Which of the following statements about enzymes is NOT true?

Introduction / Context:
Enzymes are essential biological catalysts that speed up chemical reactions in living organisms. Their structure and how they interact with substrates are central topics in biochemistry and cell biology. This question asks you to identify which statement about enzymes is not true, requiring a clear understanding of the enzyme substrate relationship and basic enzyme properties.

Given Data / Assumptions:

Enzymes have an active site where the substrate binds.
Most enzymes are proteins with complex three dimensional shapes.
They typically have optimal pH and temperature conditions.
We consider classical enzyme models such as lock and key and induced fit.

Concept / Approach:
Enzymes do not have exactly the same shape as their substrates; instead, they have an active site with a shape that is complementary to the substrate. The classical lock and key model compares the active site to a lock and the substrate to a key that fits, but they are not identical. The induced fit model adds that the active site may change shape slightly when the substrate binds. It is true that enzymes are proteins and that their three dimensional structure is critical to their function. It is also true that each enzyme has an optimal pH and temperature at which it works most efficiently. Therefore, the statement claiming that all enzymes have exactly the same shape as their substrates is incorrect.

Step-by-Step Solution:
Step 1: Recall that enzymes are mostly proteins and function as catalysts in biochemical reactions. Step 2: Recognize that each enzyme has a specific active site whose shape is complementary to its substrate, not identical. Step 3: Remember that enzymes typically have an optimum pH and temperature beyond which their activity decreases or they may denature. Step 4: Identify which statement conflicts with the concept of complementarity and induced fit by incorrectly stating that the enzyme and substrate have exactly the same shape. Step 5: Select that statement as the one that is not true.

Verification / Alternative check:
Biochemistry texts describe how the substrate fits into the active site like a key in a lock, illustrating complementarity in shape and chemical properties, not perfect identity. Structural studies of enzyme substrate complexes show that the protein folds create a pocket or groove that fits particular substrates. The entire enzyme does not mimic the shape of the substrate; only the active site region is complementary. This confirms that the claim of exactly the same shape is misleading and incorrect.

Why Other Options Are Wrong:
Enzymes are proteins: This is true for most enzymes; some catalytic RNA molecules exist but are considered ribozymes rather than typical protein enzymes.
Work best at specific pH: Each enzyme has an optimal pH and temperature; for example, pepsin is most active in acidic conditions in the stomach.
Shape allows it to do its job: The three dimensional shape of an enzyme determines its specificity and catalytic ability, so this statement is correct.

Common Pitfalls:
A frequent misunderstanding is to interpret the lock and key analogy too literally and imagine the enzyme and substrate having identical shapes. In reality, only certain parts interact closely, and the rest of the enzyme may have a very different form. Another pitfall is forgetting that denaturation changes the shape of enzymes and destroys their function, reinforcing how important structure is. Whenever exam questions ask which statement is not true, read each option carefully and compare it with known principles.

Final Answer:
The statement about enzymes that is not true is that all enzymes have exactly the same shape as their substrates; in fact, they have complementary shapes at their active sites.