Metabolism & Glycolysis — Comprehensive Study Notes Summary & Study Notes

🔥 Energy and Metabolism

Cells manage energy through coupled chemical changes. A catabolic process converts complex molecules into simpler ones, releasing free energy so that $\Delta G$ is negative; these reactions are exergonic and spontaneous. An anabolic process builds complex molecules from simple ones, requires an input of energy so that $\Delta G$ is positive; these reactions are endergonic and non-spontaneous.

⚡ Free Energy, Spontaneity, and Activation Energy

The sign of free energy change ($\Delta G$) tells whether a reaction is spontaneous, but not how fast it proceeds. Even exergonic reactions often require an initial input called activation energy to destabilize bonds before products form. For example, sucrose hydrolysis has $\Delta G = -7,\text{kcal/mol}$ (exergonic) but still requires an activation barrier because covalent bonds (e.g., between glucose and fructose) are relatively strong.

Example of ionic dissolution: $\text{NaCl} \rightarrow \text{Na}^+ + \text{Cl}^-$ has $\Delta G = -9.1,\text{kJ/mol}$; the ionic bond is easier to break in water than many covalent bonds, so dissolution proceeds more readily.

🧪 Enzymes: Catalysts of Biology

Enzymes are biological catalysts that lower the activation energy of a reaction, increasing reaction rate without being consumed. Enzymes do not change the net $\Delta G$ of a reaction — they only change how quickly equilibrium is reached.

Enzymes are typically proteins with specific 3D shapes determined by amino acid sequence. They are substrate-specific: each enzyme binds particular reactants (substrates) at an active site. Enzymes can:

• Stabilize the transition state by stretching substrates.
• Provide chemical microenvironments (e.g., acidic pockets) to facilitate reactions.
• Bring two substrates together to enable bond formation.

🧩 Enzyme Regulation and Inhibition

Cells regulate enzyme activity to control metabolic flux. Common modes:

• Competitive inhibition: inhibitor resembles the substrate and competes for the active site.
• Noncompetitive (allosteric) inhibition: inhibitor binds elsewhere and changes enzyme shape or activity.

Metabolic feedback control is common: high levels of an end product (e.g., ATP) inhibit catabolic enzymes that generate it, while low ATP (and higher ADP) relieves inhibition or activates those enzymes.

🔁 Redox Reactions and Energy Harvesting

Energy in organic molecules is often harvested through redox reactions (electron transfer). Mnemonic: OILRIG — Oxidation Is Loss, Reduction Is Gain (of electrons). Electrons are frequently transferred as hydrogen atoms (an electron plus a proton).

Direct combustion releases energy as heat and light and is inefficient for biological ATP production. Cells instead extract energy in many small, controlled redox steps so that energy can be captured for ATP synthesis.

🔋 NAD+/NADH and Controlled Electron Flow

Cells use mobile electron carriers such as $\text{NAD}^+$ and $\text{NADH}$. A dehydrogenase removes hydrogens from an organic substrate, transferring electrons and a proton to $\text{NAD}^+$ to form $\text{NADH}$ (the reduced form). This transfer loses little energy and allows electrons to be passed down a chain of carriers.

Because $\text{NAD}^+$ and $\text{NADH}$ cycle readily, they are convenient for shuttling reducing power. $\text{NAD}^+$ is derived from niacin (vitamin B3).

At the end of aerobic respiration, electrons that originated on glucose and traveled via carriers ultimately reduce oxygen to water: starting with hydrogens from glucose and $\tfrac{1}{2},O_2$ (from breathing), ending with $H_2O$ and energy release.

🧬 Cellular Respiration Overview

Cellular (aerobic) respiration fully oxidizes organic molecules using oxygen and releases a large amount of energy: for glucose oxidation $\Delta G \approx -686,\text{kcal/mol}$. About 40% of that released energy is captured as ATP; the rest is released as heat.

Variants:

• Fermentation: partial catabolism of glucose when final electron acceptors are not available; yields less energy/ATP.
• Anaerobic respiration: uses a final electron acceptor other than oxygen (e.g., nitrate or sulfate); yields more energy than fermentation but less than aerobic respiration.

🧪 Enzyme Types to Know

• Kinase: transfers a phosphate group from a high-energy donor (like ATP) to another molecule.
• Isomerase: converts one isomer into another.
• Dehydrogenase: transfers hydrogen atoms (electrons and protons) from a substrate to an electron acceptor (often $\text{NAD}^+$).

🍞 Glycolysis — "Sugar Splitting"

Glycolysis occurs in the cytosol of virtually all cells and converts one glucose into two pyruvate molecules. It proceeds in three phases:

1. Energy-investment phase

• Cells invest energy to destabilize glucose and make it easier to split. Two ATP molecules are consumed in early steps.
• Key transformations: glucose is phosphorylated (kinase), rearranged (isomerase) to fructose forms, then phosphorylated again to raise free energy and prepare for cleavage.

1. Cleavage phase

• Fructose 1,6-bisphosphate is cleaved into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
• DHAP is rapidly isomerized to a second G3P, so two G3P molecules proceed to the next phase.

1. Energy-payoff phase

• Each G3P is oxidized by a dehydrogenase, transferring electrons to $\text{NAD}^+$ to form $\text{NADH}$, and inorganic phosphate is added.
• Subsequent steps produce ATP by substrate-level phosphorylation as high-energy phosphate groups are transferred to ADP.

Summary of stoichiometry for one glucose:

• Invested: 2 ATP
• Produced: 4 ATP and 2 $\text{NADH}$
• Net yield: 2 ATP and 2 $\text{NADH}$
• End products: 2 pyruvate per glucose

Key points: ATP is produced directly by substrate-level phosphorylation in glycolysis; redox steps (e.g., G3P to 1,3-bisphosphoglycerate) generate reducing power stored in $\text{NADH}$.

🔄 Integration and Control

Glycolysis is regulated at several key enzyme steps (often the kinases and committed-step enzymes) and responds to the cell's energy status. High ATP often inhibits early steps; high ADP/AMP activates them. This feedback keeps ATP production aligned with demand.

✅ Final Remarks (Practical Takeaways)

• Enzymes speed reactions by lowering activation energy but do not change overall $\Delta G$.
• Redox chemistry and carriers like $\text{NAD}^+$/$\text{NADH}$ allow controlled extraction of energy from organic molecules.
• Glycolysis is a universal pathway that yields a small, immediate ATP payoff and provides substrates (pyruvate, $\text{NADH}$) for further energy-yielding pathways (fermentation or respiration).

Use these notes to connect concepts: thermodynamics ($\Delta G$), kinetics (activation energy and enzymes), redox flow (NAD+/NADH and electron transport), and pathway regulation (feedback inhibition and allostery).