ENZYMES

              ENZYMES

Introduction:-
Enzymes may defined as 'biocatalysts’ synthesized by living cells. They are proteins in nature(except ribozymes).They are colloidal,thermolabile and specific in action.
Nomenclature of enzymes:-
Earlier, the enzymes were given names by discovers. Eg:-pepsin, trypsin
Sometimes suffix ‘ase’ was added to substrate for naming enzymes.
These are known as ‘trivial’ names.
International Union of Biochemistry(IUB) appointed an Enzymes Commission in 1961 which studied the existing enzymes and invented basic principles for nomenclature.

Four digit Enzymes Commission (E. C.) number is given to Enzymes which represents as follows:-

•    first digit:- class
•     second digit:- sub class 
•     third digit:-sub-sub class
•     fourth digit:-individual name of enzyme

Along with E. C. number, the trivial names are commonly used and widely accepted.

Classification of Enzymes:-

According to IUB system, Enzymes are classified into six major types:-

     1.Oxidoreductases:-

Enzymes involved in oxidation-reduction reaction.

Systemic name:- L. Lactate.NAD oxidoreductase 

   Trivial name:-Lactate dehydrogenase

   Enzyme commission no:-1.1.1.27

2. Transferase:-

        Enzymes that catalyses the transfer of functional group. 

Systemic name :-Glucose-6-phosphate.ATP.transferase

Trivial name:-Glucokinase

Enzyme Commission no:-2.7.1.2

3. Hydrolases:-

   Enzymes that bring about hydrolysis of various compounds. 

Systemic name:- Urea.Amide hydrolase

Trivial name:- Urease 

Enzyme Commission no:-3.5.1.5

4. Lyase(BREAK):-

     Enzymes involved in addition or removal of water,.                 ammonia, CO2  etc.

Systemic name:- 2-oxo-acid carboxylase

Trivial name:-Pyruvate decarboxylase

Enzyme Commission no:-4.1.1.1

5. Isomerase:-

    Enzymes involved in all isomerization reaction. 

Systemic name :- Glyceraldehyde-3-phosphate

                           keto isomerase 

Trivial name:- Triosphosphate isomerase

Enzyme Commission no:- 5.3.1.1

6. Ligase(join):-

     Enzymes involved in synthetic reaction where two     molecules are joined together and ATP is consumed. 

Systemic name:- L-glutamate ammonia ligase

Trivial name:-Glutamine Synthetase

Enzymes name:-6.3.1.2

• According to place of action, it is classified in two types :-

1.Intracellular Enzymes :-

    They are functional within the cells where they are synthesized.

2. Extracellular Enzymes :-

    These enzymes are active outside the cells from where they are synthesized.

Properties of Enzymes

• Enzymes are generally soluble in water and insoluble in organic solvents. 
• Enzymes usually melt at higher temperature.
• Enzymes might be sweet, tasteless or bitter also. 
• Enzymes contain both acidic and basic group and hence they can form salts with bases and acids. 

Mechanism of action of enzyme:-
The substrate(S) combines with the enzyme(E) at the active site to form enzyme-substrate complex(ES) which results in product formation(P).

To explain mechanism few theories are given:-
1. Lock & key model (Fischer’s template theory) :-
According to this model, the structure of the enzyme is rigid.
The substrate fits to the binding site(active site) of enzyme just as a key fits into a proper lock.
The active site of an enzyme is specific,rigid and preshaped where only a specific substrate can bind.

But, this model totally fails to explain many facts of enzymatic reaction as it does not shows flexible nature of enzyme.
Eg:- Allosteric modulators.

2. Induced fit theory(Koshland’s model):-
According to this model,the active site is not rigid and preshaped.
The interaction of the substrate with the enzyme induces a fit or confirmation change in the enzyme.
Due to which the appropriate amino acid of the enzyme are repositioned to form the active site and bring about catalysis. 

3. Substrate strain theory:-

Here, the substrate is strained due to the induced confirmation change in the enzyme. 

There are chances that when a substrate binds to the preformed active site, the enzyme induces a strain to the substrate. 

