Gibbs Free Energy and Enzyme Catalysis Study Guide

Gibbs Free Energy Equation

ΔG=ΔH−TΔS

ΔG

Change in free energy

ΔH

Change in enthalpy (heat content)

T

Temperature in Kelvin

ΔS

Change in entropy (disorder)

Exergonic Reaction

ΔG < 0; Energy is released, and the reaction is spontaneous

Endergonic Reaction

ΔG > 0; Energy is absorbed, and reactions are non-spontaneous

ATP Hydrolysis

Releases energy (ΔG~-30.5 kJ/mol)

Spontaneity Prediction

Using ΔG=ΔH−TΔS to predict if a reaction is spontaneous at a given temperature

Activation Energy (Ea)

The energy barrier that must be overcome for the reaction to proceed

Change in Free Energy (ΔG)

The net difference in energy between reactants and products

Spontaneous Reaction

Spontaneous if ΔG<0 (reactants higher than product)

Non-spontaneous Reaction

Non-spontaneous if ΔG>0 (products higher than reactants)

Catalyst

Lowers the activation energy, does not change the overall ΔG, speeds up how fast the reaction reaches equilibrium

Effects of a Catalyst

Peak (Ea) gets lower, ΔG stays the same

Spontaneity vs Speed

A spontaneous reaction might still be slow if it has a high activation energy

Graph Interpretation

Identify activation energy, change in free energy, spontaneity, and effects of adding a catalyst

ΔG and Reaction Aspects

ΔG influences spontaneity, direction of reversible reactions, but not reaction rate or activation energy

Condition for ATP Coupling

ΔG < 0 for ATP coupling to be spontaneous

Condition for ATP Formation

ΔG > 0 for ATP formation to be non-spontaneous

Equilibrium Condition

ΔG = 0 indicates no net change at equilibrium

Example Calculation

ΔG= -50-(298)(0.1)=-79.8 kJ/mol; Exergonic-Spontaneous

ΔG

The change in Gibbs free energy, which determines the direction and spontaneity of a reaction.

Equilibrium

The state where the value of ΔG determines the position of equilibrium, predicting the ratio of products to reactants.

Reaction Rate

The speed of a reaction, which depends on activation energy (Ea) rather than ΔG.

Activation Energy (Ea)

The energy barrier that must be overcome to reach the transition state, independent of ΔG.

Catalyst Requirements

Catalysts are necessary for cellular reactions, even those with -ΔG, to speed up reactions that may otherwise be slow.

Thermodynamics

The study of energy changes in reactions, answering the question 'Will it happen?'

Kinetics

The study of reaction rates, answering the question 'How fast will it happen?'

Enzymes

Biological catalysts that lower activation energy, making reactions fast enough for life.

Cellular Conditions

Mild conditions under which cellular reactions occur, typically around 37°C and pH 7.

Reaction Specificity

The control enzymes have over when, where, and how reactions occur in cells.

Catabolic Pathways

Energy-releasing pathways that require enzymes for efficient energy capture and utilization.

Substrate

A specific reactant that an enzyme acts upon, binding to the enzyme to be converted into a product.

Active Site

The specific region on an enzyme where a substrate binds, characterized by a unique shape and chemical environment.

Lock and Key Model

A model describing how substrates fit into enzymes, akin to a key fitting into a lock.

Induced Fit Model

A model where the enzyme slightly changes shape to better fit the substrate upon binding.

Energy Harvesting

The process of capturing energy from spontaneous reactions, made efficient by catalysts.

Spontaneous Reactions

Reactions that occur without external input, often characterized by a negative ΔG.

Glycolysis

A series of enzymatic reactions that break down glucose, requiring enzymes at every step.

Citric Acid Cycle

A metabolic pathway that requires enzymes to efficiently process energy from nutrients.

Thermodynamics vs Kinetics

Thermodynamics indicates what can happen (ΔG), while kinetics determines what does happen (Ea).

Catalyst's Relationship to Activation Energy

Catalysts lower the activation energy required for a reaction, facilitating faster reaction rates.

Enzymes

Biological catalysts that speed up reactions.

Catalyst

A substance that lowers the activation energy (Ea) needed to start a reaction.

Activation Energy (Ea)

The amount of energy needed to start a reaction.

ΔG

The overall change in free energy of a reaction.

Key Idea of Catalysts

Catalysts don't change the energy of reactants or products; they make the hill easier to climb.

Mechanisms of Enzyme Action

Ways enzymes increase reaction rates, including orienting substrates, stabilizing transition states, providing optimal microenvironments, induced fit, covalent catalysis, and acid-base catalysis.

Orienting Substrates

Enzymes hold substrates in the perfect orientation to encourage the reaction.

Stabilizing the Transition State

Enzymes make the high-energy transition state more stable, which lowers Ea.

Providing an Optimal Microenvironment

The active site may provide conditions (acidic, basic, polar, or nonpolar) to help the reaction.

