Competitive inhibition,Michaelis constant,Ribozymes and Post translational modification

In Competitive inhibition, inhibitor competes with the common substrate for the active site of a common substrate.For the binding with the enzyme, firstly the inhibitor fits into the active site of the enzyme. The inhibition is thus due to substrate analog and it reversible inhibition. The enzyme, now cannot act upon the substrate and reaction products are not formed. Hence, the action of an enzyme may be reduced or inhibited. Since a competitive inhibitor occupies the site only temporarily, the enzyme action is not permanently affected.

Usually, such inhibitors show a close structural similarity to the substrates of the enzyme they inhibit. In such a case, inspite of enzyme-substrate complex, enzyme-inhibitor complex is formed, and enzyme activity is inhibited.

Enzyme + Inhibitor → Enzyme Inhibitor Complex

In Non-competitive inhibition, there is no competition for active sites of the enzyme by the inhibitor, and it fixed itself with some other site of the enzyme. As a result, the physical structure of the enzyme is altered. This fails to form enzyme-substrate complex and reaction fails to occur.

In non-competitive inhibition, many enzymes can be poisoned. For example, cyanide poisoning results because of inhibition of cytochrome oxidase which is essential for nearly all mammalian.

Competitive inhibition example

The Malonate inhibits the activity of the succinate dehydrogenase enzyme.The succinic acid dehydrogenase which oxidizes succinic acid to fumaric acid. If the concentration of malonic acid, is added, the activity of succinic acid dehydrogenase decreases rapidly. Hence malonic acid acts as competitive inhibitor since it has the structural resemblance to succinic acid.By increasing the concentration of the substrate, the competitive inhibition can be reversed.
Competitive inhibitors are used in the control of bacterial pathogens. Sulpha drugs work as competitive inhibitors of folic acid synthesis in bacteria as they substitute for p-amino benzoic acid.
The drugs are structurally related to p-amino benzoic acid (PABA), a vital precursor in the microbial biosynthesis of folic acid and inhibit the synthesis of the vitamin.

Michaelis constant (Km)

Km of an enzyme is the substrate concentration at which the reaction attains half of its maximum velocity.


It is the concentration of the substrate at which velocity of the enzyme reaction is half the maximal possible.The Km is different for different enzyme and is often used for the characterization of different enzyme type.

If the Km value is small it indicates, the enzymes have the greater affinity for its substrate.


In nature most of the enzymes known are proteins, but some RNA molecules act as enzymes, are known as Ribozymes.

Ribozymes are RNA molecules that exhibit enzyme (catalytic) activities. Ribozymes provide resistance to several viruses which are being applied in biotechnology in producing cDNA that encodes for the ribozyme and to integrate it into the host plant genome for developing resistant plants.

Post-translational modification (PTM)

Post-translational modification (PTM) is one of the significant steps in protein biosynthesis. Proteins are created on ribosome translating mRNA into polypeptide chains. These polypeptide chains undergo Post-translational modification, such as folding, cutting, etc., before becoming the mature protein product. Proteins synthesized by the rough endoplasmic reticulum and lipids synthesized by smooth endoplasmic then they reach into cisternae of the Golgi apparatus. Here, proteins combine with carbohydrates and turn into glycoproteins and glycolipids.This process is called glycosylation.