Energy and Enzymes: Metabolism, Thermodynamics, and Enzyme Function

CHAPTER 6: Energy and Enzymes

Overview of Energy Flow in the Human Body

The human body exchanges matter and energy with its environment through various metabolic processes. Inputs such as oxygen, water, food, and chemical energy are transformed and released as outputs including carbon dioxide, waste products, and heat.

• Inputs: Oxygen, water, dry food, chemical energy

• Outputs: Carbon dioxide, evaporated water, urine, feces, heat

• Example: Oxygen (830 g) is consumed and carbon dioxide (1,140 g) is produced daily.

Metabolism and Metabolic Pathways

Definition and Organization

Metabolism is the totality of an organism’s chemical reactions, managing the materials and energy resources of a cell. Metabolic pathways consist of a series of chemical reactions, each catalyzed by a specific enzyme, transforming a starting molecule into a final product.

• Metabolic pathway: Begins with a specific molecule, altered in a series of defined steps, resulting in a product.

• Enzymes: Biological catalysts that speed up each step in the pathway.

• Example: Glycolysis is a metabolic pathway that converts glucose to pyruvate.

Types of Metabolic Pathways

Metabolic pathways are classified based on whether they release or consume energy.

• Catabolic pathways: Release energy by breaking down complex molecules into simpler ones.

• Example: Cellular respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

• Anabolic pathways: Consume energy to build complex molecules from simpler ones.

• Example: Photosynthesis: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Thermodynamics in Biology

The First Law of Thermodynamics

The first law of thermodynamics states that the energy of the universe is constant. Energy can be transferred and transformed, but it cannot be created or destroyed.

• Key Point: Energy conservation is fundamental to biological processes.

• Example: Chemical energy in food is converted to kinetic energy and heat in animals.

The Second Law of Thermodynamics

Every energy transfer or transformation increases the disorder (entropy) of the universe. Some energy is always lost as heat during these processes.

• Key Point: Biological systems must continually obtain energy to maintain order and function.

• Example: Heat released during cellular respiration increases entropy.

Energy Coupling and ATP

Energy Coupling in Cells

Cells perform work by coupling exergonic (energy-releasing) reactions to endergonic (energy-consuming) reactions. This allows the energy released from catabolic pathways to drive anabolic processes.

• Exergonic reaction: Proceeds with a net release of free energy (e.g., respiration).

• Endergonic reaction: Absorbs free energy from surroundings (e.g., photosynthesis).

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is the molecule used for cellular work and energy coupling. ATP consists of adenine, ribose, and three phosphate groups.

• Structure: Adenine + ribose + 3 phosphates

• Hydrolysis: ATP is hydrolyzed to ADP (adenosine diphosphate) and inorganic phosphate ($P_i$), releasing energy.

• Equation: $ATP + H_2O \rightarrow ADP + P_i + \text{Energy}$

• Free energy change: $\Delta G = -7.3$ kcal/mol

Phosphorylation and Cellular Work

ATP transfers energy to other molecules by phosphorylation, destabilizing them and making them more reactive. Enzymes called kinases catalyze phosphorylation reactions.

• Example: Glutamic acid is phosphorylated and then converted to glutamine.

Enzymes: Biological Catalysts

Enzyme Function and Mechanism

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy ($E_a$) required to start a reaction. They are not consumed in the reaction and are usually named with the suffix -ase.

• Activation energy ($E_a$): The energy required to initiate a chemical reaction.

• Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Enzyme-Substrate Specificity

Enzymes are highly specific for their substrates, binding them at the active site to form an enzyme-substrate complex. The shape and charge of the substrate must fit the active site.

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

• Induced fit: Enzyme changes shape to fit the substrate more snugly, like a "clasping handshake."

Factors Affecting Enzyme Activity

Enzyme activity is influenced by temperature, pH, substrate concentration, and salinity. Each enzyme has optimal conditions for activity.

• Optimal temperature: Human enzymes typically function best at 37°C; thermophilic bacteria have enzymes with higher optimal temperatures.

• Optimal pH: Pepsin (stomach enzyme) works best at pH 2; other enzymes may have different pH optima.

Mechanisms of Catalysis

The active site of an enzyme lowers activation energy by:

• Orienting substrates correctly

• Straining substrate bonds

• Providing a favorable microenvironment

• Covalently bonding to the substrate

Enzyme Regulation

Cofactors and Coenzymes

Enzymes often require non-protein helpers called cofactors. Cofactors can be inorganic (e.g., Zn2+, Fe2+, Cu2+) or organic (coenzymes, e.g., vitamins).

• Cofactor: Inorganic helper (e.g., metal ions)

• Coenzyme: Organic helper (e.g., biotin, vitamins)

Enzyme Inhibitors

Enzyme activity can be reduced by inhibitors:

• Competitive inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive inhibitors: Bind to another part of the enzyme, causing a conformational change that makes the active site nonfunctional.

Allosteric Regulation

Allosteric regulation occurs when a regulatory molecule binds to a site other than the active site, affecting enzyme activity.

• Allosteric activator: Stabilizes the active form of the enzyme.

• Allosteric inhibitor: Stabilizes the inactive form.

Feedback Inhibition

In feedback inhibition, the end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction and conserving resources.

• Example: Cholesterol inhibits HMG-CoA reductase in its biosynthetic pathway.

Summary Table: Types of Enzyme Regulation

Type	Mechanism	Example
Competitive Inhibition	Inhibitor binds active site	Sulfa drugs inhibiting bacterial enzymes
Noncompetitive Inhibition	Inhibitor binds elsewhere, alters active site	Heavy metals inhibiting enzymes
Allosteric Regulation	Regulator binds allosteric site	ATP as allosteric inhibitor in glycolysis
Feedback Inhibition	End product inhibits pathway enzyme	Cholesterol inhibiting HMG-CoA reductase

Practice Questions (from notes)

• If an enzyme solution is saturated with substrate, the most effective way to increase product yield is to add more enzyme.

• When an enzyme is added to a solution with excess substrate, additional product will be formed.

• Thermophilic bacteria are metabolically active in hot springs because their enzymes have high optimal temperatures.

Additional info: Some explanations and examples have been expanded for clarity and completeness.