BIOB 160 Principles of Living Systems · Unit 2: Energy and the cell · on Exam 2
2.2 ATP and enzymes
Study notes for ATP and enzymes in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- ATP hydrolysis to ADP + Pᵢ releases about −7.3 kcal/mol under standard conditions. Cells use energy coupling: the phosphate is transferred to another molecule (phosphorylation), making it more reactive. ATP is regenerated from ADP by respiration.
- Enzymes are catalysts (usually proteins) that speed reactions by lowering the activation energy. They do not change ΔG or the equilibrium, and they are not used up.
- The substrate binds the active site; the fit tightens as the enzyme changes shape (induced fit). Enzymes are specific and have optimal temperature and pH; many need cofactors (metal ions) or coenzymes (organic, often vitamins).
- Competitive inhibitors bind the active site (overcome by more substrate); noncompetitive inhibitors bind elsewhere and change the enzyme’s shape. Allosteric regulation turns enzymes on or off at a separate site.
- Feedback inhibition: the end product of a pathway inhibits an early enzyme, so the cell does not overproduce it.
Key formulas
ATP hydrolysis
$$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i \quad \Delta G \approx -7.3\ \text{kcal/mol}$$
Worked example
Adding much more substrate restores an enzyme’s rate in the presence of inhibitor A but not inhibitor B. Classify each inhibitor.
Show the solution
A is competitive: it competes for the active site, so extra substrate outcompetes it. B is noncompetitive: it binds elsewhere and changes the enzyme’s shape, so more substrate does not help.
Common mistakes
- Saying enzymes make reactions more exergonic. They only lower activation energy.
- Thinking higher temperature always speeds enzyme reactions. Past the optimum, enzymes denature.
- Confusing the active site with an allosteric site.
Exam tipReading: Campbell 8.3–8.4 (pp. 150–163). Be able to sketch an energy diagram with and without an enzyme: same ΔG, lower hump.
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