Energy and Enzymes: Structure, Function, and Regulation

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.

• Substrate: The molecule upon which an enzyme acts.

• Active Site: The region of the enzyme where substrate binding and catalysis occur.

• 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 (ΔG) 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 the ratio of product to substrate at equilibrium remains unchanged.

How Enzymes Work

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.

• Example: The induced fit model describes how enzyme shape changes upon substrate binding to facilitate catalysis.

Enzyme 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 catalysis.

• Example: NAD+ acts as a coenzyme in redox reactions.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by several environmental and cellular factors.

• Substrate Concentration: Increasing substrate concentration increases reaction rate until the enzyme becomes saturated.

• Enzyme Concentration: More enzyme molecules lead to higher reaction rates, provided substrate is available.

• Temperature: Each enzyme has an optimal temperature; activity decreases if temperature is too high or too low.

• pH: Enzymes have optimal pH ranges; extreme pH can denature the enzyme and reduce activity.

• Example: Pepsin functions best in acidic conditions (stomach), while amylase works best at neutral pH (saliva).

Enzyme Kinetics

The rate of enzyme-catalyzed reactions can be described mathematically.

• Michaelis-Menten Equation:

$v = \frac{V_{max}[S]}{K_m + [S]}$

• v: Initial reaction rate

• Vmax: Maximum reaction rate

• [S]: Substrate concentration

• Km: Substrate concentration at half-maximal velocity

Regulation of Enzyme Activity

Cells regulate enzyme activity to control metabolic pathways and respond to changing conditions.

• Non-covalent Regulation:

  • Competitive Inhibition: Inhibitor competes with substrate for the active site; inhibition is concentration-dependent.

  • Non-competitive (Allosteric) Regulation: Regulatory molecule binds at a site other than the active site (allosteric site), changing enzyme activity.

• Covalent Modifications:

  • Phosphorylation: Addition of phosphate groups can activate or deactivate enzymes.

  • Changes to Amino Acid Sequence: Alterations in primary structure can affect enzyme shape and function.

• Example: Glycogen phosphorylase is activated by phosphorylation in response to hormonal signals.

Competitive vs. Non-competitive Inhibition

Type of Inhibition	Binding Site	Effect on Vmax	Effect on Km
Competitive	Active site	No change	Increases
Non-competitive (Allosteric)	Allosteric site	Decreases	No change

Feedback Inhibition in Metabolic Pathways

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

• Definition: Feedback inhibition is a regulatory mechanism in which the accumulation of an end product suppresses the activity of an enzyme involved in its synthesis.

• Example: In the synthesis of isoleucine from threonine, isoleucine inhibits the first enzyme in the pathway.

Additional info: Some details about enzyme structure, regulation, and inhibition were expanded for clarity and completeness.