Enzyme Inhibition

Enzyme Inhibition

Any substance that can diminish the velocity of an enzyme-catalyzed reaction is called an inhibitor and the process is known as inhibition.
There are two major types of enzyme inhibition, Irreversible and Reversible.

Irreversible Inhibition

The type of inhibition that can not be reversed by increasing substrate concentration or removing the remaining free inhibitor is called Irreversible inhibition
Eg. Diisopropyl & luorophosphate (DFP) Inhibits the enzyme acetyl cholinesterase, important in the transmission of nerve impulses. Acetyl cholinesterase catalyzes the hydrolysis of Acetylcholin (to acetic acid and choline) a neurotransmitter substance functioning in certain portions of the nervous system

⦁ DEP inhibits also trypsin, chymotrypsin elastase, and phosphglucomutase
Organo-phosphorus compounds like malathion, parathron pesticides-inhibits acetyl cholinesterase by the same way as DFP.
Example: Inhibition of triose phosphate dehydrogenate by iodo acetate which block the activity of the enzyme.

REVERSIBLE INHIBITION

This type of inhibition can be Competitive, Non-competitive and uncompetitive
Competitive Inhibition: This type of inhibition occurs when the inhibitor binds reversibly to the same site that the substrate would normally occupy, therefore, competes with the substrate for that site.

In competitive inhibition the inhibitor and substrate compete for the same active site on the enzyme as a result of similarity in structure. The enzyme substrate complex will be broken dawn to products (E+S«ES‹E+P) where as enzyme inhibitor complex; (EI) will not be broken down to products.

A classical example is Malonate that competes with succinate and inhibits the action of succinate dehydrogenase to produce fumarate in the Krebs cycle.

The enzyme can be also inhibited by oxalate and glutarate because of the similarity of this substance with succinate
Eg.2 Allopurinol used for the treatment of Gout
Allopurinol Inhibits Xanthine oxidase by competing with Uric acid precursors for the active site on the enzyme. This competition blocks the conversion of these precursors, and of hypoxanthine and xanthine, to uric acid and result in lower serum urate levels.

Inhibition of Enzyme Catalyzed Reactions

To avoid dealing with curvilinear plots of enzyme catalyzed reactions, biochemists Lineweaver and Burk introduced an analysis of enzyme kinetics based on the following rearrangement of the Michaelis-Menten equation:

[1/v] = [Km (1)/ Vmax[S] + (1)/Vmax]

Plots of 1/v versus 1/[S] yield straight lines having a slope of Km/Vmax and an intercept on the ordinate at 1/Vmax.

An alternative linear transformation of the Michaelis-Menten equation is the Eadie-Hofstee transformation: v/[S] = -v [1/Km] + [Vmax/Km]

and when v/[S] is plotted on the y-axis versus v on the x-axis, the result is a linear plot with a slope of -1/Km and the value Vmax/Km as the intercept on the y- axis and Vmax as the intercept on the x-axis.

Both the Lineweaver-Burk and Eadie-Hofstee transformation of the Michaelis-Menton equation are useful in the analysis of enzyme inhibition. Since most clinical drug therapy is based on inhibiting the activity of enzymes, analysis of enzyme reactions using the tools described above has been fundamental to the modern design of pharmaceuticals

Effect of Competitive inhibitors

1. Effect on Vmax: The effect of a competitive inhibitor is reversed by increasing [s]. at a sufficiently high substrate concentration, the reaction velocity reaches the Vmax. observed in the absence of inhibitor.
2. Effect on Km: A competitive inhibitor increases the apparent Km for a given substrate. This means that in the presence of a competitive inhibitor more substrate is needed to achieve ½ Vmax. 

Figure: Competitive inhibition

Non-Competitive Inhibition

In non-competitive inhibition the inhibitor binds at different site rather than the substrate-binding site. When the inhibitor binds at this site there will be a change in conformation of the enzyme molecules, which leads to the reversible inactivation of the catalytic site. Non-competitive inhibitors bind reversibly either to the free-enzyme or the ES complex to form the inactive complexes EI and ESI (Enzyme substrate Inhibition)

The most important non-competitive inhibitors are naturally occurring metabolic intermediates that can combine reversibly with specific sites on certain regulatory enzymes, that changes the activity of their catalytic sites.

An Example: is the inhibition of L-threonine dehydratase by L-isoleucine.

*Such type of Enzyme is called Allosteric Enzyme, which has a specific sites or allosteric site other than the substrate-binding site.

   Effect on Vmax.

Non-Competitive inhibition cannot be overcome by increasing the concentration of substrate. Thus, non-competitive inhibitors decrease the Vmas of the reaction.

   Effect on Km:

Non-competitive inhibitors do not interfere with the binding of substrate to enzyme. Thus, the enzyme shows the same Km in the presence or absence of the non- competitive inhibitor.

Figure: Noncompetitive inhibition 

Uncompetitive Inhibition

Uncompetitive Inhibitor binds only to ES complex at locations other than the catalytic site. Substrate binding modifies enzyme structure, making inhibitor-binding site available. Inhibition cannot be reversed by substrate. In this case apparent Vmax. and Km decreased. 

Figure: Uncompetitive inhibition