Enzyme Co-factors: Apoenzyme, Holoenzyme & Types Explained
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| Co-factors help enzymes become active by combining with apoenzymes to form functional holoenzymes. |
-Dr.Sanjaykumar Pawar
Co-factors (NCERT Class 11 Biology)
Line-by-Line Explanation Notes (NEET Preparation)
6. Co-factors
Introduction to Co-factors
Line: Enzymes are composed of one or several polypeptide chains.
Explanation:
- Enzymes are mainly made up of proteins.
- Proteins are made up of long chains of amino acids called polypeptide chains.
- Some enzymes contain only one polypeptide chain, while others contain multiple chains.
Line: However, there are a number of cases in which non-protein constituents called co-factors are bound to the enzyme to make the enzyme catalytically active.
Explanation:
- Some enzymes cannot work alone.
- They require an additional non-protein component called a co-factor.
- Co-factors attach to enzymes and help them perform their catalytic activity.
- Without the co-factor, the enzyme may become inactive.
Important NEET Point:
Co-factor + Enzyme = Active enzyme
Line: In these instances, the protein portion of the enzymes is called the apoenzyme.
Explanation:
- The protein part of an enzyme alone is called an apoenzyme.
- Apoenzyme is usually inactive without its co-factor.
Remember:
Apoenzyme (inactive) + Co-factor → Active enzyme (Holoenzyme)
Holoenzyme
Definition:
- The complete active enzyme containing both:
- Protein part (apoenzyme)
- Non-protein part (co-factor)
is called a holoenzyme.
Formula:
Apoenzyme + Co-factor = Holoenzyme
NEET Important Point:
- Apoenzyme alone → Inactive
- Holoenzyme → Active
Types of Co-factors
Line: Three kinds of cofactors may be identified: prosthetic groups, co-enzymes and metal ions.
Explanation:
There are three major types of co-factors:
- Prosthetic groups
- Co-enzymes
- Metal ions
1. Prosthetic Groups
Line: Prosthetic groups are organic compounds and are distinguished from other cofactors in that they are tightly bound to the apoenzyme.
Explanation:
- Prosthetic groups are organic molecules.
- They remain permanently attached or tightly bound to the apoenzyme.
- They do not leave the enzyme during the reaction.
Key Point:
Prosthetic group = Permanently attached co-factor
Example: Haem in Peroxidase and Catalase
Line: In peroxidase and catalase, which catalyze the breakdown of hydrogen peroxide to water and oxygen, haem is the prosthetic group and it is a part of the active site of the enzyme.
Explanation:
- Enzymes like peroxidase and catalase break down hydrogen peroxide (H₂O₂).
- Hydrogen peroxide is converted into:
Hydrogen peroxide → Water + Oxygen
2H_2O_2 \rightarrow 2H_2O + O_2
- The prosthetic group present is haem.
- Haem forms a part of the enzyme's active site.
Important NEET Point:
Haem = Prosthetic group in catalase and peroxidase
2. Co-enzymes
Line: Co-enzymes are also organic compounds but their association with the apoenzyme is only transient, usually occurring during the course of catalysis.
Explanation:
- Co-enzymes are also organic molecules.
- Unlike prosthetic groups, they are loosely attached to the enzyme.
- They attach temporarily during the enzyme reaction.
- After the reaction, they may separate from the enzyme.
Key Difference:
| Prosthetic Group | Co-enzyme |
|---|---|
| Tightly bound | Loosely bound |
| Permanent association | Temporary association |
| Example: Haem | Example: NAD |
Line: Furthermore, co-enzymes serve as co-factors in a number of different enzyme catalyzed reactions.
Explanation:
- A single co-enzyme can help many different enzymes.
- It participates in different biochemical reactions.
Line: The essential chemical components of many coenzymes are vitamins.
Explanation:
- Many co-enzymes are made from vitamins.
- Vitamins are important for proper enzyme functioning.
NEET Point:
Vitamins often act as components of co-enzymes.
Example: NAD and NADP
Line: Coenzyme nicotinamide adenine dinucleotide (NAD) and NADP contain the vitamin niacin.
Explanation:
- NAD and NADP are important co-enzymes.
- They contain niacin (Vitamin B3).
- They participate in oxidation-reduction reactions.
Remember:
Niacin (Vitamin B3) → NAD and NADP
3. Metal Ions as Co-factors
Line: A number of enzymes require metal ions for their activity.
Explanation:
- Some enzymes need metal ions to work properly.
- These metal ions act as co-factors.
Examples of Metal Ions:
- Zinc (Zn²⁺)
- Magnesium (Mg²⁺)
- Iron (Fe²⁺/Fe³⁺)
Line: Metal ions form coordination bonds with side chains at the active site.
Explanation:
- Metal ions attach to amino acid side chains present at the enzyme's active site.
- This helps maintain proper enzyme structure and activity.
Line: At the same time form one or more coordination bonds with the substrate.
Explanation:
- Metal ions also bind with the substrate.
- This helps the enzyme hold the substrate properly during the reaction.
