Enzymes

 Enzymes are biologically active and important groups of the protein. These are used to speed up the chemical reaction. Without enzymes, life processes proceed very slowly, making life impossible. These remain unchanged during the reaction and separated out after the completion of the reaction. These are made up of hundreds of amino acids. The enzyme's catalytic activity is restricted to only a small charge-bearing site called the active site. The substrate attached to the enzyme by this active site is made up of only 3-12 amino acids while the bulk of the amino acids is used to maintain the globular structure of the enzyme.

 The enzyme that requires a cofactor is active only when the cofactor is attached to it and it is called a holoenzyme. And if a cofactor is not available, then the remaining protein-containing portion is called apoenzyme. e.g Pepsinogen is secreted by the stomach wall with a polypeptide chain on its active site, so it is called an apoenzyme. And when it is exposed to HCl, then pepsinogen is converted into pepsin. which is called a holoenzyme.   Some enzymes are not made up of proteins but consist of RNA and are called ribozymes and found in ribosomes. e.g: Peptidyl transferase( which is used to make peptide bonds during protein synthesis.)

The cofactors are either organic or inorganic:

•    If the cofactor is detachable and inorganic, it is called an activator, for example, Mg ion with Hexokinase.
•     If the cofactor is detachable and organic in nature t, it coenzyme. NAD, FAD et.
•    Both activator and coenzyme are attached to the enzyme only when the substrate is already attached.
•      If the cofactor is undetachable and organic in nature then it is call, itoup. It is covalently bonded to the enzyme. Porphyrin ring in chlorophyll.

About 1000 reactions are carried out at any time in the cell. For these reactions, the energy cannot be provided by living systems as the living system works in isothermal conditions. So enzymes are used for lowering the energy of activation.

Characteristics of the enzymes:

•   These can increase the speed of the chemical reaction.
• These are required in very small amounts.
•  These are very sensitive even to minor changes in substrate concentration, PH, and temperature.
•  These can work in the cell as well as in vitro, and invivo.
•  Some enzymes require a cofactor for proper functioning.
• Lower the need for the energy of the activation.
• These do not affect the product of the chemical reaction.
• All enzymes work in aqueous media.

Mode of action of enzyme:

An enzyme and substrate are attached to each other through a definite charge-bearing site called an active site.

As the result, ES complex is produced which after some time is converted into products, and enzymes are separated and thus used again and again. An active site consists of two regions:

• Binding site:  This site helps the enzyme in substrate recognition and its attachment to the active site and to produce ES complex.
• While the catalytic site helps the transformation of the substrate into the products.
• Most enzymes do not float into the aqueous cytoplasm but attach to the specific organelles.

Emil Fischer proposed a lock and key model. According to this model,  the active site is the rigid structure. Its structure is not modified before, during, or after the chemical reaction. for example Urease, Sucrase, and Lactase. It is called absolute specificity. and Little types of enzymes follow this mechanism and are called non--regulatory proteins.

Koshland proposed an Induced fit model. According to this model, the active site is not a rigid structure. When the substrate is attached to the active site, it will induce changes to the active site, and these changes help the enzyme perform its work more efficiently. When the reaction is completed, the actiThe active site will regain its original shape when the reaction is completedxokinase follow this mechanism and are called regulatory or allosteric enzymes

Factors affecting the Rate of Enzymatic Reaction:                                                                                                        Any factor which can alter the chemistry and shape of the enzyme will affect its rate of reaction.

•      Enzyme concentration: The rate of reaction is directly proportional to the enzymes available at unlimited substrate concentration. After a specific time, further increase in enzyme concentration will not affect its activity and the rate reaction becomes saturated at this point. 
•     Substrate concentration: At low substrate concentration, the rate of reaction is directly proportional to the substrate concentration. At constant substrate concentration, the reaction is reached at a specific point which is called the saturated point.

                   

• Temperature: The rate of reaction is directly proportional to the temperature. As heat provides activation energy which in turn provides KE to the molecules of enzymes that start accelerated. With further increase in temperature, the K.E of molecules further increases and These start vibrating and the globular structure of the enzyme is lost and the rate becomes denatured. All enzymes work maximum at the particular temperature called optimum temperature, while the temperature at which the enzymes become denatured is called maximum temperature, and the temperature below which enzymes become inactive is called minimum temperature. for example the optimum temperature for human enzymes is 37C. The rate of reaction becomes double for every 10C rise in temperature.
•  Ph: Enzymes are even sensitive to minor changes in the PH. Every enzyme function to over a minor range of PH. It also cause the ionization of substrate. Under such conditions, enzyme activity is either  blocked permanently. Extreme changes cause ionic bonds break in enzymes. some enzymes  work in to acidic medium and some work in an basic medium. For example:

1. Pepsin is the enzyme that works at 2PH while tyrpsin enzyme at 8 pH.
2. Papin enzyme obtained from green papaya plants works at both acidic and basic mediums.

         

                                                                                                                                                     Inhibitors:

                       A chemical substance that can react with an enzyme in place of substrate but is not transformed into the products is called an inhibitor. for example Cyanides, pencilin, antimetabolites, antibiotics, sulpha drugs, and heavy metals. Generally, enzyme inhibition is the normal process but when external factors cause inhibition, it will be dangerous for life. There are two types of inhibitors:

  Irreversible inhibitors:   These check the rate of reaction by occupying the active site or destroying the globular structure. These form covalent bonds with the active site and these may block the active site. 

Reversible inhibitors: These form weak linkages with the active site of the enzymes. Their effect can be neutralized by increasing the concentration of the substrates. There are two types:

Competitive Inhibitors:   Because of the structural similarity,, these are selected for the binding site but are not able to activate the catalytic site. for example, Succinate is converted to fumarate by an enzyme but sometimes, malonate that is structurally similar to succinate attaches to an enzyme and causes inhibition.

Non-competitive inhibitors: These form enzyme inhibitor complexes at a site other than the active site. These alter the enzyme structure in such a way that if a genuine enzyme attaches to the active site, catalysis fails to take place.