Energy and Enzymes: Structure, Function, and Regulation

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Energy and Enzymes

Introduction to Enzymes

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They are essential for metabolism and cellular function, allowing reactions to occur at rates compatible with life.

Definition: An enzyme is a protein (or sometimes RNA) that increases the rate of a chemical reaction without being consumed in the process.
Specificity: Enzymes are highly specific for their substrates due to the unique shape of their active sites.
Example: Hexokinase catalyzes the phosphorylation of glucose in glycolysis.

What an Enzyme Can and Cannot Do

Enzymes lower the activation energy required for reactions but do not alter the overall energy change of the reaction.

Can: Speed up reactions by stabilizing the transition state.
Cannot: Change the equilibrium position or make an energetically unfavorable reaction favorable.
Example: Enzymes can convert substrate to product more quickly but cannot force a reaction to occur if it is not thermodynamically possible.

How Enzymes Work

Mechanism of Enzyme Action

Enzyme function involves several key steps, each dependent on the precise shape of the enzyme and its active site.

Initiation: Substrates bind to the enzyme's active site, forming the enzyme-substrate complex.
Transition State Facilitation: The enzyme stabilizes the transition state, lowering activation energy ().
Termination: Products are released, and the enzyme returns to its original conformation.
Key Point: Shape matters—the three-dimensional structure of the enzyme is critical for its function.

Factors Affecting Enzyme Activity

Cofactors and Coenzymes

Some enzymes require additional non-protein molecules to function properly.

Cofactors: Inorganic ions such as zinc or iron that assist enzyme activity.
Coenzymes: Organic molecules (often derived from vitamins) that participate in the reaction.
Example: NAD+ acts as a coenzyme in redox reactions.

Substrate Concentration

The rate of an enzyme-catalyzed reaction increases with substrate concentration but eventually reaches a maximum velocity () when the enzyme is saturated.

Michaelis-Menten Equation:

Key Point: At high substrate concentrations, all active sites are occupied, and the reaction rate plateaus.

Enzyme Concentration

Increasing enzyme concentration generally increases the reaction rate, provided substrate is available.

Key Point: More enzyme molecules mean more active sites for substrate binding.

Temperature and pH Effects

Enzyme activity is sensitive to temperature and pH, which affect enzyme structure and function.

Temperature: Each enzyme has an optimal temperature. Activity decreases if the temperature is too low or too high (due to denaturation).
pH: Each enzyme has an optimal pH. Extreme pH values can denature the enzyme or alter the charge of amino acids in the active site.
Example: Pepsin functions best in acidic conditions (stomach), while amylase works best at neutral pH (saliva).

Regulation of Enzyme Activity

Non-Covalent Regulation

Enzyme activity can be regulated by molecules that bind reversibly to the enzyme.

Competitive Inhibition: Inhibitor competes with substrate for the active site. Inhibition is concentration-dependent.
Non-Competitive (Allosteric) Regulation: Regulatory molecule binds to a site other than the active site (allosteric site), changing enzyme shape and activity.

Type	Binding Site	Effect on Activity
Competitive Inhibition	Active Site	Decreases (can be overcome by more substrate)
Allosteric Regulation	Allosteric Site	Can increase or decrease

Covalent Modifications

Enzyme activity can be regulated by covalent changes to the enzyme's structure, such as phosphorylation.

Phosphorylation: Addition of phosphate groups to specific amino acids can activate or deactivate enzymes.
Reversible/Irreversible: Some modifications are reversible, allowing dynamic regulation.
Example: Glycogen phosphorylase is activated by phosphorylation during energy demand.

Feedback Inhibition

Many metabolic pathways are regulated by feedback inhibition, where the end product inhibits an enzyme early in the pathway.

Key Point: Prevents overproduction of products and conserves resources.
Example: In the synthesis of isoleucine from threonine, isoleucine inhibits the first enzyme in the pathway.

Summary Table: Enzyme Regulation Mechanisms

Regulation Type	Mechanism	Effect	Example
Competitive Inhibition	Inhibitor binds active site	Decreases activity	Sulfa drugs inhibiting bacterial enzymes
Allosteric Regulation	Regulator binds allosteric site	Increases or decreases activity	ATP inhibiting phosphofructokinase
Covalent Modification	Phosphorylation/dephosphorylation	Activates or deactivates	Glycogen phosphorylase
Feedback Inhibition	End product inhibits early enzyme	Decreases pathway flux	Isoleucine synthesis pathway

Additional info: Academic context and examples have been added to clarify mechanisms and provide self-contained explanations suitable for exam preparation.