The enzymes are biomolecules, in other words molecules synthesised by living beings. Digestive enzymes are synthesised by the liver and pancreas but they are also provided by food. The other enzymes are produced by each of the body’s cells according to their needs.
The general function of an enzyme is to catalyse a chemical reaction, meaning to accelerate it. Indeed, the enzyme and the substrate bind specifically to each other. The enzyme remains unmodified at the end of the reaction process, contrary to the substrate which is generally modified to result in one or several new molecules called products.
Inside the cells, the enzymes are thus responsible both for the synthesis of new substances used to build the cell (anabolism) and for the degradation of substances used to produce energy (catabolism). Their role is vital, because the physicochemical conditions (temperature, pH) that prevail in the body prevent most reactions from occurring at a sufficient rate.
There are thousands of enzymes, many of which are not yet known. They can be classified into six categories according to the biochemical reaction they carry out:
Which catalyses the transfer of electrons from one molecule (the oxidant) to another molecule (the reductant). For example at the surface of the honey, glucose oxidase reduces atmospheric O2 to hydrogen peroxide (H2O2) which acts as an antimicrobial barrier.
Which catalyses the transfer of a functional group from one molecule (the donor) to another (the acceptor). For example DNA methyltransferase catalyses the transfer of one or several methyl groups to a DNA molecule.
Use water to break chemical bonds in order to divide a large molecule into two smaller ones. Specific hydrolases catalyse reactions that break either (C―O) bonds; carbon–nitrogen (C―N) bonds other than peptide bonds; acid anhydride bonds; carbon–carbon (C―C) bonds; or phosphorus–nitrogen (P―N) bonds.
Which break bonds but produce new ones simultaneously. For example, the cleavage of fructose 1,6-bisphosphate (F 1,6-BP) forms two compounds called glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP).
Which rearrange the functional groups of a molecule to form isomers, the end product has the same molecular formula but an alternate physical structure. An example of this kind of mechanism, i.e. ring-opening and contraction of the ring is the isomerisation of glucose to fructose.
The overall reaction causes the ring to form an aldose through acid/base catalysis and then subsequently forms cis-ethanol intermediate. The ring is closed after the formation of a ketose.
- Ligases or synthetases
Which allows the joining of two molecules. About 50 ligase enzymes are known and are often referred to as “molecular glue”.
DNA ligase is an example. It catalyses the DNA fragments’ binding by forming a phosphodiester bond between complementary ends of the DNA fragments. Thus, DNA ligase plays a critical role in repairing, replicating, and recombination of DNA.
Only the substrates and the products vary from one enzyme to another.
- Substrate: the molecule specifically undergoing enzymatic catalysis by fixing in the active site, which is a part of the enzyme in form of cavity where the interaction with the substrate occurs to form the product. For instance amylase, as a metabolic enzyme, catalyses the hydrolysis of starch into glucose. In this reaction, starch is the substrate and glucose is the product.
- Reagent: is a molecule that reacts with the substrate and enters into the balance of the final chemical equation.
- Reaction product, or products: the molecule(s) released by the enzyme after catalysis.
The most noticeable property of enzymes is their double specificity : on the one hand, an enzyme is dedicated to a specific substrate. On the other hand, an enzyme is able to catalyse only one chemical reaction for a specific substrate.
Enzymes have the same structure as proteins : a very long chain of amino acids with its own specific composition. A non-protein part, called apoenzyme can also be found in some of them. As previously mentioned, the active site is the primordial part of an enzyme, the place where reactions occur.
Each protein enzyme has a structure adapted to its function, and can be in monomeric form (composed of a simple molecule that can combine with identical or similar ones), while some on the contrary are active in dimeric form (composed of two identical monomers) or even more.
They can then associate with several protein chains coded by the same gene (homodimer) or coded by different genes (heterodimer). Note that a gene is made up of DNA, representing the basic physical and functional unit of heredity.
In cells, there are many catalytic steps in the metabolic chains between a starting substrate and the final products. If a person is not able to express a specific enzyme because of a mutation for example, a stage of the transformation is deficient. It means that the succession of catalyses is blocked, and thus the final products cannot be formed.
Reagents or reactants?
Reagents trigger chemical reactions
In a chemical reaction, a reagent is considered as a catalyst and is not consumed during this reaction, contrary to a reactant which is considered as a substrate and thus consumed at the end of the reaction.
Reagents may be compounds or mixtures. In organic chemistry, most are small organic molecules or inorganic compounds.
Examples of reagents include Grignard reagent, Tollens’ reagent, Fehling’s reagent, Collins reagent, and Fenton’s reagent.
Reagents can also be limiting
Limiting reagents stop a chemical reaction when they are used up. The chemical reaction relies on the reagent to continue processing, and stops when there is no more substance.
The limiting reagents, therefore, dictate when a certain chemical reaction does not continue. Reagents are commonly used in laboratory settings for various tests.
For example, Collins reagent is used to convert alcohols to aldehydes and ketones. As such, it can be useful to oxidise acid-sensitive compounds. Fenton’s reagent, similarly, is used in oxidation. However, Fenton’s reagent catalyses the oxidation of contaminants in water and can be used to eliminate toxic compounds, such as tetrachloroethylene.
Reagents are often used to indicate the presence of compounds by triggering changes in colours. The Fehling’s reagent is a typical example : the appearance of a reddish-brown precipitate indicates the presence of reducing sugars. The absence of the reddish precipitate or the appearance of deep blue colour indicates a lack of reducing sugars.
Millon’s reagent can be used to indicate the presence of proteins. The presence of proteins, as inferred by the presence of tyrosine residues, causes the solution to which Millon’s reagent has been added to turn reddish-brown.