Metabolism and Enzyme Function: Principles of Energy and Regulation in Cells

Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within an organism, enabling the transformation of matter and energy according to the laws of thermodynamics.

• Metabolism is the totality of an organism’s chemical reactions.

• Countless chemical reactions occur constantly within each cell.

• These reactions transform matter and energy, governed by thermodynamic principles.

Metabolic Pathways

Metabolic pathways are sequences of chemical reactions, each catalyzed by a specific enzyme, that convert a starting molecule into an end product.

• Metabolic pathway: Begins with a specific molecule and ends with a product.

• Each step is catalyzed by a specific enzyme.

• Regulation of enzymes controls the pathway’s activity.

Catabolic and Anabolic Pathways

Metabolism is divided into catabolic and anabolic pathways, which respectively break down and build up molecules.

• Catabolic pathways: Release energy by breaking down complex molecules into simpler compounds.

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

• ATP stores and transfers energy between these pathways.

• Example: Cellular respiration (catabolic) and protein synthesis (anabolic).

Energy in Biological Systems

Types of Energy

Energy exists in various forms and can be converted from one form to another in biological systems.

• Kinetic energy: Energy associated with motion.

• Heat (thermal energy): Kinetic energy from random movement of atoms or molecules.

• Potential energy: Energy matter possesses due to location or structure.

• Chemical energy: Potential energy available for release in a chemical reaction.

Energy Transformations

Energy can be converted between forms, such as potential to kinetic energy, as illustrated by a diver jumping from a platform.

• Climbing up converts kinetic energy to potential energy.

• Diving converts potential energy to kinetic energy.

Thermodynamics in Biology

Basic Concepts

Thermodynamics is the study of energy transformations. Biological systems are typically open systems, exchanging energy and matter with their surroundings.

• System: Matter under study.

• Surroundings: Everything outside the system.

• Isolated system: No exchange of energy or matter (e.g., liquid in a thermos).

• Open system: Energy and matter can be transferred (e.g., living organisms).

The First Law of Thermodynamics

The first law states that energy can be transferred and transformed, but not created or destroyed.

• Principle of conservation of energy.

• Autotrophs convert sunlight energy to chemical energy in glucose, but do not create energy.

The Second Law of Thermodynamics

Every energy transfer increases the entropy (disorder) of the universe, and some energy is lost as heat.

• Entropy: Measure of disorder.

• Examples: Salt cube (low entropy), salt dissolved in water (higher entropy).

• Heat dispersal increases entropy.

Biological Order and Disorder

Order in Cells

Cells create ordered structures from less ordered materials, but overall entropy increases in the universe.

• Anabolism: Building ordered structures at the molecular level.

• Catabolism: Replacing ordered forms with less ordered forms.

• Local entropy may decrease, but total entropy (system + surroundings) increases.

Spontaneous Reactions and Free Energy

Spontaneous Processes

Spontaneous processes occur without energy input and are energetically favorable.

• Biologists determine spontaneity by analyzing energy changes in reactions.

Free-Energy Change ($\Delta G$)

The change in free energy ($\Delta G$) indicates whether a reaction is spontaneous.

• Free energy: Energy available to do work.

• Equation: $\Delta G = \Delta H - T\Delta S$

• Only processes with $\Delta G < 0$ are spontaneous.

• $\Delta G = G_{final\ state} - G_{initial\ state}$

• Spontaneous processes can be harnessed to perform work.

Exergonic and Endergonic Reactions

Reactions are classified by their free-energy changes.

• Exergonic reactions: Net release of free energy ($\Delta G < 0$), energetically favorable, "downhill" reactions.

• Endergonic reactions: Absorb free energy from surroundings ($\Delta G > 0$), require energy input, "uphill" reactions.

• Exergonic and endergonic reactions can be coupled in cells.

Equilibrium and Cellular Work

Cells avoid equilibrium by remaining open systems, allowing continuous flow of materials and energy.

• Reactions in closed systems reach equilibrium and do no work.

• Catabolic pathways release free energy in a series of reactions.

ATP and Cellular Work

Role of ATP

ATP (adenosine triphosphate) is the primary energy currency of the cell, mediating energy coupling between exergonic and endergonic reactions.

