In the Enzymes topic, we have already discussed what these molecules are. There is a huge number of enzymes and in this topic, we will discuss the principles of enzyme classification and the main properties of individual groups.
Enzyme classification principle
There are several classification systems for enzymes, but the most popular is the classification according to the type of catalyzed reaction, which was formed in 1955. Not everyone agrees with this classification since it does not take into account the structural features of molecules, as well as their relationship and homology. In addition, enzymes with broad specificity, that is, those capable of performing different functions, hardly fit into such a classification. Therefore, in addition to the standard classification, there are others, but the classification based on the types of reactions and divided into 6 classes is most commonly used.
There are four main types of reactions — combination, decomposition, displacement, and combustion, but enzymes usually combine some of them or do some more specific "job" — for example, compose a specific molecule or perform proton transfer. So enzymes are divided into 6 classes, each class contains subclasses, which are subdivided into sub-sub classes, and then there are sub-sub-sub classes. As a result, the enzyme is assigned a code consisting of four numbers: for example, pepsin has the EC (Enzyme classification) number 3.4.23.1, and the insulin receptor 2.7.10.1. Also, the function of the enzyme can be seen in its name. However, for many enzymes, unofficial names are commonly used, but more established abbreviations: for example, alcohol dehydrogenase, instead of alcohol: NAD⁺-oxidoreductase.
EC 1: Oxidoreductases
The first class includes the class of oxidoreductases. The class of oxidoreductases includes enzymes that catalyze redox oxidation reactions. Representatives of this class are found everywhere, one of the most famous examples is alcohol dehydrogenase, which is involved in the breakdown of alcohol (say thanks to it after the holidays!).
Oxidoreductases include:
1. aerobic (reaction with oxygen, instead of anaerobic – without oxygen) dehydrogenases or oxidases that catalyze the transfer of protons (electrons) directly to oxygen;
2. anaerobic dehydrogenases, which accelerate the transfer of protons (electrons) to an additional intermediate molecule, but not to oxygen, and then to the main molecule;
3. cytochromes catalysis of the transfer of only electrons, not protons.
In total, 22 subclasses of enzymes belong to this class. This class also includes enzymes that "work" with reactive oxygen species, for example, peroxide — these are catalases and peroxidases.
EC 2: Transferases
The class of transferases includes enzymes that catalyze the reactions that transfer various atoms, groups of atoms, and radicals from one molecule to another. Their name is compiled in the form "donor: transported group – transferase". Enzymes of this class are involved in the synthesis of many important substances, for example, the amidophosphoribosyltransferase enzyme catalyzes the first reaction for the synthesis of nucleotides necessary for DNA and RNA.
There are 9 subclasses in this class. Some transferases catalyze the transfer of one-carbon residues, acyl, glycosyl, aldehyde or ketone, nucleotide residues, nitrogenous groups, phosphoric and sulfuric acid residues, etc. For example methyl transferases, acetyltransferases, aminotransferases, phosphotransferases, etc. Enzymes of the 7th subgroup of transferases that transfer a phosphoric acid residue using ATP phosphate group (adenosine triphosphate is a special molecule that provides energy for processes in living cells) as a donor are often also called kinases; while Group 6 enzymes, aminotransferases, are often referred to as transaminases.
EC 3: Hydrolases
The class of hydrolases includes a large group of enzymes that catalyze the cleavage of intramolecular bonds of organic substances with the participation of a water molecule. These are reactions in which a water molecule is attached to or split off from a substrate. Enzymes are named according to the form "substrate-hydrolase". Representatives of this class are often found among enzymes that digest food — for example, pepsin and trypsin.
There are 13 enzyme subclasses in this class. These include esterases — enzymes that catalyze the reactions of hydrolysis and synthesis of esters; glycosidases that accelerate the breaking of glycosidic bonds; phosphatases and peptide hydrolases catalyzing the hydrolysis of phosphorus anhydride and peptide bonds; amidases, which accelerate the breaking of amide bonds other than peptide bonds, etc.
EC 4: Lyases
The class of lyases includes enzymes that catalyze the breaking of C-O, C-C, C-N, and some other bonds. C-O, C-C, and C-N are the most common bonds in biological molecules (all organic life is built on C-C bonds), so lyases often take part in splitting a molecule into two or breaking bonds with some side chains — for example, carbon-nitrogen lyase releases ammonium. Also, lyases are responsible for reversible reactions of cleavage of various groups from substrates in a non-hydrolytic way. These reactions are accompanied by the formation of a double bond or the addition of groups to the double bond cleavage site. Enzymes are referred to as "substrate lyases". Enzymes of this class play an important role in energy metabolism and are involved in the synthesis of important biological compounds — for example, fructose-bisphosphate aldolase, which is involved in glycolysis.
Lyases are divided into 7 subclasses. For example, fumarate hydratase (systematic name "L-malate hydrolase") catalyzes the reversible elimination of a water molecule from malic acid to form fumaric acid. The same group includes decarboxylases (carboxy-lyases), amidine-lyases, etc.
EC 5: Isomerases
The class of isomerases includes enzymes that catalyze the interconversions of optical and geometric isomers. Their systematic name is made taking into account the type of reaction: "substrate – cis-trans-isomerase". If the isomerization involves intramolecular group transfer, the enzyme is called "mutase". These enzymes are involved, for example, in the regulation of DNA topology, relaxing its double helix, breaking, and then sewing it back together.
This class also includes racemases and epimerases that act on amino and hydroxy acids, carbohydrates, and their derivatives; intramolecular oxidoreductases catalyzing the interconversions of aldose and ketosis; intramolecular transferases that carry acyl, phosphoryl, and other groups, etc.
EC 6: Ligases
The class of ligases includes enzymes that catalyze the synthesis of organic substances from two initial molecules using the energy of the decay of ATP. Their systematic name is in the form "Substrate – 1: Substrate – 2 ligase. One of the most prominent representatives of this class is DNA ligase, which cross-links DNA chains during replication, repair, and recombination.
There are 6 subclasses in this class. An example is L-glutamate: ammonia ligase (recommended abbreviation "glutamine synthetase"), with the participation of which glutamine is synthesized from glutamic acid and ammonia in the presence of ATP.
EC 7: Translocases
This class includes enzymes that previously belonged to other classes. All enzymes in this group catalyze the transfer of ions or molecules across membranes or their separation within membranes. This class, for example, includes transmembrane transporters that carry inorganic residues (sulfates, hydrogen, etc.) or organic substances (maltose, amino acids). All enzymes, when transferred to a new class, retain their old names. This class is further divided into 6 subcategories. The best-known translocase is ATP synthetase, which creates ATP when a proton is pumped.
Conclusion
The most popular classification of enzymes divides them into 6 main and one extra class. All classes are divided into subclasses, and those are categorized further into sub-sub classes, and so on, as a result of which each enzyme has an individual identification number consisting of 4 digits. This classification is not perfect, thus constantly revised (this is how the new 7th class of translocases was added).