycolysis	2 Pyruvate, 2 NADH, 2 ATP (net) Acetyl- CoA makes 2 ATP	2
Krebs Cycle	6 NADH, 2 FADH2, 2 ATP, 4 CO2 (2264, FAN Company) (1:1:3)	2
ETC & Oxidative Phosphorylation	~34 ATP, H2O	34

Oxidative Phosphorylation and Electron Transport

Oxidative phosphorylation is the process by which ATP is synthesized as electrons are transferred through the electron transport chain to a final electron acceptor (usually oxygen in aerobic respiration). NADH and FADH2 drop of their electrons at the ETC, these electrons then move up to the intermembrane through electron carriers, creating a high concentration gradient (Chemiosmosis). The electrons then go through the ATP synthesis to make ATP.

• Electron Transport Chain (ETC): Series of protein complexes in the membrane that transfer electrons and pump protons to create a proton gradient.

• ATP Synthase: Enzyme that uses the proton gradient to convert ADP and inorganic phosphate into ATP. (Proton motive force)

$\text{ADP} + \text{P}_i \xrightarrow{\text{ATP synthase}} \text{ATP}$

Beta-Oxidation: Fatty acids are split into two-carbon fragments.

Deamination: Removal of amino group from amino acids.

Carbohydrate Metabolism in Different Conditions

Microorganisms can metabolize carbohydrates aerobically or anaerobically, depending on the availability of oxygen.

• Aerobic Respiration: Uses oxygen as the final electron acceptor, producing the most ATP.

• Anaerobic Respiration: Uses inorganic molecules other than oxygen (e.g., nitrate, sulfate) as final electron acceptors, yielding less ATP.

• Fermentation: Occurs in the absence of a suitable electron acceptor; organic molecules serve as both electron donors and acceptors, producing less ATP.

Terminal Electron Acceptors

Different metabolic processes use different terminal electron acceptors:

• Aerobic Respiration: Oxygen (O2)

• Anaerobic Respiration: Nitrate (NO3-), sulfate (SO42-), or other inorganic molecules

• Fermentation: Organic molecules (e.g., pyruvate, acetaldehyde) Ethanol (alcohol), Lactate (muscle cells)

• Obligate aerobes: Absolutely require Oxygen.

• Obligate anaerobe: Don't survive with Oxygen.

• Facultative anaerobe: Can survive with or without oxygen.

• Aerotolerant: Tolerates oxygen but prefer no oxygen.

Biochemical Tests in Microbiology

Biochemical tests are used to identify bacteria based on their metabolic characteristics. These tests detect the presence of specific enzymes or metabolic pathways.

• Examples: Catalase test, oxidase test, fermentation of sugars, utilization of citrate.

Photosynthesis: Light and Dark Reactions

Photosynthetic microorganisms use light energy to synthesize organic compounds.

• Light Reactions: Capture light energy to produce ATP and NADPH. Photosystem 1 (Cyclic P700nm) Produces only NADH and no ATP, Photosystem 2 (non-cyclic P680nm) Produces ATP and NADH, Oxygen id the byproduct.

• Dark Reactions (Calvin Cycle): Use ATP and NADPH to fix CO2 into organic molecules. CO2 enters the reaction and leaves as sugar.

• Carbon fixation, Reduction, Regeneration of CO2.

• Photo - Light

• Chemo - Chemical

• Autotroph - CO2

• Heterotroph - Organic Carbon i.e. carbohydrates lipids (CHO)

Photophosphorylation vs. Oxidative Phosphorylation

Feature	Photophosphorylation	Oxidative Phosphorylation
Energy Source	Light	Chemical (from redox reactions)
Location	Thylakoid membrane (chloroplasts or cyanobacteria)	Cell membrane (prokaryotes), mitochondria (eukaryotes)
Final Electron Acceptor	NADP+	O2 (aerobic) or other molecules (anaerobic)

Amphibolic Pathways

Amphibolic pathways are metabolic pathways that function in both catabolism and anabolism, providing intermediates for biosynthesis and energy production.

• Example: The Krebs cycle is amphibolic because its intermediates are used in both energy production and biosynthetic reactions.