Topic 4-Metabolism_New (1)

METABOLISM

Metabolism Overview

Metabolism encompasses all chemical reactions that occur within living organisms, essential for maintaining life. These reactions are divided into two main categories: catabolism, which breaks down molecules to release energy, and anabolism, which builds larger molecules from smaller units using energy.

CHEMICAL ENERGY

Types of Chemical Energy Sources

• Carbohydrates: Serve as a primary energy source, providing quick energy through glucose breakdown.

• Fats: Store energy more efficiently than carbohydrates; metabolized during prolonged exercise and fasting.

• Proteins: Can be used for energy but primarily serve structural and functional roles within cells.

• Other organic compounds: Includes vitamins and hormones that contribute to metabolic processes.

ATP (Adenosine Triphosphate)

ATP is known as the body's energy currency. It is vital for energy transfer in biological systems, acting as a reservoir of energy due to its high-energy phosphate bonds.

CHEMICAL WASTE

Byproducts of Metabolism

• Carbon Dioxide: A product of cellular respiration that must be excreted by organisms, particularly during aerobic respiration.

• Water: Produced in various metabolic pathways, including cellular respiration and dehydration synthesis.

• Heat: Byproduct of metabolic reactions, contributes to maintaining body temperature in warm-blooded animals.

BASIC CHEMICAL REACTIONS UNDERLYING METABOLISM

Two Major Classes of Metabolic Reactions

• Catabolism: Involves pathways that break larger molecules into smaller products, typically exergonic reactions that release energy usable by cells.

• Anabolism: Includes metabolic pathways that synthesize larger molecules from small precursors, requiring energy input (endergonic reactions).

OXIDATION AND REDUCTION REACTIONS

Overview of Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules, occurring simultaneously. They are crucial in energy production and cellular respiration.

Electron Carriers

• NAD+ (Nicotinamide adenine dinucleotide): Functions in redox reactions as an electron carrier, essential in energy metabolism.

• NADP+ (Nicotinamide adenine dinucleotide phosphate): Similar to NAD+, used mainly in anabolic reactions.

• FAD (Flavin adenine dinucleotide): Serves as a redox cofactor, often involved in the Krebs cycle.

ATP PRODUCTION AND ENERGY STORAGE

Energy Release and Storage

Organisms release energy from nutrients by creating high-energy phosphate bonds in ATP. This process is pivotal in transferring energy for various cellular functions.

Phosphorylation

The process of adding an inorganic phosphate group to a substrate, forming ATP.

Three Methods of ATP Production:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate.

• Oxidative phosphorylation: Generates ATP through the electron transport chain by using oxygen as the final electron acceptor.

• Photophosphorylation: Occurs in photosynthetic organisms, converting light energy into chemical energy.

ENZYMES IN METABOLISM

Role of Enzymes

Enzymes function as organic catalysts that significantly increase the likelihood of biochemical reactions, allowing metabolism to occur at a rate sufficient to support life.

Naming and Classifying Enzymes

Enzymes are categorized based on the type of reaction they catalyze:

• Hydrolases: Catalyze hydrolysis reactions.

• Isomerases: Facilitate isomerization, rearranging atoms within a molecule.

• Ligases or polymerases: Join two molecules together, commonly in DNA and RNA synthesis.

• Lyases: Add or remove groups from substrates to form double bonds.

• Oxidoreductases: Catalyze oxidation-reduction reactions, transferring electrons between molecules.

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

COFACTORS OF ENZYMES

Enzyme Activity Enhancers

Many enzymes require non-protein cofactors to function properly, known as holoenzymes.

Types of Cofactors:

• Inorganic ions: e.g., Mg2+, Zn2+, which are critical in a range of enzymatic reactions.

• Organic coenzymes: e.g., NAD+, FAD, which assist in various metabolic pathways.

ENZYME ACTIVITY AND REGULATION

Factors Affecting Enzyme Activity

• Temperature: Enzyme activity typically increases with temperature to a point, beyond which denaturation occurs.

• pH: Each enzyme has an optimal pH range; deviations can affect activity and stability.

• Concentration of enzyme and substrates: Enzyme activity can be increased by higher concentrations of substrate until a saturation point is reached.

• Presence of inhibitors: Compounds that reduce enzyme activity, which can be competitive or non-competitive.

Allosteric Activation

Activation occurs when cofactors bind to sites other than the active site, altering the enzyme's shape and enhancing its activity.

Feedback Inhibition

A regulatory mechanism where the final product of a metabolic pathway inhibits an upstream process, maintaining homeostasis and preventing overproduction.

CARBOHYDRATE CATABOLISM

Overview

Carbohydrates, especially glucose, are oxidized to produce energy mainly through two key processes:

• Cellular respiration: Aerobic process converting glucose and oxygen into ATP, CO2, and water.

• Fermentation: Anaerobic process that partially oxidizes glucose in absence of oxygen.

Glycolysis

Occurs in the cytoplasm; glucose is split into two three-carbon molecules (pyruvate), producing a net gain of 2 ATP and 2 NADH.

