Biochemical Reactions and Enzymes

Overview

This lecture covers activation energy, the laws of thermodynamics, ATP's role in cells, and how enzymes and catalysts affect biochemical reactions.

Activation energy is the initial energy needed to start a chemical reaction.
Molecules must reach a transition state, which is unstable and requires energy.
In non-living systems, heat overcomes activation energy; in cells, catalysts (mostly enzymes) lower activation energy.
Catalysts speed up reactions by lowering activation energy but do not change ΔG (Gibbs free energy) of the reaction.
Without activation energy barriers, molecules like sucrose would decompose spontaneously, making storage impossible.

First Law: Energy cannot be created or destroyed, only converted between forms.
Second Law: Each energy transfer increases entropy (disorder), with some energy lost as heat.
Organized living systems require constant energy input to maintain order and combat increasing entropy.

ATP (adenosine triphosphate) is the primary energy currency in cells.
ATP consists of adenine, ribose, and three phosphate groups (α, β, γ).
The high potential energy in phosphate bonds makes ATP unstable and ready to release energy via hydrolysis.
ATP hydrolysis releases a phosphate group and energy, powering endergonic (energy-requiring) reactions.
The sodium-potassium pump uses ATP hydrolysis to move ions against concentration gradients.

Enzymes are biological catalysts, mostly proteins, that speed up reactions by lowering activation energy.
Enzymes are highly specific to their substrates and reactions.
Most enzymes are proteins, but some RNA molecules (ribozymes) function as enzymes.
The active site is where a substrate binds to the enzyme, forming an enzyme-substrate complex.
Enzyme activity depends on 3D structure, which is sensitive to temperature and pH.
Induced fit: enzyme active site changes shape to better fit the substrate, enhancing catalysis.
Enzymes return to their original state after the reaction and can be reused.

Activation energy — Minimum energy needed for a chemical reaction to proceed.
Catalyst — Substance that lowers activation energy, speeding up reactions.
ΔG (Gibbs free energy) — Energy change of a reaction, indicating if it is spontaneous.
Entropy — Measure of disorder or randomness in a system.
ATP (adenosine triphosphate) — Main energy carrier molecule in cells.
Enzyme — Protein that acts as a biological catalyst.
Active site — Region on an enzyme where substrates bind and reactions occur.
Induced fit — Model where enzyme changes shape to better accommodate the substrate.