Microbial Metabolism: Chemical Reactions and Pathways

스터디 가이드 - 스마트 노트

자료에 맞춘 맞춤형 노트, 핵심 정의, 예시, 맥락을 확장해 제공합니다.

Microbial Metabolism

Overview of Metabolism

Metabolism encompasses all controlled biochemical reactions within a microbe, serving the ultimate function of reproducing the organism. It is guided by a series of elementary statements that describe nutrient acquisition, energy utilization, and cellular growth.

Metabolism: The sum of all chemical reactions in a cell.
Cells acquire nutrients and use energy from light or catabolism.
Energy is stored in adenosine triphosphate (ATP).
Catabolism forms precursor metabolites; anabolism uses these, ATP, and enzymes to build macromolecules.
Cells grow and reproduce by assembling macromolecules.

Diagram of metabolism showing the relationship between catabolism and anabolism Cellular diagram showing catabolism and anabolism, energy flow, and macromolecule formation

Catabolism and Anabolism

Metabolic reactions are divided into two major classes: catabolic and anabolic pathways.

Catabolic pathways: Break down larger molecules into smaller products; these reactions are exergonic (release energy).
Anabolic pathways: Synthesize large molecules from smaller products; these reactions are endergonic (require energy).
ATP is central to energy transfer between catabolism and anabolism.

Oxidation and Reduction Reactions

Oxidation-reduction (redox) reactions involve the transfer of electrons from an electron donor to an electron acceptor. These reactions are fundamental to energy production in cells.

Redox reactions always occur simultaneously.
Cells use electron carriers such as NAD+, NADP+, and FAD to transport electrons.
Electron carriers often carry electrons in the form of hydrogen atoms.

Diagram of oxidation and reduction reactions Alternative diagram of oxidation and reduction reactions

ATP Production and Energy Storage

Organisms release energy from nutrients and store it in high-energy phosphate bonds of ATP. ATP is produced by phosphorylation of ADP in three ways:

Substrate-level phosphorylation: Direct transfer of phosphate between substrates.
Oxidative phosphorylation: Uses energy from electron transport chain.
Photophosphorylation: Uses light energy (in photosynthetic organisms).

The Roles of Enzymes in Metabolism

Enzyme Structure and Function

Enzymes are organic catalysts that increase the likelihood of chemical reactions. They are classified based on their mode of action:

Hydrolases: Catalyze hydrolysis reactions.
Isomerases: Catalyze isomerization changes.
Ligases/Polymerases: Join molecules together.
Lyases: Break bonds without water.
Oxidoreductases: Catalyze oxidation-reduction reactions.
Transferases: Transfer functional groups.

Enzymes may require cofactors for activity:

Apoenzyme: Protein portion, inactive without cofactors.
Holoenzyme: Active enzyme with cofactors (inorganic ions or coenzymes).
Some enzymes are RNA molecules called ribozymes.

Structure of a holoenzyme showing apoenzyme, coenzyme, and inorganic cofactor

Enzyme Activity and Regulation

Enzyme activity is influenced by several factors:

Temperature: Optimal range for activity; extreme temperatures can denature enzymes.
pH: Each enzyme has an optimal pH.
Enzyme and substrate concentrations: Affect reaction rates.
Inhibitors: Block enzyme active sites; include competitive and noncompetitive inhibitors.

Graphs showing effects of temperature, pH, and substrate concentration on enzyme activity Comparison of functional and denatured protein structures Diagram of competitive inhibition of enzyme activity Diagram of allosteric inhibition and activation of enzymes

Carbohydrate Catabolism

Glucose Catabolism

Glucose is the primary carbohydrate used by many organisms for energy. It is catabolized by cellular respiration and fermentation.

Glycolysis: Occurs in the cytoplasm; splits glucose into two pyruvic acid molecules.
Net gain: 2 ATP, 2 NADH, and pyruvic acid.
Stages: Energy-investment, lysis, and energy-conserving.

Cellular Respiration

Cellular respiration completely oxidizes pyruvic acid to produce ATP via three stages:

Synthesis of acetyl-CoA
Krebs cycle
Electron transport chain (ETC)

Formation of acetyl-CoA from pyruvic acid Diagram of the Krebs cycle

Fermentation

Fermentation provides cells with an alternate source of NAD+ when cellular respiration is not possible. It involves partial oxidation of sugar using an organic molecule as the final electron acceptor.

Produces various products such as lactic acid, ethanol, and propionic acid.
Important in food and industrial microbiology.

Diagram of fermentation pathways Fermentation products and organisms that produce them

Other Catabolic Pathways

Lipid Catabolism

Lipids are hydrolyzed into glycerol and fatty acids. Fatty acids undergo beta-oxidation to produce acetyl-CoA, which enters the Krebs cycle.

Diagram of fat molecule catabolism

Protein Catabolism

Proteins are broken down into amino acids, which are deaminated and converted into metabolites that enter the Krebs cycle.

Diagram of protein catabolism

Photosynthesis

Photosynthetic Structures and Chemicals

Photosynthesis is the process by which organisms synthesize organic molecules from inorganic carbon dioxide using light energy.

Chlorophylls: Pigments that capture light energy; have a hydrocarbon tail and a magnesium-centered active site.
Photosystems: Arrangements of chlorophyll and other pigments in thylakoid membranes.
Prokaryotes: Thylakoids are invaginations of the cytoplasmic membrane.
Eukaryotes: Thylakoids are in chloroplasts, arranged in stacks called grana.

Photosynthetic structures in a prokaryote Reaction center of photosystem

Light-Dependent and Light-Independent Reactions

Photosynthesis consists of light-dependent reactions (require light) and light-independent reactions (Calvin-Benson cycle).

Light-dependent: Use energy to pump protons and generate ATP via photophosphorylation.
Light-independent: Use ATP and NADPH to fix carbon dioxide into glucose.

Simplified diagram of the Calvin-Benson cycle

Other Anabolic Pathways

Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. Many anabolic pathways are the reverse of catabolic pathways and are termed amphibolic.

Diagram of gluconeogenesis

Biosynthesis of Fat and Amino Acids

Fatty acids and amino acids are synthesized from intermediates of glycolysis and the Krebs cycle.

Biosynthesis of fat Synthesis of amino acids by amination and transamination

Biosynthesis of Nucleotides

Nucleotides are synthesized from intermediates of glycolysis, the pentose phosphate pathway, and the Krebs cycle.

Biosynthesis of nucleotides

Integration and Regulation of Metabolic Function

Regulation Mechanisms

Cells regulate metabolism by controlling enzyme synthesis and activity, isolating pathways in organelles, and using feedback inhibition.

Control of gene expression: Regulates amount and timing of enzyme production.
Control of metabolic expression: Regulates activity of enzymes once produced.
Feedback inhibition slows or stops anabolic pathways when products are abundant.

Summary Table: Metabolic Pathways and Their Functions

Pathway	Main Function	Key Products
Glycolysis	Breakdown of glucose	ATP, NADH, pyruvic acid
Krebs Cycle	Oxidation of acetyl-CoA	ATP, NADH, FADH2, CO2
Electron Transport Chain	ATP synthesis via oxidative phosphorylation	ATP, H2O
Fermentation	Regeneration of NAD+	Lactic acid, ethanol, other products
Photosynthesis	Synthesis of carbohydrates from CO2	Glucose, O2
Gluconeogenesis	Synthesis of glucose	Glucose

Additional info: Amphibolic pathways are those that can proceed in both directions, serving both catabolic and anabolic functions. Regulation of metabolism is essential for cellular efficiency and adaptation to environmental changes.