Stages of Glycolysis:

• Energy-investment stage: Initial use of 2 ATP to prepare glucose for further breakdown.

• Lysis stage: Splitting of glucose into two three-carbon molecules.

• Energy-conserving stage: Generation of ATP and NADH through substrate-level phosphorylation.

CELLULAR RESPIRATION

Stages of Cellular Respiration

1. Synthesis of acetyl-CoA: Links glycolysis to the Krebs cycle.

2. Krebs cycle (Citric Acid Cycle): Takes place in the matrix of mitochondria; produces ATP, NADH, and FADH2, utilizing acetyl-CoA as the starting molecule, and releasing CO2.

3. Electron Transport Chain (ETC): Located in the inner mitochondrial membrane; utilizes a series of redox reactions to produce ATP through oxidative phosphorylation, facilitated by a proton gradient.

METABOLIC DIVERSITY

Alternative Pathways

• Entner-Doudoroff pathway: An alternative to glycolysis found in some prokaryotes, producing fewer energy carriers.

• Pentose Phosphate Pathway: Operates alongside glycolysis, producing precursors for nucleic acids and redox equivalents while not generating ATP directly.

FERMENTATION

Overview

Provides an alternative source of NAD+ when oxygen is absent, allowing for partial oxidation of glucose to produce energy without fully entering aerobic pathways.

Types of Fermentation:

• Lactic acid fermentation: Occurs in muscle cells and certain bacteria, converting glucose to lactic acid, producing ATP.

• Alcoholic fermentation: Performed by yeast and some bacteria, converting glucose to ethanol and carbon dioxide, producing ATP and a variety of alcohols relevant to brewing and baking.

PHOTOSYNTHESIS

Basics of Photosynthesis

Photosynthesis is the process by which light energy is converted into chemical energy, transforming inorganic CO2 into organic molecules to produce carbohydrates.

Light-Dependent vs Light-Independent Reactions

• Light-Dependent reactions: Occur in the thylakoid membranes of chloroplasts; convert light energy into ATP and NADPH while generating O2 as a byproduct.

• Light-Independent reactions (Calvin Cycle): Utilize ATP and NADPH to fix carbon into glucose, occurring in the stroma of chloroplasts.

Competitive inhibition and non-competitive inhibition are two types of enzyme inhibition that affect enzyme activity:

Competitive Inhibition:

• This occurs when an inhibitor competes with the substrate for binding to the active site of the enzyme.

• The inhibitor resembles the substrate's structure, allowing it to bind to the active site, thus preventing the substrate from binding.

• This type of inhibition can be overcome by increasing the concentration of the substrate, which reduces the chance of the inhibitor binding to the active site.

Non-Competitive Inhibition:

• In this case, the inhibitor binds to a site other than the active site (called an allosteric site), leading to a change in the enzyme's shape and function.

• This binding does not prevent substrate binding but reduces the efficiency of the enzyme, lowering the maximum reaction rate.

• Non-competitive inhibition cannot be overcome by simply increasing substrate concentration because the inhibitor affects the enzyme's ability to catalyze the reaction regardless of whether the substrate is present.

Amphibolic Pathways:

• Amphibolic pathways are metabolic pathways that involve both catabolic and anabolic processes.

• They play a crucial role in cellular metabolism, as they allow organisms to adapt to various energy and biomass requirements by using the same set of reactions to both break down and synthesize molecules depending on the cell's needs.

• For example, the citric acid cycle is amphibolic because it helps to break down carbohydrates and fats for energy while also providing intermediates for the synthesis of amino acids and other vital compounds.

Competitive inhibition and non-competitive inhibition are two types of enzyme inhibition that affect enzyme activity:

Competitive Inhibition:

• This occurs when an inhibitor competes with the substrate for binding to the active site of the enzyme.

• The inhibitor resembles the substrate's structure, allowing it to bind to the active site, thus preventing the substrate from binding.

• This type of inhibition can be overcome by increasing the concentration of the substrate, which reduces the chance of the inhibitor binding to the active site.

Non-Competitive Inhibition:

• In this case, the inhibitor binds to a site other than the active site (called an allosteric site), leading to a change in the enzyme's shape and function.

• This binding does not prevent substrate binding but reduces the efficiency of the enzyme, lowering the maximum reaction rate.

• Non-competitive inhibition cannot be overcome by simply increasing substrate concentration because the inhibitor affects the enzyme's ability to catalyze the reaction regardless of whether the substrate is present.

Amphibolic Pathways:

• Amphibolic pathways are metabolic pathways that involve both catabolic and anabolic processes.

• They play a crucial role in cellular metabolism, as they allow organisms to adapt to various energy and biomass requirements by using the same set of reactions to both break down and synthesize molecules depending on the cell's needs.

• For example, the citric acid cycle is amphibolic because it helps to break down carbohydrates and fats for energy while also providing intermediates for the synthesis of amino acids and other vital compounds.