Example: Zinc as a co-factor
Line: Zinc is a cofactor for the proteolytic enzyme carboxypeptidase.
Explanation:
- Zinc ion helps carboxypeptidase perform its function.
- Carboxypeptidase breaks peptide bonds in proteins.
NEET Point:
Zn²⁺ → Cofactor of carboxypeptidase
Importance of Co-factors
Line: Catalytic activity is lost when the co-factor is removed from the enzyme.
Explanation:
- If the co-factor is removed, the enzyme cannot perform its function.
- The enzyme loses its catalytic ability.
Line: This testifies that they play a crucial role in the catalytic activity of the enzyme.
Explanation:
- Co-factors are essential for enzyme action.
- They help enzymes convert substrates into products.
Important NEET Revision Points
1. Apoenzyme
- Protein part of enzyme.
- Inactive alone.
2. Holoenzyme
- Complete active enzyme.
- Apoenzyme + Co-factor.
3. Types of Co-factors
- Prosthetic groups
- Co-enzymes
- Metal ions
Quick Comparison Table
| Feature | Prosthetic Group | Co-enzyme | Metal Ion |
|---|---|---|---|
| Nature | Organic compound | Organic compound | Inorganic ion |
| Binding | Tightly bound | Temporarily bound | Coordination bonds |
| Example | Haem | NAD, NADP | Zn²⁺ |
| Function | Helps enzyme activity | Transfers chemical groups/electrons | Helps substrate binding |
Easy Memory Trick
"PC-M" = Three Co-factors
P → Prosthetic group
C → Co-enzyme
M → Metal ions
One-Line NEET Summary
Co-factors are non-protein components required by some enzymes for activity; they include prosthetic groups, co-enzymes and metal ions, and their removal makes enzymes inactive.
Class 11 Biology (CBSE) Question Bank
Topic: Co-factors (Enzymes)
MCQs, Very Short, Short, Long Answers, Assertion-Reason, Fill in the Blanks, Case Study, Statement-Based Questions & Match the Column
A. Multiple Choice Questions (MCQs)
1. The protein part of an enzyme without a co-factor is called:
A. Holoenzyme
B. Apoenzyme
C. Coenzyme
D. Prosthetic group
Answer: B. Apoenzyme
2. The complete active enzyme is known as:
A. Apoenzyme
B. Coenzyme
C. Holoenzyme
D. Substrate
Answer: C. Holoenzyme
3. Which of the following is a type of co-factor?
A. Carbohydrate
B. Lipid
C. Prosthetic group
D. Nucleic acid
Answer: C. Prosthetic group
4. Haem acts as a prosthetic group in:
A. Amylase and lipase
B. Catalase and peroxidase
C. DNA polymerase
D. Pepsin
Answer: B. Catalase and peroxidase
5. Co-enzymes are generally:
A. Inorganic ions
B. Organic compounds
C. Proteins
D. Amino acids
Answer: B. Organic compounds
6. NAD and NADP contain which vitamin?
A. Vitamin A
B. Vitamin B1
C. Niacin (Vitamin B3)
D. Vitamin C
Answer: C. Niacin (Vitamin B3)
7. Which metal ion acts as a co-factor for carboxypeptidase?
A. Iron
B. Zinc
C. Calcium
D. Sodium
Answer: B. Zinc
8. Removal of co-factor from an enzyme results in:
A. Increased activity
B. Loss of catalytic activity
C. Formation of substrate
D. Protein breakdown
Answer: B. Loss of catalytic activity
9. A co-factor that is tightly bound to an enzyme is called:
A. Coenzyme
B. Prosthetic group
C. Metal ion
D. Substrate
Answer: B. Prosthetic group
10. The active form of enzyme is:
A. Apoenzyme
B. Holoenzyme
C. Protein only
D. Co-factor only
Answer: B. Holoenzyme
B. Very Short Answer Questions (1 Mark)
1. What are co-factors?
Answer:
Co-factors are non-protein components required by some enzymes for their catalytic activity.
2. Name the protein part of an enzyme.
Answer:
Apoenzyme.
3. What is a holoenzyme?
Answer:
A holoenzyme is a complete active enzyme consisting of an apoenzyme and a co-factor.
4. Name the three types of co-factors.
Answer:
- Prosthetic groups
- Co-enzymes
- Metal ions
5. Which vitamin is present in NAD and NADP?
Answer:
Niacin (Vitamin B3).
6. Give one example of a metal ion co-factor.
Answer:
Zinc (Zn²⁺).
C. Short Answer Questions (2–3 Marks)
1. Differentiate between apoenzyme and holoenzyme.
Answer:
| Apoenzyme | Holoenzyme |
|---|---|
| Protein part of enzyme | Complete active enzyme |
| Inactive alone | Catalytically active |
| Requires co-factor | Contains apoenzyme and co-factor |
2. What are prosthetic groups? Give an example.
Answer:
- Prosthetic groups are organic co-factors that are tightly bound to the apoenzyme.
- They remain attached during enzyme action.
- Example: Haem in catalase and peroxidase.
