In 1913, scientists Michael and Menten expressed the theory about the mechanism of action of enzymes called Michaelis-Menten theory. This doctrine is similar to the doctrine of lock and key. The doctrine is discussed.
An enzyme is a type of protein made up of numerous amino acids. The site of apoenzyme where the substrate binds and causes the reaction is called the active site. Each enzyme has one or more active sites or reaction centers. The substrate binds to the enzyme’s active site or reaction center to form an enzyme-substrate complex. Enzymes dissociate after product formation. The rate of enzyme reaction is related to the concentration of substrate. As the concentration of substrate increases, the rate of enzyme action increases. The Michaelis constant (Km) of the reaction is called the Michaelis constant (Km) of the reaction half the maximum speed due to increase in substrate concentration. The Michaelis constant is a special feature of enzymes. The rate of reaction is determined by the value of Km.

In 1898, the German scientist Emil Fisher published the lock-key theory about the mechanism of action of enzymes. According to this theory, a specific enzyme binds to a specific substrate in a lock-and-key manner to form an enzyme-substrate complex, so named. The lock-key doctrine is discussed.
Enzymes are special types of proteins made up of numerous amino acids. Each enzyme molecule has a specific shape. The site of their apoenzyme where the substrate binds and causes the reaction is called the active site. Each enzyme has one or more active sites or reaction centers. The size of the substrate and the size of the enzyme’s active site are always the same. According to the lock-key theory, a specific substrate molecule or molecules are attached like a key to the active site or reaction center of a specific enzyme. This is called induced fit. The substrate molecule then joins with the enzyme molecule by hydrogen bonding to form the enzyme-substrate complex. A single substrate binds to an active site of an enzyme. More than one substrate can never bind to an active site. Just as a lock does not open without a specific key, a specific enzyme does not act on any substrate other than a specific substrate. When an enzyme-substrate complex is formed, the enzyme easily cleaves the substrate or bonds between the molecules to form larger molecules. At the end of the reaction, the product is freed from the bond and moved away and the enzyme is released intact. The free enzyme takes part in the new reaction.
According to the lock-key theory, the shape of the substrate molecule participating in the reaction must be suitable for joining the active site or reaction center of the enzyme. Even the slightest change in the size of the enzyme and substrate results in a change in activity.
Modern studies have revealed that some enzymes act on multiple substrates. Pepin enzyme is reactive on 60-180 substrate molecules. For this reason, many scientists question the lock-key theory.

An enzyme which converts a substance into its epimer is called an epimerase enzyme. Epimer molecules differ only by the configuration of one carbon atom. Eg – Epimerase.

An enzyme which adds phosphate group to a substance or removes phosphate group from a substance is called phosphorylase enzyme. For example – phosphofructokinase, phosphorylase, pyruvic acid kinase etc.

An enzyme which combines CO2 with a substance or releases CO2 from a substance is called a carboxylase enzyme. Eg – carboxylase.

Enzymes which act on carbon-carbon, carbon-oxygen and carbon-nitrogen bonds of substances are called lyases. Such as isocitrate, lyase, citric synthetase, aldolase, fumarase, decarboxylase, dehydratase, hydrolyase etc.

Enzyme which joins two or more substrates to form a new compound by taking energy from ATP is called ligase. Such as glutamic synthetase, acetyl Co-A synthetase, aspartic synthetase, succinic thiokinase, pyruvic carboxylase etc.

An enzyme which converts a substance into its isomer is called an isomerase enzyme. Eg – phosphoglucoisomerase, phosphoglucomutase, racemase, mutase etc.

An enzyme that releases a group from a substance and attaches it to another substance is called a transferase enzyme. Such as hexokinase, kinase, trans amylase, trans acetylase, decarboxylase, methylase, protein kinase, polymerase etc.

An enzyme that breaks down a substance by binding water molecules to specific bonds is called a hydrolytic enzyme. For example, protease, sucrase, phosphatase, esterase, lipase, carbohydrase, nuclease, pepsin, invertase, urease, amylase, maltase, trypsin etc.