Nature of Enzyme Action and Factors Affecting Enzyme Activity
Enzymes are remarkable biological catalysts that speed up the chemical reactions sustaining life without being consumed in the process. This quiz explores the four-step catalytic cycle of enzyme action, from substrate binding to product release, and examines how temperature, pH, substrate concentration and inhibitors govern enzyme activity.
You will encounter key concepts such as:
- The enzyme-substrate (ES) and enzyme-product (EP) complexes
- Optimum temperature and optimum pH
- The saturation curve and the meaning of ##V_{max}##
- Competitive inhibition, illustrated by succinic dehydrogenase and malonate
Work carefully through each question and read the explanations to strengthen your understanding of enzyme kinetics.
Q1. The formation of which complex is described as essential for catalysis?
The chapter states that the formation of the enzyme-substrate (ES) complex is essential for catalysis; the enzyme has a substrate binding site that produces this highly reactive complex.
Q2. In the catalytic cycle, what happens immediately after the substrate binds to the active site of the enzyme?
Step 2 of the catalytic cycle states that the binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate.
Q3. Which sequence correctly represents the overall enzyme-catalysed reaction as given in the lesson?
The lesson gives the sequence ##\ce{E + S -> ES -> EP -> E + P}##, with intermediate formation of the enzyme-product complex.
Q4. In the fourth step of the catalytic cycle, what happens to the enzyme after the products are released?
Step 4 states that the enzyme releases the products and the free enzyme is ready to bind to another molecule of the substrate and run through the catalytic cycle once again.
Q5. The highest activity of an enzyme at a particular temperature and pH is referred to as:
Each enzyme shows its highest activity at a particular temperature and pH called the optimum temperature and optimum pH; activity declines both below and above the optimum value.
Q6. What effect does low temperature have on an enzyme?
The lesson states that low temperature preserves the enzyme in a temporarily inactive state, whereas high temperature destroys activity because proteins are denatured by heat.
Q7. Why does high temperature destroy enzymatic activity?
High temperature destroys enzymatic activity because proteins are denatured by heat, altering the tertiary structure of the enzyme.
Q8. As substrate concentration increases, the velocity of an enzymatic reaction:
With increasing substrate concentration the velocity rises at first and ultimately reaches a maximum velocity ##V_{\max}## which is not exceeded by any further rise in substrate concentration.
Q9. Why does the reaction velocity not exceed ##V_{\max}## even when more substrate is added?
Enzyme molecules are fewer than substrate molecules; after saturation of these molecules, there are no free enzyme molecules to bind with the additional substrate molecules.
Q10. When the binding of a specific chemical shuts off enzyme activity, the process is called:
When the binding of the chemical shuts off enzyme activity, the process is called inhibition and the chemical is called an inhibitor.
Q11. A competitive inhibitor is best described as a molecule that:
A competitive inhibitor closely resembles the substrate in molecular structure and competes with the substrate for the substrate-binding site, so the substrate cannot bind and enzyme action declines.
Q12. Which pair correctly illustrates competitive inhibition as described in the lesson?
The lesson gives the example of inhibition of succinic dehydrogenase by malonate, which closely resembles the substrate succinate in structure.
Q13. Which of the following is NOT listed in the lesson as a factor affecting enzyme activity?
The lesson lists temperature, pH, change in substrate concentration and binding of specific chemicals as factors affecting enzyme activity; molecular weight of the product is not among them.
Q14. Enzymes lower the energy barrier of a reaction. This barrier is known as:
The difference in average energy content of the substrate from that of the transition state is called activation energy, and enzymes bring down this energy barrier.
Q15. Competitive inhibitors, such as those resembling a substrate, are often used in:
The lesson states that such competitive inhibitors are often used in the control of bacterial pathogens.