Questions - Answers

Very Short Answer Type Questions

1. How are prosthetic groups are different from co-factors?

A: Prosthetic groups are organic factors which are tightly bound to the Apoenzyme.

Co-factors are metallic or organic and tightly or loosely bound to Apoenzyme.

2. What is meant by feed back inhibition?

A: The end products formed in enzyme catalysed matabolic biochemical reactions must be utilised by the organism immediately. If they are not used and accumulated they act back and inhibit the action of enzyme participating in the first step of the chain of reactions. It is called feed-back metabolism. It is a kind of homeostatic control of metabolism.

3. Why are ‘Oxido - reductases’ so named?

A: These enzymes catalyse oxidation and reduction of substances, so they are named as Oxido reductases. It involves mainly hydrogen transfer.

4. Distinguish between Apoenzyme and co-factor?

A: Apoenzyme is the protein part of conjugate or Holoenzyme where as the co-factor is the non-protein part. 

5. What are competitive enzyme inhibitors? Mention one example.

A: The chemical substances which are structurally similar to substrate molecules and compete for the active site on the enzyme and inhibit the enzyme action are called Competitive inhibitors.

e.g.: Malonic acid (Malanate) resembles Succinic acid (Succinate). During Krebs Cycle, Malonate competes with Succinate and occupies active site on Succinic dehydrogenase. As a result Fumaric acid is not formed & Krebs Cycle stops abruptly.

6. What are non-competitive Enzyme inhibitors? Give one example.

A: The inhibitors have no structural similarity to the substrate. They sit on a place on the enzyme other than the active site and change the Globular structure of the enzyme. The enzyme becomes inactive.

e.g.: Salts of heavy metals like Copper, Silver, Mercury and Cyanide.

7. What do the four digits of an enzyme code indicate?

A: An enzyme code has 4 digits

     I . Digit indicates major class (It is never more than 6).

     II. Digit indicates sub - class.

     III. Digit indicates sub sub - class.

     IV. Digit indicates the serial number of the enzyme.

8. Who proposed ‘Lock and Key’ hypothesis and induced fit hypothesis?

A: Emil Fischer proposed ‘Lock and Key’ hypothesis (1898) and it was elaborated by Paul Fields and D.D. Woods (1934). According to this theory Lock = Substrate and Key = Enzyme. It emphasises the ‘Enzyme Specificity’. Induced fit hypothesis was proposed by Koshland (1973). According to this, the binding of the substrate induces the enzyme to alter its shape for fitting more tightly around the substrate.

9. What is Michaelis Constant?

A: Substrate concentration required to cause half the maximal reaction rate is termed as Michaelis - Menten constant (Km). It represents approximate inverse

measures of the affinity of the enzyme for a given substance 

Short Answer Type Questions

1. Write briefly about Enzyme inhibitors.

A: Any chemical substance other than substrate, attaches to the enzyme and shuts off enzyme activity is called inhibitor and the phenomenon is called inhibition. The inhibitors are of following types

1) Competitive Inhibitors: The inhibitor resembles the substrate molecule in structure and shape. The inhibitor competes with substrate and occupies the active site on the enzyme. It inhibits the action of the enzyme. Product is not formed.

e.g.: Malonate resembles Succinate. So it inhibits the function of Succinic dehydrogenase. Fumaric acid is not formed.

2) Non - competitive Inhibitors: The inhibitor do not resembles the substrate. It occupies a site on enzyme other than active site. Thus it induces a change in the structure of enzyme due to which active site of the enzyme changes. Enzyme becomes inactive.

e.g.: Salts of heavy metals like Silver, Copper, Mercury, Cyanide.

3) Feed - back inhibition:  The end products incase not utilised immediately, will be accumulated. They inhibit the action of the enzyme participating in first step. It is called feed back inhibition.

‘E’ acts back on E1 & stops further formation end product of E (End products).

2. Explain different types of co-factors.

A: The non - protein part of the Holoenzyme or conjugate enzyme is called co-factors. These are 2 types

        1) Metallic co-factors

        2) Organic co-factors

1) Metallic Co-factors: These are tightly bound to Apoenzyme. Such enzymes are called metallic enzymes. The metallic cofactor activates enzyme. If metallic cofactor is separated the enzyme becomes inactive.

e.g.:  Cu²⁺   Cytochrome - C - Oxidase, Ascorbic acid Oxidase.

         Fe²⁺   Catalase, Peroxidase, Aconitase

         Mg²⁺  Hexokinase, Phosphotases

         Mn²⁺  I A A Oxidase

         Mo²⁺  Nitrate reductase, Nitrogenase

         Zn²⁺   Carbonic anhydrase, Alcohol dehydrogenase

2) Organic co-factors: These are 2 types

a) Co-enzymes b) Prosthetic groups

a) Co enzymes: Organic cofactor which is loosely bound to Apoenzyme is called Co-enzyme.

e.g.: FAD, NAD, TPP, Co-A etc.

b) Prosthetic groups: Smaller, non-protein part, organic in nature and tightly attached to Apoenzyme by covalent bonds is called as the prosthetic group.

e.g.:  FAD (Co enzyme) - Succinic dehydrogenase

         FAD (Prosthetic) - Complex II of ETS

         FMN - Complex I of ETS

         Haem - Peroxidase

EXERCISES

1. Explain how pH affects enzyme activity with the help of a graphical representation?

A: The Hydrogen ion concentration controls the enzymatic activity. Due to their protein nature, they are confined to narrow ranges of pH. Any shift towards alkaline or acidic side results in a decrease in enzyme activity. Every enzyme has its own optimum pH. Most of the enzymes work at their best in neutral pH. The effect of pH on enzyme activity can be shown as follows in the form of a graph.

2. Explain the importance of (E - S) complex formation.

A: Every enzyme has more than one active site. The number of active sites depends up on the capacity of enzyme. Though enzyme is made of several amino acids only few amino acids directly come in contact with substrate molecules at reaction site.

A specific substrate molecule sits in the active site of enzyme. E - S Complex is formed. It is active and short lived. The activation energy of the formation of E - S complex is low. Many molecules of the substrate participate. Later products are formed. Enzyme is set free.