Which of the following is not true concerning enzyme activity?

Options

A. Enzymes are considered organic catalysts
B. Most enzymes work with a wide variety of substrates
C. Enzymes are not altered in the course of their action
D. Enzymes form temporary compounds with their substrates

Explanation

To determine which statement is not true concerning enzyme activity, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Enzymes as Organic Catalysts**:
- Enzymes are indeed organic catalysts. They facilitate chemical reactions without being consumed or altered in the process. This means they can be reused multiple times.
- **Conclusion**: This statement is true.

2. **Enzymes Working with a Wide Variety of Substrates**:
- Enzymes have specific active sites that are tailored to bind specific substrates. This means that most enzymes are selective and work with only a limited range of substrates, not a wide variety.
- **Conclusion**: This statement is not true.

3. **Enzymes Not Altered in the Course of Their Action**:
- Enzymes do not change their structure permanently during the reaction. They participate in the reaction and return to their original state after the reaction is complete.
- **Conclusion**: This statement is true.

4. **Enzymes as Temporary Compounds with Their Substrates**:
- When a substrate binds to an enzyme, they form an enzyme-substrate complex. This complex is temporary, as the enzyme will release the product after the reaction.
- **Conclusion**: This statement is true.

### Final Answer:
The statement that is **not true** concerning enzyme activity is: **"Most enzymes work with a wide variety of substrates."**