Induced Fit

Enzyme changes shape slightly to grip the substrate better and align functional groups.

Covalent Catalysis

Some enzymes form a temporary covalent bond with the substrate to help with electron transfer.

Acid-Base Catalysis

Enzymes may donate or accept protons to make bonds break/form more easily.

Competitive Inhibition

Inhibitor competes with the substrate for binding to the active site of the enzyme.

Effects of Competitive Inhibition on Km

Increases - more substrate is needed to outcompete the inhibitor.

Effects of Competitive Inhibition on Vmax

No change - can be reached if enough substrate is added.

Reversibility of Competitive Inhibition

Yes - adding more substrate can outcompete the inhibitor.

Noncompetitive Inhibition

Inhibitor binds to an allosteric site and changes the enzyme's shape/function.

Effects of Noncompetitive Inhibition on Km

No change - substrate can still bind normally.

Effects of Noncompetitive Inhibition on Vmax

Decreases - enzyme activity is reduced regardless of substrate amount.

Reversibility of Noncompetitive Inhibition

Yes - but adding more substrate doesn't help overcome the inhibition.

Allosteric Regulation

Occurs when a molecule binds to an allosteric site, causing a conformational shape change in the enzyme.

Allosteric Effector

A molecule that binds to an allosteric site on the enzyme.

Allosteric Activation

Activator binds to the allosteric site, changing the enzyme's shape to enhance binding at the active site or improve catalysis.

Result of Allosteric Activation

Increases catalytic activity, causing the enzyme to work faster or bind substrate better.

Example of Allosteric Activation

ADP is an allosteric activator of some glycolysis enzymes, speeding up energy production when the cell needs it.

Allosteric Inhibition

Inhibitor binds to the allosteric site, causing the enzyme to change shape.

Effects of Allosteric Inhibition

The active site becomes less effective at binding substrate or the enzyme becomes less catalytically efficient.

Result of Allosteric Inhibition

Decreased catalytic activity, leading to a slowdown of the reaction.

Example of Allosteric Inhibition

ATP can act as an allosteric inhibitor of glycolytic enzymes when energy is abundant.

Allosteric Regulation

Reversible and tunable, allowing enzymes to respond dynamically to changing conditions.

Role of Allosteric Regulation

Often plays a role in feedback loops, helping maintain balance (homeostasis).

Cellular Catalysts

Usually enzymes (proteins, sometimes with multiple subunits), occasionally ribozymes (RNA).

Enzymes

Proteins (amino acids) that are the most common biological catalysts.

Ribozymes

RNA molecules that are rare RNA-based enzymes, e.g., in ribosomes.

Chemical Properties of Enzymes

Made of amino acids, fold into complex 3D shapes, and have specific active sites.

Enzyme Specificity

Each enzyme usually catalyzes one specific reaction or acts on one specific substrate.

Active Sites

Regions where the substrate binds and the reaction happens.

Allosteric Sites

May also have allosteric sites for regulation.

Cofactors and Coenzymes

May require metals (e.g., Zn²⁺, Mg²⁺) or organic molecules (e.g., vitamins, NAD⁺) for function.

Environmental Sensitivity of Enzymes

Enzymes are sensitive to environmental conditions, requiring optimal temperature and pH.

Denaturation of Enzymes

Enzymes may denature if conditions are extreme.

Chemical Properties of Ribozymes

Made of ribonucleotides (RNA) and fold into specific 3D shapes through base pairing and hydrogen bonding.

Ribozyme Function

Catalyze specific RNA reactions, such as self-splicing introns and peptidyl transferase activity in ribosomes.

Versatility of Ribozymes

Less versatile than proteins due to fewer chemical groups for catalysis.

Ancient Origin of Ribozymes

Evidence supports an early 'RNA World' where ribozymes catalyzed reactions before proteins evolved.

Coupled Reaction

A coupled reaction is when an endergonic reaction is paired with an exergonic reaction, making the overall process spontaneous.

Endergonic Reaction

Requires energy, with ΔG > 0.

Exergonic Reaction

Releases energy, with ΔG < 0.

ATP hydrolysis

A strongly exergonic reaction with ΔG≈−30.5 kJ/mol.

Spontaneous reaction

A reaction that proceeds with a negative total ΔG.

Exergonic reaction

A reaction that releases energy, resulting in a negative ΔG.

Endergonic reaction

A reaction that requires energy input, resulting in a positive ΔG.

Glucose Phosphorylation

An endergonic reaction where Glucose + Pi → Glucose-6-Phosphate (ΔG > 0).

Coupled reaction

A reaction where an endergonic process is powered by an exergonic one.

Exergonic reaction example

ATP → ADP + Pi (ΔG < 0).

Coupled reaction example

Glucose + ATP → Glucose-6-Phosphate + ADP (ΔGnet < 0).

Effect of coupling to exergonic reaction

Rate increases as the overall ΔG becomes negative, making the reaction spontaneous.