Enzymes and Cellular Respiration: Key Concepts in General Biology

Concept 8.4 - Enzymes

Definition and Function of Enzymes

Enzymes are biological catalysts, typically proteins, that accelerate chemical reactions in living organisms by lowering the activation energy barrier. They do not alter the free energy change ($\Delta G$) of a reaction, but increase the rate at which equilibrium is reached.

• Enzymes: Catalytic proteins that facilitate biochemical reactions.

• Activation Energy ($E_A$): The energy required to initiate a chemical reaction. Enzymes lower $E_A$.

• Free Energy ($\Delta G$): The difference in energy between reactants and products; unaffected by enzymes.

Example: The graph below illustrates how enzymes lower the activation energy required for a reaction, making it proceed faster.

With Enzyme	Without Enzyme
Lower $E_A$	Higher $E_A$
Faster reaction rate	Slower reaction rate
$\Delta G$ unchanged	$\Delta G$ unchanged

Enzyme-Substrate Interaction

Enzymes act on specific molecules called substrates. The region where the substrate binds is known as the active site. The binding is highly specific and often involves an induced fit, where the enzyme changes shape to better accommodate the substrate, enhancing its catalytic ability.

• Substrate: The reactant molecule upon which an enzyme acts.

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

• Enzyme-Substrate Complex: The temporary association formed when an enzyme binds its substrate.

• Induced Fit: The conformational change in the enzyme upon substrate binding that increases catalytic efficiency.

Example: The enzyme hexokinase binds glucose at its active site, forming an enzyme-substrate complex and catalyzing the phosphorylation of glucose.

Cellular Respiration and Fermentation

Overview of Energy Conversion in Cells

Cells convert energy stored in food molecules into usable chemical energy (ATP) through cellular respiration and fermentation. Photosynthesis in plants captures light energy and stores it in organic molecules, which are then broken down in mitochondria to release energy for cellular work.

• Photosynthesis: Converts light energy into chemical energy stored in glucose.

• Cellular Respiration: Breaks down organic molecules to generate ATP, releasing CO2 and H2O.

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

Example: In plant cells, photosynthesis produces glucose, which is then used in cellular respiration to generate ATP for cellular activities.

Cellular Respiration

Terminology and General Descriptions

Cellular respiration is a series of metabolic pathways that break down proteins, carbohydrates, and lipids to release energy. It involves electron transfer and can occur via aerobic respiration or fermentation.

• Aerobic Respiration: Requires oxygen; complete breakdown of organic compounds.

• Fermentation: Occurs in the absence of oxygen; partial breakdown of sugars with end products such as alcohols or acids.

Aerobic Respiration vs Fermentation

Aerobic respiration and fermentation are two distinct metabolic pathways for energy production.

Process	Oxygen Required?	End Products	ATP Yield
Aerobic Respiration	Yes	CO2, H2O	High
Fermentation	No	Alcohols, Acids	Low

Equation for Aerobic Respiration:

$\text{Organic Compounds} + O_2 \rightarrow CO_2 + H_2O + \text{Energy (ATP)}$

Fermentation: Partial breakdown of sugars in the absence of oxygen.

Electron Transfer, Oxidation, and Reduction

Cellular respiration involves redox reactions, where electrons are transferred from one molecule to another. Oxidation is the loss of electrons, while reduction is the gain of electrons. The molecule that loses electrons is the reducing agent; the one that gains electrons is the oxidizing agent.

• Oxidation: Loss of electrons (and often hydrogen).

• Reduction: Gain of electrons (and often hydrogen).

• Reducing Agent: Donates electrons and becomes oxidized.

• Oxidizing Agent: Accepts electrons and becomes reduced.

Example: In the oxidation of methane:

$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$

Methane is oxidized (loses electrons), oxygen is reduced (gains electrons).

Electron Carriers and Coenzymes

Electron carriers such as NAD+ (nicotinamide adenine dinucleotide) play a crucial role in cellular respiration by shuttling electrons between reactions. NAD+ is a coenzyme that cycles between oxidized (NAD+) and reduced (NADH) forms.

• NAD+: Oxidized form; accepts electrons.

• NADH: Reduced form; donates electrons to the electron transport chain.

• Coenzyme: Non-protein molecule that assists enzyme function.

Reaction:

$\text{C-OH} + NAD^+ \xrightarrow{\text{Dehydrogenase}} \text{C=O} + NADH + H^+$

ATP Production: Substrate-level vs Oxidative Phosphorylation

ATP is produced in cellular respiration by two main mechanisms: substrate-level phosphorylation and oxidative phosphorylation.

• Substrate-level Phosphorylation: Direct transfer of a phosphate group to ADP from a substrate; occurs in glycolysis and the citric acid cycle.

• Oxidative Phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis; produces the majority of ATP.

Example: Most ATP in cellular respiration is generated via oxidative phosphorylation in mitochondria.

Main Stages of Cellular Respiration

Cellular respiration consists of three main stages:

1. Glycolysis: Occurs in the cytosol; breaks down glucose into two molecules of pyruvate.

2. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose derivatives, producing NADH and FADH2.

3. Electron Transport Chain and Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to generate ATP.

Glycolysis

Overview and Reaction

Glycolysis is the first stage of cellular respiration, occurring in the cytosol. It converts one molecule of glucose into two molecules of pyruvate, generating a small amount of ATP and NADH.

• Location: Cytosol

• Products: 2 Pyruvate, 2 ATP (net), 2 NADH

Overall Reaction:

$\text{Glucose} \rightarrow 2\ \text{Pyruvate} + 2\ \text{ATP} + 2\ \text{NADH}$

Pyruvate Oxidation and Citric Acid Cycle

Pyruvate Oxidation

After glycolysis, pyruvate is transported into the mitochondrion and converted to acetyl CoA, which enters the citric acid cycle. This step links glycolysis to the citric acid cycle.

• Location: Mitochondrial matrix

• Products: Acetyl CoA, CO2, NADH

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of acetyl CoA, generating NADH and FADH2 for the electron transport chain. Each turn of the cycle produces CO2, ATP, NADH, and FADH2.

• Key Steps: Acetyl CoA combines with oxaloacetate to form citrate; citrate is decomposed back to oxaloacetate through a series of reactions.

• Products per glucose: 6 NADH, 2 FADH2, 2 ATP, 4 CO2

Oxidative Phosphorylation

Electron Transport Chain and Chemiosmosis

Oxidative phosphorylation occurs in the inner mitochondrial membrane and involves the electron transport chain and chemiosmosis. Electrons from NADH and FADH2 are transferred through protein complexes, driving the production of ATP.

• Electron Transport Chain: Series of protein complexes that transfer electrons and pump protons (H+) across the membrane.

• Chemiosmosis: The movement of H+ back across the membrane through ATP synthase, driving ATP synthesis.

• Proton-Motive Force: The H+ gradient across the membrane that powers ATP production.

ATP Yield: About 28 ATP per glucose molecule are produced via oxidative phosphorylation.

Integration of Metabolism

Catabolism of Proteins, Carbohydrates, and Fats

Proteins, carbohydrates, and fats can all be broken down and enter cellular respiration at various points, contributing to ATP production.

• Proteins: Broken down into amino acids, which can enter glycolysis or the citric acid cycle after deamination.

• Carbohydrates: Enter as glucose or other sugars in glycolysis.

• Fats: Broken down into glycerol (enters glycolysis) and fatty acids (enter citric acid cycle as acetyl CoA).

Example: Fatty acids undergo beta-oxidation to produce acetyl CoA, which enters the citric acid cycle.