Which of the following statements is incorrect?
I. Nearly 1/3rd of all enzymes requires the presence of metal ions for catalytic function
II. Metal ions form coordinate bond with side chain at active site of metalloenzyme and at the same time form one or more coordinate bonds with the substrate
III. NAD and NAD (coenzymes) contain niacin (vit.)
IV. Coenzyme are organic compounds but their association with the apoenzyme is only transient, usually occurring during catalysis

Options

A. All
B. Only III and IV
C. Only IV
D. None of the above

Answer

To determine which statement is incorrect, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement I**: "Nearly 1/3rd of all enzymes requires the presence of metal ions for catalytic function."
- This statement is **correct**. Many enzymes, known as metalloenzymes, require metal ions as cofactors to perform their catalytic functions. It is estimated that about one-third of all enzymes need metal ions.

2. **Statement II**: "Metal ions form coordinate bond with side chain at active site of metalloenzyme and at the same time form one or more coordinate bonds with the substrate."
- This statement is also **correct**. Metal ions in metalloenzymes do indeed form coordinate bonds with the side chains of amino acids at the active site and can also interact with the substrate, facilitating the catalytic process.

3. **Statement III**: "NAD and NADP (coenzymes) contain niacin (vit.)."
- This statement is **correct**. NAD (Nicotinamide adenine dinucleotide) and NADP (Nicotinamide adenine dinucleotide phosphate) are coenzymes that are derived from niacin (vitamin B3).

4. **Statement IV**: "Coenzymes are organic compounds but their association with the apoenzyme is only transient, usually occurring during catalysis."
- This statement is **correct**. Coenzymes are indeed organic molecules that associate with enzymes (apoenzymes) transiently, typically during the catalytic process.

### Conclusion:
Since all four statements are correct, it appears there is a misunderstanding in the question. However, if we assume that the question requires us to identify the least accurate or misleading statement, we can conclude that all statements are accurate based on the information provided.

### Final Answer:
None of the statements are incorrect; all are correct.

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