What are Enzymes?

A substance that acts as a catalyst to regulate the reaction rate in the living organism's metabolic pathways without itself getting altered is an enzyme. Most of the biological reactions and metabolic pathways in the living systems are carried out by enzymes. They are specific for their works and work in particular conditions. It maintains the best possible rate of reaction in the most stable state. The enzymes have distinct properties as they can proceed with the reaction in any direction, their particular binding sites, pH specificity, temperature specificity required in very few amounts.

What is the Regulation of Enzymes?

The enzymes have various functions to perform in the body, so it is important to control their conditions and work with regulatory enzymes. The enzyme activity needs to be regulated by certain other molecules called regulatory enzymes to control the chemical reactions in the body. Those molecules which control the enzyme activity are called regulatory enzymes. These regulatory enzymes generate responses to the presence of other certain biomolecules.

Regulatory Molecules

The activity of the enzyme can be increased or decreased by activators and inhibitors, respectively. They bind specifically to the enzyme. These molecules affect the enzyme function by different pathways. There are two types of inhibiting mechanisms - competitive and non-competitive. When an inhibitor binds to an enzyme and avoids substrate attachment by attaching to the active site is called competitive inhibition. In this way, the inhibitor competes with the substrate for the enzyme, which means that only one can bound out of the inhibitor and substrate. When the inhibitor doesn't bind to the substrate's same binding site, it attaches somewhere else but blocks the enzyme's job; it is called non-competitive inhibition. Here both the inhibitor and substrate are bound to the enzyme at the same time. In the presence of competitive inhibition, the reaction rate will decrease maximally when there is less or no substrate and can also be out-competed when there are a lot of substrates. In the non-competitive inhibition condition, the reaction rate will never be maximum as the bounding of the inhibitor will always poison the reaction even if there is a lot amount of substrate.

Allosteric Regulation

It is a type of regulation when an activator or inhibitor binds to an enzyme at a place other than the active site. There can be allosteric inhibition or allosteric activation depending on the bounding substance. The certain enzymes which are controlled and regulated by the allosteric regulation are called allosteric enzymes. Allosteric enzymes have many active sites present in different locations on protein subunits. The binding of the allosteric inhibitor on the allosteric enzyme at the allosteric site will bring conformational change in the amino acids slightly so that it can work less efficiently. at the same time; allosteric activators will increase the function of the amino acid. When the substrate is also the allosteric activator, it is called cooperativity. This is called regulation in that attachment of allosteric activator will cause a conformational change of other active sites on the protein moiety.

Covalent modifications

The modifications made in the structure of enzymes by covalent interactions are another regulatory method. This includes adenylation, methylation, phosphorylation, ribosylation of the enzymes. Separate enzymes will be needed for doing these covalent modifications. These are reversible reactions.

Proteolytic Cleavage

Enzymes by this method are produced as inactive forms; the inactive form is called zymogen. The binding sites will be masked; hence there will be no enzyme activity unless there is the removal of certain enzyme parts on the masked part by proteolytic cleavage. The certain enzymes regulated by this regulatory method are chymotrypsinogen to chymotrypsin, pepsinogen to pepsin, procollagen to collagen, etc.

Isoenzyme

This is an indirect method of enzyme regulation. Isoenzymes have a different amino acid sequence but do the identical catalytic function. However, they have different kinetic, Km, Vmax values. They make the cell catalyze the same reaction in different conditions due to different kinetics. 

Cofactors

Some enzymes only become activated when they are attached to some non-protein molecules. Those non-protein molecules are the cofactors. They are attached to the enzyme temporarily by hydrogen and ionic bonds. Certain cofactors are magnesium and iron. For instance, the DNA polymerase needs magnesium to make a DNA molecule. The other cofactors are coenzymes; they are the organic cofactor molecules—a few vitamins act as coenzymes. For instance, vitamin C is used in making collagen.

Compartmentalization

To avoid any damage to the enzyme, they can be stored in certain compartments. This also helps to provide the best conditions for the enzyme. In easy language, compartmentalization keeps enzymes in places where they can act more specifically, making sure that the substrate is readily available and best environmental conditions. For instance, the lysosome's digestive enzyme works best at a pH of 5.0 in the interior of the lysosome, but in the cytosol, it works at pH 7.2. thus, the lysosomal enzymes work at a low activity at higher pH in the cytosol. This will help avoid the cell being digested by the lysosomal enzymes even if the lysosome bursts, as the pH will not be appropriate for enzymatic actions.

Feedback Inhibition

The feedback inhibition mechanism is a product controlling mechanism that means the enzyme's activity is inhibited by the end product it makes. It way ensures that an appropriate amount of product is only produced. At the time when the product is little, the enzyme will not be inhibited. But as soon as the production of the product crosses the threshold value, the enzyme will be inhibited by the product to stop producing it. Thus, it acts as the first commanding step in a pathway. It can also occur at multiple steps in a reaction, but it usually occurs at the first rate-limiting step. For instance, the ATP energy carrier molecule is an allosteric inhibitor of many enzymes, including cellular respiration; it happens in the manner that when there are lots of ATPs, the production of ATP is stopped. More ATP means more breakdown of ATP to ADP.

Common Mistakes

• All enzymes are proteins, but all proteins are not enzymes.
• Every biological enzyme has its regulatory pathway.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in chemistry
• Master of Science in biochemistry
• Biotechnology

Related concepts

1. Configuration of enzymes
2. Activation energy