• Cells perform chemical (endergonic), transport (active transport), and mechanical (movement) work.

• Energy coupling: Use of exergonic processes to drive endergonic ones, usually via ATP.

Structure and Hydrolysis of ATP

ATP consists of adenine, ribose, and three phosphate groups. Hydrolysis of the terminal phosphate releases energy.

• Bonds between phosphate groups can be broken by hydrolysis.

• Energy is released when the terminal phosphate bond is broken.

• Release of energy is due to the chemical change to a state of lower free energy, not the bond itself.

ATP in Coupled Reactions

ATP hydrolysis is often coupled to endergonic reactions, making them energetically favorable.

• Example: Glutamic acid conversion to glutamine (endergonic) is coupled with ATP hydrolysis (exergonic).

• Net $\Delta G$ for coupled reaction is negative, allowing the process to proceed spontaneously.

Reaction	$\Delta G$ (kcal/mol)
Glutamic acid + NH3 → Glutamine	+3.4
ATP hydrolysis	-7.3
Coupled reaction	-3.9

ATP in Cellular Work

ATP powers transport and mechanical work by transferring phosphate groups to proteins or binding to motor proteins.

• Transport work: ATP phosphorylates transport proteins.

• Mechanical work: ATP binds to motor proteins and is hydrolyzed.

Regeneration of ATP

ATP is regenerated from ADP and inorganic phosphate ($P_i$) using energy from catabolic reactions.

• ATP is a renewable resource.

• Energy from catabolism (exergonic) is used to regenerate ATP.

Enzymes and Catalysis

Enzyme Function

Enzymes are biological catalysts that speed up reactions by lowering activation energy barriers.

• Catalyst: Chemical agent that speeds up a reaction without being consumed.

• Enzyme: Catalytic protein.

• Example: Sucrase hydrolyzes sucrose into glucose and fructose.

Activation Energy

Activation energy ($E_a$) is the initial energy required to start a chemical reaction.

• All reactions require activation energy, supplied as thermal energy from surroundings.

• Enzymes lower $E_a$, making reactions proceed faster.

Enzyme-Substrate Interaction

Enzymes bind substrates specifically at the active site, often changing shape to fit the substrate (induced fit).

• Induced fit: Enzyme changes shape when substrate binds.

• Active site lowers $E_a$ by orienting substrates, straining bonds, providing a favorable environment, or forming covalent bonds.

Enzyme Specificity

Enzymes are highly specific, often acting on only one substrate or type of bond.

• Sucrase acts only on sucrose.

• Lipase acts on ester linkages in fats.

• Enzyme names often end in -ase or -zyme.

Enzyme Activity and Regulation

Enzyme activity is influenced by environmental factors and substrate concentration.

• Optimal temperature and pH favor the most active enzyme shape.

• Rate increases with substrate concentration until saturation is reached.

Enzyme	Optimal Temperature (°C)	Optimal pH
Human enzyme	37	Varies
Thermophilic bacteria	77	Varies
Pepsin (stomach)	37	2
Trypsin (intestine)	37	8

Enzyme Saturation

At high substrate concentrations, enzymes become saturated and the reaction rate plateaus.

• Maximum rate is reached when all enzyme active sites are occupied.

Cofactors and Coenzymes

Many enzymes require nonprotein helpers for activity.

• Cofactors: Inorganic (e.g., metal ions) or organic molecules.

• Coenzymes: Organic cofactors, often derived from vitamins.

• Apoenzyme: Protein part of the enzyme.

• Holoenzyme: Complete, functional enzyme (apoenzyme + cofactor).

Enzyme Inhibition

Enzyme activity can be inhibited by specific molecules.

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

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

Allosteric Regulation and Feedback Inhibition

Enzymes can be regulated by molecules binding at sites other than the active site (allosteric regulation), and by feedback inhibition.

• Allosteric regulation: Regulatory molecule binds allosteric site, stabilizing active or inactive form.

• Cooperativity: Substrate binding increases enzyme activity for additional substrates.

• Feedback inhibition: End product of a pathway inhibits an earlier step, preventing overproduction.

Compartmentalization

Cellular structures help organize metabolic pathways, with enzymes localized to specific organelles or membrane regions.

• Example: Enzymes for cellular respiration are located in mitochondria.