3. Explain co-enzymes with examples.
Answer:
- Co-enzymes are organic co-factors that bind temporarily with enzymes during reactions.
- They help enzymes perform catalytic activity.
- Examples:
- NAD
- NADP
4. Explain the role of metal ions in enzymes.
Answer:
- Metal ions act as co-factors for many enzymes.
- They form coordination bonds with the enzyme's active site and substrate.
- Example: Zinc acts as a co-factor for carboxypeptidase.
D. Long Answer Questions (5 Marks)
1. Explain different types of co-factors with examples.
Answer:
Co-factors are non-protein components required by some enzymes for catalytic activity. They combine with enzymes to make them active.
Types of co-factors:
1. Prosthetic Groups
- Organic compounds.
- Tightly bound to apoenzyme.
- Remain permanently attached.
- Example: Haem in catalase and peroxidase.
2. Co-enzymes
- Organic compounds.
- Temporarily associated with enzymes.
- Often derived from vitamins.
- Example:
- NAD
- NADP
- Both contain niacin (Vitamin B3).
3. Metal Ions
- Inorganic cofactors.
- Form coordination bonds with enzymes and substrates.
- Example:
- Zinc in carboxypeptidase.
Removal of co-factors results in loss of enzyme activity, showing their importance in catalysis.
E. Assertion and Reason Questions
1. Assertion (A): Apoenzyme alone is inactive.
Reason (R): Apoenzyme requires a co-factor for catalytic activity.
A. Both A and R are true and R explains A
B. Both A and R are true but R does not explain A
C. A is true but R is false
D. Both are false
Answer: A
2. Assertion (A): Prosthetic groups are tightly bound to enzymes.
Reason (R): They remain attached during enzyme action.
Answer: A. Both A and R are true and R explains A
3. Assertion (A): NAD is a metal ion co-factor.
Reason (R): NAD contains niacin.
Answer: D. Assertion is false but Reason is true
4. Assertion (A): Removal of co-factor decreases enzyme activity.
Reason (R): Co-factors are necessary for catalytic function.
Answer: A
F. Fill in the Blanks
-
The inactive protein part of enzyme is called ______.
Answer: apoenzyme -
Apoenzyme combined with co-factor forms ______.
Answer: holoenzyme -
Haem is a ______ group.
Answer: prosthetic -
NAD and NADP contain vitamin ______.
Answer: B3 (niacin) -
Zinc acts as a co-factor for ______.
Answer: carboxypeptidase -
Co-factors are ______ components of enzymes.
Answer: non-protein
G. Statement-Based Questions
1. Statement I: Co-factors are non-protein components.
Statement II: All enzymes require co-factors for activity.
A. Both statements are correct
B. Both statements are incorrect
C. Statement I is correct but Statement II is incorrect
D. Statement I is incorrect but Statement II is correct
Answer: C
2. Statement I: Co-enzymes are organic compounds.
Statement II: They are always permanently attached to enzymes.
Answer: C. Statement I is correct but Statement II is incorrect
3. Statement I: Metal ions can act as co-factors.
Statement II: Zinc is a co-factor for carboxypeptidase.
Answer: A. Both statements are correct
H. Match the Following
| Column A | Column B |
|---|---|
| 1. Apoenzyme | A. Organic co-factor |
| 2. Holoenzyme | B. Zinc |
| 3. NAD | C. Protein part |
| 4. Metal ion | D. Active enzyme |
| 5. Haem | E. Prosthetic group |
Answers:
1 → C
2 → D
3 → A
4 → B
5 → E
I. Case Study-Based Questions
Case Study:
A researcher studies an enzyme that is inactive alone. When a non-protein component is added, the enzyme becomes active. The added component remains permanently attached to the enzyme and contains haem.
Questions:
1. What is the protein part of this enzyme called?
Answer:
Apoenzyme.
2. What is the complete active enzyme called?
Answer:
Holoenzyme.
3. Identify the type of co-factor present.
Answer:
Prosthetic group.
4. Name the component present in the co-factor.
Answer:
Haem.
Case Study 2:
An enzyme requires Zn²⁺ ions for activity. The zinc ion binds with the active site and helps in substrate conversion.
Questions:
1. What type of co-factor is Zn²⁺?
Answer:
Metal ion co-factor.
2. Give an example of an enzyme requiring zinc.
Answer:
Carboxypeptidase.
3. Why does enzyme activity decrease after removal of Zn²⁺?
Answer:
Because Zn²⁺ is essential for catalytic activity.
J. Important CBSE Exam Questions
1. Define co-factor.
Answer:
A co-factor is a non-protein component required by some enzymes for their catalytic activity.
2. Write the equation showing enzyme activation.
Answer:
Apoenzyme + Co-factor → Holoenzyme (Active enzyme)
3. Why are vitamins important in enzyme activity?
Answer:
Many vitamins act as essential components of co-enzymes and help enzymes perform their functions.
4. Explain why co-factors are essential for enzymes.
Answer:
Co-factors help enzymes become catalytically active. Removal of co-factors results in loss of enzyme activity.
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