Metabolism, Enzymes, and Energy Transformations in Cells

Metabolism and Enzymes

Introduction to Metabolism

Metabolism refers to all chemical reactions that occur within living organisms to maintain life. These reactions are organized into metabolic pathways, which can be either catabolic (breaking down molecules to release energy) or anabolic (building complex molecules from simpler ones).

• Catabolic pathways: Break down molecules and release energy (e.g., cellular respiration).

• Anabolic pathways: Build complex molecules and require energy input (e.g., protein synthesis).

• Enzymes: Biological catalysts that speed up metabolic reactions by lowering activation energy.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It consists of adenine, ribose (a sugar), and three phosphate groups. Energy is released when ATP is hydrolyzed to adenosine diphosphate (ADP) and inorganic phosphate (Pi).

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

• Dephosphorylation: Removal of a phosphate group releases energy for cellular work.

• Spontaneous/Exergonic: ATP hydrolysis is a spontaneous, exergonic reaction that releases free energy ($\Delta G < 0$).

Table: Comparison of Reaction Types

Reaction Type	Spontaneity	Energy Change	Free Energy
Endergonic	Nonspontaneous	Requires energy input	Increases free energy ($\Delta G > 0$)
Exergonic	Spontaneous	Releases energy	Decreases free energy ($\Delta G < 0$)

Coupled Reactions and Free Energy

Cells often couple exergonic reactions (like ATP hydrolysis) with endergonic reactions to drive processes that require energy. The overall free energy change ($\Delta G$) determines if the coupled reaction will occur spontaneously.

• Example: If $A + B \rightarrow AB$ has $\Delta G = +15$ kJ (endergonic), and $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$ has $\Delta G = -30.5$ kJ (exergonic), then the coupled reaction:

$\text{ATP} + \text{H}_2\text{O} + A + B \rightarrow AB + \text{ADP} + \text{P}_i$

$\Delta G = -15.5$ kJ$

• Conclusion: The overall negative $\Delta G$ means the reaction will happen spontaneously.

Enzyme Structure and Function

How Enzymes Work

Enzymes are usually proteins that bind specific reactants called substrates at their active site. The enzyme-substrate complex facilitates the conversion of substrates into products.

• Lowering Activation Energy: Enzymes lower the activation energy ($E_a$) required for a reaction, allowing it to proceed at biological temperatures.

• Enzyme-Substrate Complex: $\text{Enzyme} + \text{Substrate} \rightleftharpoons \text{Enzyme-Substrate Complex} \rightarrow \text{Enzyme} + \text{Products}$

• Enzyme is not consumed: The enzyme is unchanged after the reaction and can be reused.

Mechanisms of Enzyme Action

• Enzymes may bring multiple substrates together in favorable orientations.

• They can stretch and bend chemical bonds to make them easier to break.

• Provide a favorable microenvironment (e.g., acidic or basic conditions) for the reaction.

• May participate directly in the reaction by temporarily forming covalent bonds with substrates.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature. Human enzymes typically function best at 37°C, while enzymes from thermophilic bacteria may have higher optimal temperatures.

• pH: Enzymes also have optimal pH ranges. For example, pepsin (stomach enzyme) works best at low pH, while trypsin (intestinal enzyme) works best at neutral to slightly basic pH.

Table: Optimal Conditions for Enzyme Activity

Enzyme	Optimal Temperature (°C)	Optimal pH
Human Enzyme	37	Varies (often 7)
Thermophilic Bacteria Enzyme	~70	Varies
Pepsin	37	2
Trypsin	37	8

Enzyme Inhibition and Regulation

Types of Enzyme Inhibition

• Competitive Inhibition: Inhibitor binds to the active site, blocking substrate binding.

• Noncompetitive Inhibition: Inhibitor binds to a different site (allosteric site), changing the enzyme's shape and reducing activity.

Table: Comparison of Enzyme Inhibition Types

Type	Binding Site	Effect on Enzyme
Competitive	Active site	Blocks substrate directly
Noncompetitive	Allosteric site	Alters enzyme conformation

Feedback Inhibition

Feedback inhibition is a regulatory mechanism in which the end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction of the product.

• Example: Isoleucine inhibits threonine deaminase in its biosynthetic pathway.

Energy Flow in Ecosystems

Photosynthesis and Cellular Respiration

Energy enters ecosystems as sunlight and leaves as heat. Photosynthesis converts light energy into chemical energy, while cellular respiration releases energy from organic molecules.

• Photosynthesis: $\text{Energy} + 6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Cellular Respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$

Redox Reactions in Cellular Respiration

Cellular respiration involves a series of oxidation-reduction (redox) reactions, where electrons are transferred from glucose to oxygen, releasing energy.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Example: $\text{Na} + \text{Cl} \rightarrow \text{Na}^+ + \text{Cl}^-$ (Na is oxidized, Cl is reduced)

Electron Carriers: NAD+ and NADH

Nicotinamide adenine dinucleotide (NAD+) acts as an electron carrier, accepting electrons during metabolic reactions to become NADH. NADH later donates electrons to the electron transport chain.

• Reduction: $\text{NAD}^+ + 2e^- + 2H^+ \rightarrow \text{NADH} + H^+$

• Oxidation: $\text{NADH} \rightarrow \text{NAD}^+ + 2e^- + 2H^+$

Pathways of Cellular Respiration

Major Stages

Cellular respiration consists of three main pathways that break down glucose and other fuels to produce ATP.

• Glycolysis: Occurs in the cytosol; breaks down glucose into pyruvate.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondria; completes the breakdown of glucose derivatives.

• Electron Transport Chain and Oxidative Phosphorylation: Occurs in the mitochondria; uses oxygen to produce ATP.

Additional info: The notes reference class activities and questions about enzyme inhibition, redox reactions, and metabolic regulation, which are standard topics in introductory biology courses.