Enzymes and Energy in Biological Systems: Structure, Function, and Regulation

Review of Biological Molecules and Proteins

Building Blocks of Life

All living organisms are composed of fundamental units called cells. Within cells, four major classes of biological macromolecules serve as the building blocks:

• Nucleic acids (e.g., DNA, RNA)

• Lipids (e.g., fats, phospholipids)

• Carbohydrates (e.g., glucose, starch)

• Proteins (e.g., enzymes, structural proteins)

These macromolecules are polymers made from smaller units called monomers.

Proteins: Structure and Function

Proteins are found throughout the cell and perform a wide variety of functions. Their structure is organized into four levels:

• Primary structure: Linear sequence of amino acids.

• Secondary structure: Local folding into α-helices and β-sheets, stabilized by hydrogen bonds.

• Tertiary structure: Three-dimensional folding driven by interactions such as hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.

• Quaternary structure: Association of multiple polypeptide chains.

Factors affecting protein folding include:

• Amino acid sequence

• Interactions between amino acids

• Temperature

• pH

Functions of proteins:

• Structural support (e.g., collagen)

• Transport (e.g., hemoglobin)

• Muscle contraction (e.g., actin, myosin)

• pH regulation

• Energy source (via catabolism)

• Catalysis (enzymes)

Enzymes: Structure, Function, and Regulation

Definition and Properties of Enzymes

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions without being consumed. An exception is the ribozyme, an RNA molecule with catalytic activity. Enzymes are typically named by adding the suffix "-ase" to the substrate or reaction type (e.g., lactase, sucrase, DNAse).

• Enzymes are substrate-specific: they catalyze only specific reactions.

Active Site and Substrate Specificity

The three-dimensional folding of an enzyme creates an active site, a groove or pocket formed by a cluster of amino acids. The active site binds the substrate with high affinity and specificity, allowing the enzyme to catalyze a particular reaction.

The Catalytic Cycle of an Enzyme

The catalytic cycle describes how enzymes facilitate reactions:

1. The active site is available for a substrate molecule.

2. The substrate binds to the active site, forming an enzyme-substrate complex.

3. The enzyme catalyzes the conversion of the substrate to products.

4. The products are released, and the enzyme is free to catalyze another reaction.

Example: The enzyme sucrase binds sucrose, breaks it into glucose and fructose, and releases the products.

Mechanism of Enzyme Action

Enzymes lower the activation energy (Ea) required for a reaction to proceed, making it easier for reactants to reach the transition state. They do this by:

• Orienting substrates correctly

• Straining substrate bonds

• Providing a favorable microenvironment

• Participating directly in the reaction

Rate of Enzymatic Reactions: The Michaelis-Menten Model

The rate of an enzyme-catalyzed reaction depends on substrate concentration and can be described by the Michaelis-Menten equation:

$V_0 = \frac{V_{max}[S]}{K_m + [S]}$

• Vmax: Maximum reaction rate (enzyme saturated with substrate)

• Km: Substrate concentration at which the reaction rate is half of Vmax

Interpretation of Km:

• High Km: Low substrate binding affinity (more substrate needed to reach half-maximal rate)

• Low Km: High substrate binding affinity

Factors Affecting Enzyme Activity

• Temperature: Enzyme activity increases with temperature up to an optimum (usually ~37°C in humans), then decreases due to denaturation.

• pH: Each enzyme has an optimal pH (often 6-8, but can be lower or higher depending on the enzyme).

• Salt concentration: Physiological salt concentrations are optimal for most enzymes; extreme salinity can denature enzymes.

• Cofactors: Non-protein helpers required for enzyme activity. Can be inorganic (e.g., Cu2+, Zn2+, Mn2+) or organic (coenzymes, such as vitamins and ATP).

Regulation of Enzymatic Reactions

Cells regulate enzyme activity to control metabolic pathways. Major mechanisms include:

• Competitive inhibition: Inhibitor resembles the substrate and binds to the active site, blocking substrate binding. Reversible by increasing substrate concentration.

• Noncompetitive inhibition: Inhibitor binds to a site other than the active site, causing a conformational change that reduces enzyme activity. Not reversible by increasing substrate concentration.

• Allosteric regulation: Regulatory molecules bind to allosteric sites, stabilizing either the active or inactive form of the enzyme. Often seen in enzymes with multiple subunits.

• Cooperativity: Binding of one substrate molecule increases the affinity of other subunits for the substrate (e.g., oxygen binding to hemoglobin).

• Feedback inhibition: End product of a metabolic pathway inhibits an enzyme early in the pathway, preventing overproduction.

Table: Types of Enzyme Inhibition

Type	Binding Site	Effect on Vmax	Effect on Km	Reversibility
Competitive	Active site	No change	Increases	Yes (by increasing [substrate])
Noncompetitive	Allosteric site	Decreases	No change	No

Energy in Biological Systems

Catabolic vs. Anabolic Pathways

Metabolic pathways are classified as:

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., breakdown of proteins, glycogen, triglycerides).

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., synthesis of proteins, glycogen, triglycerides).

Exergonic vs. Endergonic Reactions

• Exergonic reactions: Release free energy; spontaneous (ΔG < 0).

• Endergonic reactions: Require input of energy; non-spontaneous (ΔG > 0).

Free Energy Change (ΔG) in Reactions

The change in free energy (ΔG) determines whether a reaction is spontaneous:

$\Delta G = G_{products} - G_{reactants}$

• ΔG < 0: Reaction is spontaneous (exergonic)

• ΔG > 0: Reaction is non-spontaneous (endergonic)

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is the primary energy carrier in cells. Energy is stored in the high-energy phosphate bonds. Hydrolysis of ATP to ADP releases energy:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

The standard free energy change for ATP hydrolysis is:

$\Delta G^\circ = -7.3\ \text{kcal/mol}$

Cells use ATP hydrolysis to drive endergonic reactions by coupling them to exergonic reactions.

Example: Coupling Reactions

The synthesis of glutamine from glutamic acid and ammonia is endergonic, but can proceed when coupled to ATP hydrolysis:

• Step 1: ATP phosphorylates glutamic acid, increasing its free energy.

• Step 2: Ammonia displaces the phosphate, forming glutamine.

The overall coupled reaction is exergonic (ΔG negative), allowing it to occur spontaneously.

Summary Table: Catabolic vs. Anabolic Pathways

Pathway	Direction	Energy	Examples
Catabolic	Breakdown	Releases energy	Glycogen → Glucose, Proteins → Amino acids
Anabolic	Synthesis	Consumes energy	Glucose → Glycogen, Amino acids → Proteins

Key Terms

• Enzyme: Protein catalyst that speeds up biochemical reactions.

• Active site: Region of enzyme where substrate binds and reaction occurs.

• Substrate: Reactant molecule acted upon by an enzyme.

• Cofactor: Non-protein molecule required for enzyme activity.

• Coenzyme: Organic cofactor (often a vitamin or derived from vitamins).

• Allosteric site: Site on enzyme where regulatory molecules bind, affecting activity.

• ATP: Adenosine triphosphate, main energy currency of the cell.

• ΔG: Change in free energy during a reaction.