Biological Activation and Enzymes

Overview

This lecture covers activation energy, the laws of thermodynamics, ATP structure and function, and how enzymes catalyze biological reactions.

Activation energy is the initial energy input required to start a chemical reaction.
The transition state is a temporary, unstable arrangement molecules must reach for reactions to proceed.
In cells, heat cannot be used to overcome activation energy; instead, catalysts lower activation energy.
Catalysts speed up reactions by lowering activation energy without changing the Gibbs free energy (ΔG).
Activation energy explains the stability of substances like table sugar, which would break down quickly if no energy was required.

The first law: Energy cannot be created or destroyed, only transformed.
The second law: Entropy (disorder) always increases, with some energy lost as heat in every reaction.
Living cells require a constant energy input to maintain order and organization.

ATP stands for adenosine triphosphate and is the main energy currency in cells.
ATP’s structure includes adenine (a nitrogenous base), ribose (a five-carbon sugar), and three phosphate groups (alpha, beta, gamma).
The bonds between the phosphate groups have high potential energy due to repulsion between negative charges.
Hydrolysis of ATP (removal of the gamma phosphate) releases energy for cellular work.
The hydrolysis reaction is efficient, with most energy captured and some lost as heat.
ATP can be regenerated by adding a phosphate group to ADP (adenosine diphosphate).

ATP hydrolysis powers cellular processes like the sodium-potassium pump (active transport).
The released phosphate temporarily binds to proteins (such as pumps), causing conformational changes that enable work.

Enzymes are biological catalysts, mostly proteins, that lower activation energy.
Enzymes bind specific substrates at the active site, forming an enzyme-substrate complex.
Most enzymes catalyze only one specific reaction and depend on their 3D shape for function.
The catalytic process is sensitive to temperature and pH; extremes can denature enzymes.
Some catalysts, called ribozymes, are made from RNA.
The induced fit model describes how enzymes change shape to fit substrates, unlike the old lock-and-key model.
Enzymes return to their original state after catalysis and can be reused.

Activation Energy — energy needed to initiate a chemical reaction.
Catalyst — substance that speeds up a reaction by lowering activation energy.
Gibbs Free Energy (ΔG) — the amount of energy available to do work in a system.
Entropy — measure of disorder or randomness in a system.
ATP (Adenosine Triphosphate) — molecule that stores and supplies energy in cells.
Enzyme — a protein catalyst that speeds up biochemical reactions.
Active Site — part of an enzyme where the substrate binds.
Induced Fit — model where enzymes change shape to better fit their substrates.
Ribozyme — RNA molecule with catalytic activity.