This strained substrate leads to formation of product

• Factors affecting the enzymes activity:-

1. Concentration of enzyme:-

The concentration of enzyme is directly proportional to the velocity of the enzyme reaction i.e as the concentration of the enzyme increases, the velocity of the enzyme reaction proportionally increases.

2. Concentration of substrate:- 

Increase in concentration of substrate gradually increases the velocity of enzyme reaction but within the limited range of substrate levels. 

It shows rectangular hyperbola curve.

3. Effect of temperature:-

Increase in temperature causes increase in velocity of enzyme reaction upto a maximum level and then declines. 

It shows bell shaped curve. The optimum temperature for maximum activity of enzyme is 40-45 0c.

4. Effect of pH:- 

As the hydrogen ion concentration (pH) increases, it considerably influences the enzyme activity. 

It shows bell shaped curve. Every enzymes has its it’s own optimum pH at which it’s velocity is maximum.  

5. Effect of product concentration:- 

Accumulation of reaction product usually decreases the enzyme velocity. 

6. Effect of activators:-

Some enzyme require specific inorganic metallic cations like Mg2+,Ca2+,Na+,K+ for their optimum activity. But anions are rarely needed(eg:-chloride ion needed for amylase).  

7. Effect of time:-

Under ideal conditions (such as optimum pH, temperature etc.) the time required for enzyme reaction is less. 

Variation in time of enzyme reaction usually occurs due to alteration in pH and temperature.

8. Effect of light and radiation:-

Some enzymes inactivated due to exposure to ultraviolet,beta rays,gamma rays,X-ray’s because of formation of peroxides. Eg:- ultraviolet rays inhibits salivary amylase activity. 

• Structure of Enzyme:-

It is made up of proteinous and non-proteinous part. 

The functional unit of the enzyme is known as HOLOENZYME. 

Co-enzymes can be separated from Apoenzyme by DIALYSIS. 

There are several coenzymes which covalently bond with apoenzymes and hence cannot be separated. They are called as PROSTHETIC GROUP. 

Enzymes which are made up of single polypeptide are called as monomeric enzymes while which are made up of more than one polypeptide is called as oligomeric enzymes. 

   Eg:- Monomeric enzyme:-Trypsin, Ribonuclease. 

         Oligomeric enzyme:- Lactate dehydrogenase. 

Several Enzymes contain specific sites to catalyse different reactions in a sequence which is called as multienzyme complexes. 

Enzyme Inhibition:-

Enzyme inhibitor can be defined as a substance which binds with the enzyme and decreases the catalytic activity of that enzyme.

The inhibitor can be organic or inorganic in nature. 

The process is called as enzyme inhibition.

There are three types of enzyme inhibition :-

1.Reverse Inhibition:-

Here,the inhibitor binds to the enzyme non-covalently.

Hence the enzyme inhibition can be reversed,if the inhibitor is removed. 

It is of two types:-

Competitive inhibition:-

    The inhibitor competes with the substrate and binds at the active site of the enzyme non-covalently. 

It doesn’t undergo any catalysis. 

As long as competitive inhibitor holds the active site of the enzyme,the enzyme is unable to bind with substrate. 

Competitive inhibition can be overcome by increase in substrate concentration.

Eg:- Malonic Acid, Oxalic Acid are structurally similar with succinic acid. Hence they compete with succinic acid for binding at the active site of succinate dehdrogenase(SDH).

Non-competitive inhibition:- 

    The inhibitor binds at another site than the active site of the enzyme surface. 

This binding unable the enzyme activity.

The inhibitor has no structural similarity with the substrate. 

Hence the inhibitor does not interfere with the enzyme-substrate binding site. 

The catalysis is prevented because of distortion in the enzyme structure. 

Eg:- Heavy metal ions(Hg2+,Ag+,etc.) can non-competitively inhibit the enzyme by binding with cysteinyl sulfhydryl group’s. 

2. Irreversible Inhibition:- 

     The inhibitor binds covalently with the active site of the  enzyme and makes them inactive,which is irreversible.

    These are mainly poisonous sustances.