Enzymes and Metabolic Pathways

Catabolic vs. Anabolic Pathways

Metabolic pathways are series of chemical reactions in cells, classified as either catabolic or anabolic based on their function and energy flow.

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy. Example: Cellular respiration.

• Anabolic Pathways: Build complex molecules from simpler ones, consuming energy. Example: Protein synthesis.

Kinetic vs. Potential Energy

Energy exists in different forms within biological systems:

• Kinetic Energy: Energy of motion; associated with moving objects (e.g., movement of molecules).

• Potential Energy: Stored energy due to position or structure (e.g., chemical bonds in glucose).

Exergonic vs. Endergonic Reactions

Chemical reactions are classified by their energy changes:

• Exergonic Reactions: Release free energy; spontaneous. Example: ATP hydrolysis.

• Endergonic Reactions: Consume free energy; non-spontaneous. Example: Synthesis of glucose from CO2 and H2O.

Key Equation:

• Change in free energy:

• Exergonic:

• Endergonic:

Thermodynamics and Life

The Second Law of Thermodynamics

The second law states that every energy transfer or transformation increases the entropy (disorder) of the universe.

• Entropy (S): A measure of disorder or randomness.

• Living organisms do not violate this law because they are open systems, exchanging energy and matter with their environment. Local decreases in entropy (order) are offset by greater increases in the surroundings.

ATP and Cellular Work

ATP: The Cell's Energy Currency

Adenosine triphosphate (ATP) powers cellular work by coupling exergonic and endergonic reactions.

• ATP hydrolysis releases energy:

• This energy is used for mechanical, transport, and chemical work in cells.

Enzyme Structure and Function

What Are Enzymes?

Enzymes are catalytic proteins that speed up biochemical reactions by lowering activation energy.

• Most enzymes are proteins; some RNA molecules (ribozymes) also have catalytic activity.

Types of Enzymes

• Anabolic Enzymes: Catalyze synthesis (building) reactions.

• Catabolic Enzymes: Catalyze breakdown reactions.

Enzyme Mechanisms: Lowering Activation Energy

Enzymes lower the activation energy required for reactions by:

• Providing an active site where substrates bind.

• Induced fit: The enzyme changes shape slightly to fit the substrate more closely.

• Aligning substrates correctly for reaction.

• Bending substrate bonds to facilitate breaking/forming bonds.

• Providing a favorable microenvironment (e.g., pH, charge).

• Forming temporary covalent bonds with substrates.

Active Site and Induced Fit

• Active Site: The specific groove or pocket on the enzyme where the substrate binds.

• Induced Fit: The enzyme changes shape to better fit the substrate upon binding, enhancing catalysis.

Functions of Enzymes

• Breakdown and synthesis of molecules.

• Alignment and orientation of substrates.

• Stabilization of transition states.

• Creation of optimal conditions for reaction.

Enzyme Denaturation

Enzyme function depends on its three-dimensional structure:

• Denaturation: Loss of protein structure due to heat, pH, or chemicals, resulting in loss of enzyme activity.

• The active site is formed by the enzyme's tertiary and quaternary structure; disruption alters function.

Enzyme Regulation

Enzyme Inhibition

• Competitive Inhibition: Inhibitor competes with substrate for the active site, blocking substrate binding.

• Noncompetitive Inhibition: Inhibitor binds elsewhere on the enzyme, changing its shape and reducing activity.

Allosteric Regulation

Allosteric regulators bind to sites other than the active site, affecting enzyme activity:

• Allosteric Activators: Stabilize the active form of the enzyme, increasing activity.

• Allosteric Inhibitors: Stabilize the inactive form, decreasing activity.

Ribozymes

Ribozymes are RNA molecules with catalytic activity, functioning as biological catalysts for specific RNA reactions.

Summary Table: Types of Reactions and Energy Changes

Key Terms

• Catabolic Pathway: Metabolic pathway that releases energy by breaking down complex molecules.

• Anabolic Pathway: Metabolic pathway that consumes energy to build complex molecules.

• Enzyme: Protein (or RNA) that acts as a catalyst to speed up a reaction.

• Active Site: Region on the enzyme where the substrate binds.

• Induced Fit: Change in enzyme shape to fit the substrate more closely.

• Denaturation: Loss of protein structure and function.

• Allosteric Regulation: Regulation of enzyme activity by binding of molecules at sites other than the active site.

• Ribozyme: Catalytic RNA molecule.

Additional info: Expanded explanations and examples were added for clarity and completeness, including the summary table and key equations.