Key Points: Enzyme Fundamentals

What Enzymes Do:

• Enzymes = biological catalysts. Mostly proteins; EXCEPTION: Ribozymes (RNA-based catalysts)
• Function: Lower ACTIVATION ENERGY. They do NOT change: equilibrium, $\Delta G$, reaction direction, enthalpy, entropy
• Enzymes are NOT consumed in reactions — regenerated after each catalytic cycle
• A non-spontaneous reaction ($\Delta G$ > 0) CANNOT be made spontaneous by an enzyme

Models of Enzyme Action:

• Lock-and-Key model (Emil Fischer, 1894): rigid active site, static complementarity
• Induced Fit model (Daniel Koshland, 1958): flexible active site reshapes upon substrate binding — NOW MORE ACCEPTED

Active Site and Specificity:

• Active site = pocket/cleft where substrate binds and reaction occurs
• Enzyme specificity = geometric and chemical complementarity between active site and substrate
• One enzyme → one substrate or class of substrates (absolute vs. relative specificity)
• Changing one key amino acid in the active site can abolish enzyme activity

Six Enzyme Classes (OTHLIL Mnemonic):

• EC 1 Oxidoreductases → redox reactions (e.g., alcohol dehydrogenase)
• EC 2 Transferases → group transfer (e.g., transaminase)
• EC 3 Hydrolases → hydrolysis with water (e.g., lipase)
• EC 4 Lyases → non-hydrolytic cleavage (e.g., aldolase)
• EC 5 Isomerases → molecular rearrangement (e.g., phosphoglucose isomerase)
• EC 6 Ligases → bond formation using ATP (e.g., DNA ligase)

Key Examples to Know:

• DNA ligase → Ligase (EC 6) → joins Okazaki fragments with ATP
• Lipase → Hydrolase (EC 3) → triglyceride hydrolysis with water
• Aldolase → Lyase (EC 4) → non-hydrolytic C-C cleavage in glycolysis