Apoenzymes



 Co_ factors:

Enzymes are composed of one or several polypeptide chains. 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.Cofactors may be simple metal ions, e.g., Mg++ or K++, or complex organic compounds.In these instances, the protein portion of the enzymes is called the apoenzymes. 


Apoenzymes:

1) It is an enzyme which acts only in the presence of a cofactor.

2): It is a protein, hence a macromolecule.

3): It is specific for an enzyme system which it makes work.

4): Aloenzymes conduct the enzymatic activity.

5): Aproenzyme is thermolabile

Three kinds of cofactors may be identified:
1): Prosthetic group, 

2): Co_ enzymes 

3):Metal ions.




Prosthetic groups are organic compounds and are distinguished from other cofactors in that they are tightly bound to the apoenzymes. The  nonprotein organic cofactor that is tightly bound to  and non_ dissociable from the enzyme is termed a prosthetic group,

For example,  biotic of carboxylases, in peroxide to water and oxygen, haem of haemoglobin is the prosthetic group and it is a part of the active site of the enzyme.


Co_ enzymes  are also organic compounds but their associated with the apoenzyme is only transient ,usually occurring during the course of catalysis. Furthermore, co_ enzymes serve as co_ factors in a number of different enzyme catalyzed reactions. The essential chemical components of many coenzymes are vitamins,An organic nonprotein cofactor which is  easily separable from the apoenzyme is called coenzymes, 

Coenzymes:

1): It may be some metal ion or a complex organic compound that makes an apoenzyme functional.

2): It is some ion or a non_ protein compound, often having some vitamin, hence a micromolecule.

3): It can work with many enzymes  which catalyse particular kind of reactions.

4): Coefactors mould the enzyme or coenzyme, or carry the groups removed from the substrates.

5): Cofactor is thermostable.

 Examples:
 coenzyme nicotinamide adenine dinucleotide ( NAD) and NADP contain the vitamin niacin.



  
  A number of enzymes require metal ions for their activity which form coordinates bonds with side chains at the active site and at the same time form one or more coordination bonds with the substrate, 

Examples:
zinc is a cofactor for the proteolytic enzyme carboxpeptidase.

 Catalytic activity is lost when the co _ factor is removed  from the enzyme which testifies that they play a crucial role in  the catalytic activity of the enzyme.

The working combination of an apoenzyme and cofactor ( mineral ion, prosthetic group or coenzymes) is called enzymes system, or holoenzyme.

Apoenzyme+ Mineral Ion/ Prosthetic group / Coenzyme➡️Enzyme  System or Holoenzyme 

The organic cofactors often contain some vitamins, such as thiamine, niacin, riboflavin, etc., as a part of their molecule. For example, the coenzymes NAD and FAD contain nicotinamide and flavin in them. The cofactors are needed in very small amounts because, like the enzymes, they can be used over and over again. The micronutrient or trace elements essential for the health of the organism mostly act as cofactors for enzymes. Thus, the vitamins and trace elements have the same role. They aid in biochemical functioning of the cells. The mineral ions perhaps mould the apoenzyme or  its substrate to form  a proper enzyme _ substrate complex.



    
  The organic cofactors directly reduce the activation energy, that is, they form a functional part of the active site of the enzyme. They act as carriers of chemical groups, atoms or electrons removed from the substrate during reactions.

Thus, there are three recognized types of cofactors : inorganic ions, prosthetic  groups and coenzymes. 

Structure of co_ factors of enzymes:



 Co_ factors are small organic or inorganic molecules that bind to enzymes and are required for their  optimal activity. They can be loosely or tightly bound to the enzyme and are involved in the catalytic process.The 
structure of co_ factors  can vary widely depending on the specific enzyme and its function.Here are some examples  of common co_ factors and their structures. 


   
  1): NAD+ ( nicotinamide adenine dinucleotide):  NAD + is  a co_ factor involved in redox reactions, acting as an electron acceptor. It consists of two nucleotides, adenine  and nicotinamide, connected by a ribose molecule.

2): FAD ( flavin adenine dinucleotide): FAD is another co_ factor involved in redox reactions. It contains a flavin ring structure derived from riboflavin ( vitamins B2) and is  bound to adenine and ribose molecules.


3): Coenzymes A ( CoA) : CoA is a co_ factor involved in acyl group transfer reactions, such as fatty acid synthesis and oxidative metabolism .It consists of a pantothenic acid moiety  linked to a 4' - phosphopantetheine group.

4): ATP:( adenosine triphosphate): ATP is a co_ factor involved in energy transfer and acts as a molecular currency for cellular processes.It consists of an adenine molecule,  ribose sugar and three phosphate groups.



 In addition to  these organic co_ factor, enzymes may also require metal ions as co_ factors.These include ions like magnesium, calcium, zinc, copper ,iron, and manganese, which can act as factors by participating in catalysis or by stabilizing enzyme _ substrate complexes. 


It is important to note that the structure of co_ factors can change during catalysis , either by transferring functional groups or by accepting or donating electrons. This allows them to participate in the  enzymes catalytic reaction and be regenerated for further catalysis 




Function of co _ factors of enzymes:

Co _ factors are non_ protein molecules requires for the proper functioning of enzymes. They play a crucial role in catalytic activity by assisting enzymes in catalyzing biochemical reactions. 


The main functions of co_ factors in enzymes are:

1): Activiting the enzyme: Some co_ factors bind to the enzyme and help in the proper folding and activation of the enzyme, ensuring that it is in the correct conformation for catalysis.


2): Providing chemical groups for catalysis: Many co_ factors act as carriers of specific chemical groups such as electron or functional groups like methyle or acetyle groups. These groups are transferred to the substrate during the  enzymatic reaction, allowing the reaction to occur.


3): Participating in redox reactions: Co_ factors like NAD + ( nicotinamide adenine dinucleotide) and FAD  ( flavin adenine dinucleotide) are involved in redox reactions. They accept or denote electrons during the reaction, facilitating the transfer of electrons between reactions. 


4): Acting as electron carriers: Some co_ factors, such as coenzyme Q or cytochromes play a crucial role in electron transport chains. They shuttle electrons between different enzymes or protein complexes, allowing the transfer of energy and the generation of ATP.


5): Regultating enzyme Activity: Co_ factors can also modulate the activity of enzymes by binding to specific sites on the enzyme, changing its conformation and there by affecting its enzymatic activity. 


6): Binding and stabilizing substrate or transition states: Co_ factors may bind to the substrate or the transition state of the reaction, stabilizing the molecules in a favourable conformation for catalysis.


Overall, Co_ factor play an essential role in enzymes catalysis by providing critical chemical groups ; participating in redox reactions, regulating enzyme activity and stabilizing substrates or transition states. They are often required for the proper functioning of many enzymes, and their absence can lead to the impairment or complete loss of catalytic activity.

 











































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